clocksource: Reduce watchdog readout delay limit to prevent false positives

The "valid" readout delay between the two reads of the watchdog is larger than the valid delta between the resulting watchdog and clocksource intervals, which results in false positive watchdog results.

Assume TSC is the clocksource and HPET is the watchdog and both have a uncertainty margin of 250us (default). The watchdog readout does:

1) wdnow = read(HPET);
2) csnow = read(TSC);
3) wdend = read(HPET);

The valid window for the delta between #1 and #3 is calculated by the uncertainty margins of the watchdog and the clocksource:

   m = 2 * watchdog.uncertainty_margin + cs.uncertainty margin;

which results in 750us for the TSC/HPET case.

The actual interval comparison uses a smaller margin:

   m = watchdog.uncertainty_margin + cs.uncertainty margin;

which results in 500us for the TSC/HPET case.

That means the following scenario will trigger the watchdog:

Watchdog cycle N:

 1)       wdnow[N] = read(HPET);
 2)       csnow[N] = read(TSC);
 3)       wdend[N] = read(HPET);

Assume the delay between #1 and #2 is 100us and the delay between #1 and #3 is within the 750us margin, i.e. the readout is considered valid.

Watchdog cycle N + 1:

 4)       wdnow[N + 1] = read(HPET);
 5)       csnow[N + 1] = read(TSC);
 6)       wdend[N + 1] = read(HPET);

If the delay between #4 and #6 is within the 750us margin then any delay between #4 and #5 which is larger than 600us will fail the interval check and mark the TSC unstable because the intervals are calculated against the previous value:

    wd_int = wdnow[N + 1] - wdnow[N];
    cs_int = csnow[N + 1] - csnow[N];

Putting the above delays in place this results in:

    cs_int = (wdnow[N + 1] + 610us) - (wdnow[N] + 100us);

-> cs_int = wd_int + 510us;

which is obviously larger than the allowed 500us margin and results in marking TSC unstable.

Fix this by using the same margin as the interval comparison. If the delay between two watchdog reads is larger than that, then the readout was either disturbed by interconnect congestion, NMIs or SMIs.

Fixes 4ac1dd3245b9 ("clocksource: Set cs_watchdog_read() checks based on .uncertainty_margin")

Reported-by Daniel J Blueman <[email protected]>

Signed-off-by Thomas Gleixner <[email protected]>

Link https://lore.kernel.org/lkml/[email protected]/ open here

---
 kernel/time/clocksource.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

kernel/time/clocksource.c +2 -2

@@ -252,7 +252,7 @@ enum wd_read_status {
 
 static enum wd_read_status cs_watchdog_read(struct clocksource *cs, u64 *csnow, u64 *wdnow)
 {
-	int64_t md = 2 * watchdog->uncertainty_margin;
+	int64_t md = watchdog->uncertainty_margin;
 	unsigned int nretries, max_retries;
 	int64_t wd_delay, wd_seq_delay;
 	u64 wd_end, wd_end2;

@@ -285,7 +285,7 @@ static enum wd_read_status cs_watchdog_r
 		 * watchdog test.
 		 */
 		wd_seq_delay = cycles_to_nsec_safe(watchdog, wd_end, wd_end2);
-		if (wd_seq_delay > md)
+		if (wd_seq_delay > 2 * md)
 			goto skip_test;
 	}
 

Quoted 4 lines · Thomas Gleixner

The "valid" readout delay between the two reads of the watchdog is larger than the valid delta between the resulting watchdog and clocksource intervals, which results in false positive watchdog results.

Quoted 6 lines

Assume TSC is the clocksource and HPET is the watchdog and both have a uncertainty margin of 250us (default). The watchdog readout does:

1) wdnow = read(HPET);
2) csnow = read(TSC);
3) wdend = read(HPET);

4) wd_end2 = read(HPET);

Quoted 6 lines

The valid window for the delta between #1 and #3 is calculated by the uncertainty margins of the watchdog and the clocksource:

   m = 2 * watchdog.uncertainty_margin + cs.uncertainty margin;

which results in 750us for the TSC/HPET case.

Yes, because this interval includes two watchdog reads (#1 and #3 above) and one clocksource read (#2 above). We therefore need to allow two watchdog uncertainties and one clocksource uncertainty.

Quoted 5 lines

The actual interval comparison uses a smaller margin:

   m = watchdog.uncertainty_margin + cs.uncertainty margin;

which results in 500us for the TSC/HPET case.

This is the (wd_seq_delay > md) comparison, righr? If so, the reason for this is because it is measuring only a pair of watchdog reads (#3 and #4). There is no clocksource read on the latency recheck, so we do not include the cs->uncertainty_margin value, only the pair of watchdog uncertainty values.

If this check fails, that indicates that the watchdog clocksource is much slower than expected (for example, due to memory-system overload affecting HPET on multicore systems), so we skip this measurement interval.

Quoted 10 lines

That means the following scenario will trigger the watchdog:

Watchdog cycle N:

 1)       wdnow[N] = read(HPET);
 2)       csnow[N] = read(TSC);
 3)       wdend[N] = read(HPET);

Assume the delay between #1 and #2 is 100us and the delay between #1 and #3 is within the 750us margin, i.e. the readout is considered valid.

Yes. We expect at most 250us for #1, another 250us for #2, and yet another 250us for #3.

Quoted 11 lines

Watchdog cycle N + 1:

 4)       wdnow[N + 1] = read(HPET);
 5)       csnow[N + 1] = read(TSC);
 6)       wdend[N + 1] = read(HPET);

If the delay between #4 and #6 is within the 750us margin then any delay between #4 and #5 which is larger than 600us will fail the interval check and mark the TSC unstable because the intervals are calculated against the previous value:

Quoted 2 lines

    wd_int = wdnow[N + 1] - wdnow[N];
    cs_int = csnow[N + 1] - csnow[N];

Except that getting 600us latency between #4 and #5 is not consistent with a 250us uncertainty. If that is happening, the uncertainty should instead be at least 300us.

Quoted 7 lines

Putting the above delays in place this results in:

    cs_int = (wdnow[N + 1] + 610us) - (wdnow[N] + 100us);

-> cs_int = wd_int + 510us;

which is obviously larger than the allowed 500us margin and results in marking TSC unstable.

Agreed, but due to the ->uncertainty_margin values being too small.

Quoted 6 lines

Fix this by using the same margin as the interval comparison. If the delay between two watchdog reads is larger than that, then the readout was either disturbed by interconnect congestion, NMIs or SMIs.

Fixes 4ac1dd3245b9 ("clocksource: Set cs_watchdog_read() checks based on .uncertainty_margin")

Reported-by Daniel J Blueman <[email protected]>

If this is happening in real life, we have a couple of choices:

1. Increase the ->uncertainty_margin values to match the objective
universe.

2. In clocksource_watchdog(), replace "(abs(cs_nsec - wd_nsec) > md)"

	with "(abs(cs_nsec - wd_nsec) > 2 * md)".

	The rationale here is that the ->uncertainty_margin values are
	two-tailed, as in the clocksource might report a value that is
	->uncertainty_margin and ->uncertainty_margin too late.  When I
	was coding this, I instead assumed that ->uncertainty_margin
	covered the full range, centered on the correct time value.

You would know better than would I.

My concern is that the patch below would force needless cs_watchdog_read() retries.

Quoted 4 lines · Paul E. McKenney

Quoted 3 lines · Thomas Gleixner

The "valid" readout delay between the two reads of the watchdog is larger than the valid delta between the resulting watchdog and clocksource intervals, which results in false positive watchdog results.

Quoted 7 lines

Quoted 6 lines

Assume TSC is the clocksource and HPET is the watchdog and both have a uncertainty margin of 250us (default). The watchdog readout does:

1) wdnow = read(HPET);
2) csnow = read(TSC);
3) wdend = read(HPET);

4) wd_end2 = read(HPET);

That's completely irrelevant for the problem at hand.

Quoted 5 lines

Quoted 5 lines

The valid window for the delta between #1 and #3 is calculated by the uncertainty margins of the watchdog and the clocksource:

   m = 2 * watchdog.uncertainty_margin + cs.uncertainty margin;

Quoted 5 lines

Quoted 1 line

which results in 750us for the TSC/HPET case.

Yes, because this interval includes two watchdog reads (#1 and #3 above) and one clocksource read (#2 above). We therefore need to allow two watchdog uncertainties and one clocksource uncertainty.

That's a made up and broken theory based on ill defined heuristics.

The uncertainty of a clocksource is defined by:

1) The frequency it runs with, which affects the accuracy of the time readout.

2) The access time

Your implementation defines that the uncertainty is at least 50us, with a default of 125us and an upper limit of 1ms. That's just made up numbers pulled of of thin air which have nothing to do with reality.

Declaring that TSC access time of up to 1ms and at least 50us is hillarious.

Adding up these made up margins and then double the watchdog margin to validate the readout is just a hack to make it "work for me" and thereby breaking the whole machinery for existing problematic systems. See below.

Quoted 11 lines

Quoted 5 lines

The actual interval comparison uses a smaller margin:

   m = watchdog.uncertainty_margin + cs.uncertainty margin;

which results in 500us for the TSC/HPET case.

This is the (wd_seq_delay > md) comparison, righr? If so, the reason for this is because it is measuring only a pair of watchdog reads (#3 and #4). There is no clocksource read on the latency recheck, so we do not include the cs->uncertainty_margin value, only the pair of watchdog uncertainty values.

No. This is the check which does:

	int64_t md = 2 * watchdog->uncertainty_margin;
        ...

                *wdnow = watchdog->read(watchdog);
		*csnow = cs->read(cs);
		wd_end = watchdog->read(watchdog);
                ...
                
		wd_delay = cycles_to_nsec_safe(watchdog, *wdnow, wd_end);
		if (wd_delay <= md + cs->uncertainty_margin) {
                          ...
                          return WR_READ_SUCCESS;
                }

It has nothing to do with the wd_seq_delay check.

Quoted 29 lines

If this check fails, that indicates that the watchdog clocksource is much slower than expected (for example, due to memory-system overload affecting HPET on multicore systems), so we skip this measurement interval.

Quoted 10 lines

That means the following scenario will trigger the watchdog:

Watchdog cycle N:

 1)       wdnow[N] = read(HPET);
 2)       csnow[N] = read(TSC);
 3)       wdend[N] = read(HPET);

Assume the delay between #1 and #2 is 100us and the delay between #1 and #3 is within the 750us margin, i.e. the readout is considered valid.

Yes. We expect at most 250us for #1, another 250us for #2, and yet another 250us for #3.

Quoted 11 lines

Watchdog cycle N + 1:

 4)       wdnow[N + 1] = read(HPET);
 5)       csnow[N + 1] = read(TSC);
 6)       wdend[N + 1] = read(HPET);

If the delay between #4 and #6 is within the 750us margin then any delay between #4 and #5 which is larger than 600us will fail the interval check and mark the TSC unstable because the intervals are calculated against the previous value:

Quoted 6 lines

Quoted 2 lines

    wd_int = wdnow[N + 1] - wdnow[N];
    cs_int = csnow[N + 1] - csnow[N];

Except that getting 600us latency between #4 and #5 is not consistent with a 250us uncertainty. If that is happening, the uncertainty should instead be at least 300us.

That's utter nonsense.

Quoted 9 lines

Quoted 7 lines

Putting the above delays in place this results in:

    cs_int = (wdnow[N + 1] + 610us) - (wdnow[N] + 100us);

-> cs_int = wd_int + 510us;

which is obviously larger than the allowed 500us margin and results in marking TSC unstable.

Agreed, but due to the ->uncertainty_margin values being too small.

You seriously fail to understand basic math.

Let me draw you a picture:

H = HPET read
T = TSC read
RW = read valid window
IW = interval valid window

RW-------------------------------------------RW IW-------------------------IW

HT H <- Read 1 valid
H TH <- Read 2 valid
|---------------------------------| <- Interval too large

Q: How is increasing the uncertainty values fixing the underlying math
problem of RW > IW?

A: Not at all. It just papers over it and makes the false positive case
more unlikely by further weakening the accuracy.

Quoted 7 lines

Quoted 6 lines

Fix this by using the same margin as the interval comparison. If the delay between two watchdog reads is larger than that, then the readout was either disturbed by interconnect congestion, NMIs or SMIs.

Fixes 4ac1dd3245b9 ("clocksource: Set cs_watchdog_read() checks based on .uncertainty_margin")

Reported-by Daniel J Blueman <[email protected]>

Quoted 4 lines

If this is happening in real life, we have a couple of choices:

1. Increase the ->uncertainty_margin values to match the objective
universe.

Which universe? The universe of made up math? See above.

Quoted 8 lines

2. In clocksource_watchdog(), replace "(abs(cs_nsec - wd_nsec) > md)"

	with "(abs(cs_nsec - wd_nsec) > 2 * md)".

	The rationale here is that the ->uncertainty_margin values are
	two-tailed, as in the clocksource might report a value that is
	->uncertainty_margin and ->uncertainty_margin too late.  When I
	was coding this, I instead assumed that ->uncertainty_margin
	covered the full range, centered on the correct time value.

So you propose the opposite of what I'm doing, which weakens the watchdog even further.

Quoted 4 lines

You would know better than would I.

My concern is that the patch below would force needless cs_watchdog_read() retries.

That's not the end of the world and way better than degrading the watchdog further.

It's already useless for the original purpose to detect even slow skew of TSC between CPUs because it is relative and the uncertainty margins are insanely big.

I just booted an old machine where the BIOS "hides" SMI time by saving the TSC value on entry and restoring it on exit. The original watchdog implementation caught that. Now it happily continues and I can observe time going backwards between CPUs in user space. After an hour the difference between the two CPUs is > 1sec and the system still claims that everything is fine. And no, TSC_ADJUST does not catch that on this machine because the CPU does not support it.

IOW, this whole big machine scalability hackery broke basic functionality for existing systems.

We really need to go back to the drawing board and rethink this machinery from scratch.

There are two main aspects to the watchdog:

1) Detect general frequency skew by comparing it against a known (assumed to be) stable clocksource

This addresses the problems on older machines where the BIOS fiddles with the CPU frequency behind the kernels back.

That's a non-issue on modern CPUs because the TSC is constant frequency.

2) Detect inter CPU skew

This addresses the problems where

A) the BIOS fiddles with the TSC behind the kernels back (see above) and the modification cannot be detected by the TSC_ADJUST MSR due to its non-existance

Not an issue on modern CPUs

B) the sockets are not synchronized and the TSC frequency on them drifts apart

C) hardware failure (something you mentioned back then when you hacked this uncertainty magic up)

As I just validated on that old machine the whole "scalability" hackery broke #2A and #2B unless the modifications or drifts are massive. But if they are not it leaves both user space and kernel with inconsistent time.

That means we really have to look at this in two ways:

I) On old hardware where TSC is not constant frequency, the validation against HPET is required.

II) On new hardware with constant frequency TSC and TSC_ADJUST_MSR the
HPET comparison is not really useful anymore as it does not detect low drift issues between unsynchronized sockets.

What can be done instead?

- Make CPU0 the watchdog supervisor, which runs the timer and orchestrates the machinery.

- Limit the HPET comparison to CPUs which lack constant frequency, which makes the whole legacy I/O issue on large systems go away.

Run this comparision only on CPU0

- Instead of moving the timer around CPUs, utilize a SMP function call similar to what the TSC synchronization mechanism does, i.e.

CPU0 CPUN
sync() sync()
t1 = rdtsc()
handover()
t2 = rdtsc()
handover()
t3 = rdtsc()
....

Each step validates that tN <= TN+1, which detects all issues of #2 above for both old and contemporary machines.

Of course this mechanism is also affected by interconnect congestion, but the only side effect of interconnect congestion is that the handover becomes slow, which means that the accuracy of detecting time going backwards suffers temporarily.

With that all these made up uncertainty heuristics go completely away or can be turned into something which actually reflects the reality of the universe, i.e. the accuracy of the clocks and their access time.

I doubt it's needed because the original implementation worked just fine and it's only relevant for the actual HPET/TSC comparison case. That is then limited to museum pieces which are not really affected by the scalability issues of todays machines. Especially not as the HPET/TSC check is limited to CPU0, which is the one closest to the legacy peripherals which contain the HPET or in real old machines the ACPI_PM timer.

I'll go and utilize my copious spare time to implement this, but don't expect it to materialize before 2026 :)

Quoted 8 lines · Thomas Gleixner

Quoted 7 lines · Paul E. McKenney

Quoted 6 lines · Thomas Gleixner

Assume TSC is the clocksource and HPET is the watchdog and both have a uncertainty margin of 250us (default). The watchdog readout does:

1) wdnow = read(HPET);
2) csnow = read(TSC);
3) wdend = read(HPET);

Quoted 3 lines

Quoted 1 line

4) wd_end2 = read(HPET);

That's completely irrelevant for the problem at hand.

It is exactly one of the problems at hand. If we didn't have HPET readout performance issues, we could dispense with that piece of the puzzle.

By the way, did this issue really occur on real hardware? Or are we having a strictly theoretical discussion?

Quoted 5 lines

Quoted 5 lines

Quoted 5 lines

The valid window for the delta between #1 and #3 is calculated by the uncertainty margins of the watchdog and the clocksource:

   m = 2 * watchdog.uncertainty_margin + cs.uncertainty margin;

Quoted 7 lines

Quoted 5 lines

Quoted 1 line

which results in 750us for the TSC/HPET case.

Yes, because this interval includes two watchdog reads (#1 and #3 above) and one clocksource read (#2 above). We therefore need to allow two watchdog uncertainties and one clocksource uncertainty.

That's a made up and broken theory based on ill defined heuristics.

If we want better heuristics, we either restrict ourselves to timer hardware that has reasonable access latencies or we add something like an ->access_time field to struct clocksource.

Otherwise, you will just be cutting off one end of the heuristic "blanket" and sewing on to another.

Accounting more directly for vCPU preemption would also help a lot, as that was the root cause of much (but by no means all) of the problem on Meta's fleet.

Quoted 10 lines

The uncertainty of a clocksource is defined by:

1) The frequency it runs with, which affects the accuracy of the time readout.

2) The access time

Your implementation defines that the uncertainty is at least 50us, with a default of 125us and an upper limit of 1ms. That's just made up numbers pulled of of thin air which have nothing to do with reality.

These were set using test data from real hardware.

Quoted 2 lines

Declaring that TSC access time of up to 1ms and at least 50us is hillarious.

Yes, in theory we could instead have an ->access_time that is used to reject bad reads, so that ->uncertainty_margin would only be used to detect timer skew. The discussion of such an addition didn't go far last time around, but maybe now is a better time. Though I suspect that the issues around determining what ->access_time should be set to are still with us. But I would love to be proven wrong.

Quoted 3 lines

Adding up these made up margins and then double the watchdog margin to validate the readout is just a hack to make it "work for me" and thereby breaking the whole machinery for existing problematic systems. See below.

Nope.

The readout #4 above into wd_end2 wasn't something my employer needed. For our use cases, #1, #2, and #3 sufficed. It was instead needed by other users running memory-bandwidth-heavy workloads on multi-socket systems. This resulted in HPET access times that were astoundingly large.

One might naively hope that the HPETs in a given system could synchronize themselves so that each CPU could read the HPET nearest it instead of everyone reading CPU 0's HPET. Or that some other means would allow reasonable access times. Hey, I can dream, can't I?

Quoted 28 lines

Quoted 11 lines

Quoted 5 lines

The actual interval comparison uses a smaller margin:

   m = watchdog.uncertainty_margin + cs.uncertainty margin;

which results in 500us for the TSC/HPET case.

This is the (wd_seq_delay > md) comparison, righr? If so, the reason for this is because it is measuring only a pair of watchdog reads (#3 and #4). There is no clocksource read on the latency recheck, so we do not include the cs->uncertainty_margin value, only the pair of watchdog uncertainty values.

No. This is the check which does:

	int64_t md = 2 * watchdog->uncertainty_margin;
        ...

                *wdnow = watchdog->read(watchdog);
		*csnow = cs->read(cs);
		wd_end = watchdog->read(watchdog);
                ...
                
		wd_delay = cycles_to_nsec_safe(watchdog, *wdnow, wd_end);
		if (wd_delay <= md + cs->uncertainty_margin) {
                          ...
                          return WR_READ_SUCCESS;
                }

Quoted 1 line

It has nothing to do with the wd_seq_delay check.

Eh? Here wd_delay is twice watchdog uncertainty plus clocksource uncertainty.

Quoted 14 lines

Let me draw you a picture:

H = HPET read
T = TSC read
RW = read valid window
IW = interval valid window

RW-------------------------------------------RW IW-------------------------IW

HT H <- Read 1 valid
H TH <- Read 2 valid
|---------------------------------| <- Interval too large

Quoted 5 lines

Q: How is increasing the uncertainty values fixing the underlying math
problem of RW > IW?

A: Not at all. It just papers over it and makes the false positive case
more unlikely by further weakening the accuracy.

Yep. Given the current lack of something like ->access_time to go along with the current ->uncertainty_margin, there are limits to what we can do.

Again, was the report from a real workload running on real hardware, or are we simply having a theoretical discussion of what might happen?

OK, that is at least a real problem, even if on antique hardware. A problem that surprises me, but I don't have that sort of hardware, so I will take your word for it. Though you would need what, a 62.5 millisecond SMI for the old watchdog to catch it, right?

(Yes, yes, I have seen far longer SMIs. Don't get me started...)

Quoted 10 lines

We really need to go back to the drawing board and rethink this machinery from scratch.

There are two main aspects to the watchdog:

1) Detect general frequency skew by comparing it against a known (assumed to be) stable clocksource

This addresses the problems on older machines where the BIOS fiddles with the CPU frequency behind the kernels back.

Fair enough.

Quoted 2 lines

That's a non-issue on modern CPUs because the TSC is constant frequency.

Give or take the occasional messed-up motherboard or firmware, where the frequency is constant---but wrong. :-/

Quoted 10 lines

2) Detect inter CPU skew

This addresses the problems where

A) the BIOS fiddles with the TSC behind the kernels back (see above) and the modification cannot be detected by the TSC_ADJUST MSR due to its non-existance

Not an issue on modern CPUs

Quoted 5 lines

B) the sockets are not synchronized and the TSC frequency on them drifts apart

C) hardware failure (something you mentioned back then when you hacked this uncertainty magic up)

3) Deal with insanely slow readout times for some clocksources. Which your smp_call_function() approach describe below should handle.

Quoted 4 lines

As I just validated on that old machine the whole "scalability" hackery broke #2A and #2B unless the modifications or drifts are massive. But if they are not it leaves both user space and kernel with inconsistent time.

Agreed. Imperfect though the current setup might be, there were reasons why we changed it.

Quoted 9 lines

That means we really have to look at this in two ways:

I) On old hardware where TSC is not constant frequency, the validation against HPET is required.

II) On new hardware with constant frequency TSC and TSC_ADJUST_MSR the
HPET comparison is not really useful anymore as it does not detect low drift issues between unsynchronized sockets.

Quoted 4 lines

What can be done instead?

- Make CPU0 the watchdog supervisor, which runs the timer and orchestrates the machinery.

Fine on x86, but last I knew some systems can run without CPU 0. Maybe the boot CPU? Though that can be offlined on some systems. Handoff? For NO_HZ_FULL people, confine to the housekeeping CPUs?

Or is x86 the only system with more than one timer?

Quoted 2 lines

- Limit the HPET comparison to CPUs which lack constant frequency, which makes the whole legacy I/O issue on large systems go away.

I thought that was already the default, and that you need to use the tsc=watchdog kernel boot parameter to override that default in order to use a known-stable TSC as the watchdog for HPET and/or ACPI PM timer.

Quoted 14 lines

Run this comparision only on CPU0

- Instead of moving the timer around CPUs, utilize a SMP function call similar to what the TSC synchronization mechanism does, i.e.

CPU0 CPUN
sync() sync()
t1 = rdtsc()
handover()
t2 = rdtsc()
handover()
t3 = rdtsc()
....

Quoted 7 lines

Each step validates that tN <= TN+1, which detects all issues of #2 above for both old and contemporary machines.

Of course this mechanism is also affected by interconnect congestion, but the only side effect of interconnect congestion is that the handover becomes slow, which means that the accuracy of detecting time going backwards suffers temporarily.

Is this going to be OK for real-time workloads? I guess that you have at least as good a chance as I would to convince them. The more aggressive NO_HZ_FULL people might have concerns. Especially these guys:

https://arxiv.org/abs/2509.03855

I have similar code in the current clocksource watchdog, but it doesn't run until clocksource skew is detected, in other words, only after something has likely gone sideways.

Quoted 11 lines

With that all these made up uncertainty heuristics go completely away or can be turned into something which actually reflects the reality of the universe, i.e. the accuracy of the clocks and their access time.

I doubt it's needed because the original implementation worked just fine and it's only relevant for the actual HPET/TSC comparison case. That is then limited to museum pieces which are not really affected by the scalability issues of todays machines. Especially not as the HPET/TSC check is limited to CPU0, which is the one closest to the legacy peripherals which contain the HPET or in real old machines the ACPI_PM timer.

Give or take firmware/hardware bugs that cause the TSC to at a constant but incorrect rate. Maybe interact with userspace facilities such as NTP?

Quoted 2 lines

I'll go and utilize my copious spare time to implement this, but don't expect it to materialize before 2026 :)

The current implementation is working for us, so your schedule is our schedule. ;-)

Quoted 5 lines · Thomas Gleixner

Quoted 2 lines

My concern is that the patch below would force needless cs_watchdog_read() retries.

That's not the end of the world and way better than degrading the watchdog further.

But what you proposed is just a further tweak of the heuristics you so energetically decry above.

One might naively hope that the HPETs in a given system could synchronize themselves so that each CPU could read the HPET nearest it instead of everyone reading CPU 0's HPET. Or that some other means would allow reasonable access times. Hey, I can dream, can't I?

Quoted 4 lines · Paul E. McKenney

One might naively hope that the HPETs in a given system could synchronize themselves so that each CPU could read the HPET nearest it instead of everyone reading CPU 0's HPET. Or that some other means would allow reasonable access times. Hey, I can dream, can't I?

Sounds more like a nightmare TBH.

Quoted 7 lines · Paul E. McKenney

Quoted 5 lines · Thomas Gleixner

Quoted 2 lines

My concern is that the patch below would force needless cs_watchdog_read() retries.

That's not the end of the world and way better than degrading the watchdog further.

Quoted 2 lines

But what you proposed is just a further tweak of the heuristics you so energetically decry above.

I fully agree that it is a bandaid fix, but it makes the machinery consistent and correct. It prevents false positives, which are inevitable in the current design. No?

Quoted 11 lines · Paul E. McKenney

Quoted 10 lines · Thomas Gleixner

Quoted 8 lines · Paul E. McKenney

Quoted 6 lines · Thomas Gleixner

Assume TSC is the clocksource and HPET is the watchdog and both have a uncertainty margin of 250us (default). The watchdog readout does:

1) wdnow = read(HPET);
2) csnow = read(TSC);
3) wdend = read(HPET);

4) wd_end2 = read(HPET);

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That's completely irrelevant for the problem at hand.

It is exactly one of the problems at hand. If we didn't have HPET readout performance issues, we could dispense with that piece of the puzzle.

It's part of the "make it work" hack, but unrelated to the fact that the readout valid window is larger than the interval valid window.

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By the way, did this issue really occur on real hardware? Or are we having a strictly theoretical discussion?

It has been reported by Daniel and it happens on real hardware.

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The valid window for the delta between #1 and #3 is calculated by the uncertainty margins of the watchdog and the clocksource:

   m = 2 * watchdog.uncertainty_margin + cs.uncertainty margin;

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which results in 750us for the TSC/HPET case.

Yes, because this interval includes two watchdog reads (#1 and #3 above) and one clocksource read (#2 above). We therefore need to allow two watchdog uncertainties and one clocksource uncertainty.

That's a made up and broken theory based on ill defined heuristics.

If we want better heuristics, we either restrict ourselves to timer hardware that has reasonable access latencies or we add something like an ->access_time field to struct clocksource.

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Otherwise, you will just be cutting off one end of the heuristic "blanket" and sewing on to another.

Accounting more directly for vCPU preemption would also help a lot, as that was the root cause of much (but by no means all) of the problem on Meta's fleet.

The goal is to get rid of the heuristics completely.

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The uncertainty of a clocksource is defined by:

1) The frequency it runs with, which affects the accuracy of the time readout.

2) The access time

Your implementation defines that the uncertainty is at least 50us, with a default of 125us and an upper limit of 1ms. That's just made up numbers pulled of of thin air which have nothing to do with reality.

These were set using test data from real hardware.

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Declaring that TSC access time of up to 1ms and at least 50us is hillarious.

Yes, in theory we could instead have an ->access_time that is used to reject bad reads, so that ->uncertainty_margin would only be used to detect timer skew. The discussion of such an addition didn't go far last time around, but maybe now is a better time. Though I suspect that the issues around determining what ->access_time should be set to are still with us. But I would love to be proven wrong.

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Adding up these made up margins and then double the watchdog margin to validate the readout is just a hack to make it "work for me" and thereby breaking the whole machinery for existing problematic systems. See below.

Nope.

The readout #4 above into wd_end2 wasn't something my employer needed. For our use cases, #1, #2, and #3 sufficed. It was instead needed by other users running memory-bandwidth-heavy workloads on multi-socket systems. This resulted in HPET access times that were astoundingly large.

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One might naively hope that the HPETs in a given system could synchronize themselves so that each CPU could read the HPET nearest it instead of everyone reading CPU 0's HPET. Or that some other means would allow reasonable access times. Hey, I can dream, can't I?

Latest at that point it should have been clear that heuristics aren't going to cut it.

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The actual interval comparison uses a smaller margin:

   m = watchdog.uncertainty_margin + cs.uncertainty margin;

which results in 500us for the TSC/HPET case.

This is the (wd_seq_delay > md) comparison, righr? If so, the reason for this is because it is measuring only a pair of watchdog reads (#3 and #4). There is no clocksource read on the latency recheck, so we do not include the cs->uncertainty_margin value, only the pair of watchdog uncertainty values.

No. This is the check which does:

	int64_t md = 2 * watchdog->uncertainty_margin;
        ...

                *wdnow = watchdog->read(watchdog);
		*csnow = cs->read(cs);
		wd_end = watchdog->read(watchdog);
                ...
                
		wd_delay = cycles_to_nsec_safe(watchdog, *wdnow, wd_end);
		if (wd_delay <= md + cs->uncertainty_margin) {
                          ...
                          return WR_READ_SUCCESS;
                }

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It has nothing to do with the wd_seq_delay check.

Eh? Here wd_delay is twice watchdog uncertainty plus clocksource uncertainty.

Correct. As implemented by Paul McKenney, no?

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Let me draw you a picture:

H = HPET read
T = TSC read
RW = read valid window
IW = interval valid window

RW-------------------------------------------RW IW-------------------------IW

HT H <- Read 1 valid
H TH <- Read 2 valid
|---------------------------------| <- Interval too large

Q: How is increasing the uncertainty values fixing the underlying math
problem of RW > IW?

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A: Not at all. It just papers over it and makes the false positive case
more unlikely by further weakening the accuracy.

Yep. Given the current lack of something like ->access_time to go along with the current ->uncertainty_margin, there are limits to what we can do.

Well, we can at least limit the damage by making RW and IW consistent, no?

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Again, was the report from a real workload running on real hardware, or are we simply having a theoretical discussion of what might happen?

It's real hardware and again even in theory it's entirely clear that having a readout validation window which is larger than the interval validation window leads exactly to the problem reported by Daniel. No?

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OK, that is at least a real problem, even if on antique hardware. A problem that surprises me, but I don't have that sort of hardware, so I will take your word for it. Though you would need what, a 62.5 millisecond SMI for the old watchdog to catch it, right?

(Yes, yes, I have seen far longer SMIs. Don't get me started...)

It's a SMI plagued horrorshow where BIOS writers did value add!

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We really need to go back to the drawing board and rethink this machinery from scratch.

There are two main aspects to the watchdog:

1) Detect general frequency skew by comparing it against a known (assumed to be) stable clocksource

This addresses the problems on older machines where the BIOS fiddles with the CPU frequency behind the kernels back.

Fair enough.

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That's a non-issue on modern CPUs because the TSC is constant frequency.

Give or take the occasional messed-up motherboard or firmware, where the frequency is constant---but wrong. :-/

We still can enforce the frequency check even for constant frequency systems.

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2) Detect inter CPU skew

This addresses the problems where

A) the BIOS fiddles with the TSC behind the kernels back (see above) and the modification cannot be detected by the TSC_ADJUST MSR due to its non-existance

Not an issue on modern CPUs

B) the sockets are not synchronized and the TSC frequency on them drifts apart

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C) hardware failure (something you mentioned back then when you hacked this uncertainty magic up)

3) Deal with insanely slow readout times for some clocksources. Which your smp_call_function() approach describe below should handle.

That's an implementation detail, but not part of what the watchdog should actually detect, i.e. frequency change and inter CPU skew.

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As I just validated on that old machine the whole "scalability" hackery broke #2A and #2B unless the modifications or drifts are massive. But if they are not it leaves both user space and kernel with inconsistent time.

Agreed. Imperfect though the current setup might be, there were reasons why we changed it.

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That means we really have to look at this in two ways:

I) On old hardware where TSC is not constant frequency, the validation against HPET is required.

II) On new hardware with constant frequency TSC and TSC_ADJUST_MSR the
HPET comparison is not really useful anymore as it does not detect low drift issues between unsynchronized sockets.

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What can be done instead?

- Make CPU0 the watchdog supervisor, which runs the timer and orchestrates the machinery.

Fine on x86, but last I knew some systems can run without CPU 0. Maybe the boot CPU? Though that can be offlined on some systems. Handoff? For NO_HZ_FULL people, confine to the housekeeping CPUs?

Sure, that's a detail where you assign it to.

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Or is x86 the only system with more than one timer?

The watchdog is only enabled on x86 and MIPS, but none of the MIPS clocksources sets the MUST_VERIFY flag. PARISC sets the MUST_VERIFY flag, but does not enable the watchdog. So it's effectively x86.

It seems that everything else either has a sane timer or at least believes so.

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- Limit the HPET comparison to CPUs which lack constant frequency, which makes the whole legacy I/O issue on large systems go away.

I thought that was already the default, and that you need to use the tsc=watchdog kernel boot parameter to override that default in order to use a known-stable TSC as the watchdog for HPET and/or ACPI PM timer.

Correct.

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Run this comparision only on CPU0

- Instead of moving the timer around CPUs, utilize a SMP function call similar to what the TSC synchronization mechanism does, i.e.

CPU0 CPUN
sync() sync()
t1 = rdtsc()
handover()
t2 = rdtsc()
handover()
t3 = rdtsc()
....

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Each step validates that tN <= TN+1, which detects all issues of #2 above for both old and contemporary machines.

Of course this mechanism is also affected by interconnect congestion, but the only side effect of interconnect congestion is that the handover becomes slow, which means that the accuracy of detecting time going backwards suffers temporarily.

Is this going to be OK for real-time workloads? I guess that you have at least as good a chance as I would to convince them. The more aggressive NO_HZ_FULL people might have concerns. Especially these guys:

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https://arxiv.org/abs/2509.03855

I'm sure that the wishful thinking departement which ignores the system immanent latencies and demonstrates great results on a system which is tuned to run only the demonstrator will go apeshit.

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I have similar code in the current clocksource watchdog, but it doesn't run until clocksource skew is detected, in other words, only after something has likely gone sideways.

Of course we can exclude the nohz full CPUs, but that's again an implementation detail and does not change the underlying design.

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With that all these made up uncertainty heuristics go completely away or can be turned into something which actually reflects the reality of the universe, i.e. the accuracy of the clocks and their access time.

I doubt it's needed because the original implementation worked just fine and it's only relevant for the actual HPET/TSC comparison case. That is then limited to museum pieces which are not really affected by the scalability issues of todays machines. Especially not as the HPET/TSC check is limited to CPU0, which is the one closest to the legacy peripherals which contain the HPET or in real old machines the ACPI_PM timer.

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Give or take firmware/hardware bugs that cause the TSC to at a constant but incorrect rate. Maybe interact with userspace facilities such as NTP?

Either we enforce HPET/ACPI_PM comparison on the supervisor CPU or let user space deal with it. If NTP detects time going sideways, but being consistent between CPUs, then it can switch the clocksource to HPET or ACPI_PM already today.

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I'll go and utilize my copious spare time to implement this, but don't expect it to materialize before 2026 :)

The current implementation is working for us, so your schedule is our schedule. ;-)

Thanks for confirming that this is based on "works for me" :)

Thanks,

tglx

The "valid" readout delay between the two reads of the watchdog is larger than the valid delta between the resulting watchdog and clocksource intervals, which results in false positive watchdog results.

Assume TSC is the clocksource and HPET is the watchdog and both have a uncertainty margin of 250us (default). The watchdog readout does:

1) wdnow = read(HPET);
2) csnow = read(TSC);
3) wdend = read(HPET);

The valid window for the delta between #1 and #3 is calculated by the uncertainty margins of the watchdog and the clocksource:

   m = 2 * watchdog.uncertainty_margin + cs.uncertainty margin;

which results in 750us for the TSC/HPET case.

The actual interval comparison uses a smaller margin:

   m = watchdog.uncertainty_margin + cs.uncertainty margin;

which results in 500us for the TSC/HPET case.

That means the following scenario will trigger the watchdog:

Watchdog cycle N:

 1)       wdnow[N] = read(HPET);
 2)       csnow[N] = read(TSC);
 3)       wdend[N] = read(HPET);

Assume the delay between #1 and #2 is 100us and the delay between #1 and #3 is within the 750us margin, i.e. the readout is considered valid.

Watchdog cycle N + 1:

 4)       wdnow[N + 1] = read(HPET);
 5)       csnow[N + 1] = read(TSC);
 6)       wdend[N + 1] = read(HPET);

If the delay between #4 and #6 is within the 750us margin then any delay between #4 and #5 which is larger than 600us will fail the interval check and mark the TSC unstable because the intervals are calculated against the previous value:

    wd_int = wdnow[N + 1] - wdnow[N];
    cs_int = csnow[N + 1] - csnow[N];

Putting the above delays in place this results in:

    cs_int = (wdnow[N + 1] + 610us) - (wdnow[N] + 100us);

-> cs_int = wd_int + 510us;

which is obviously larger than the allowed 500us margin and results in marking TSC unstable.

Fix this by using the same margin as the interval comparison. If the delay between two watchdog reads is larger than that, then the readout was either disturbed by interconnect congestion, NMIs or SMIs.

Fixes 4ac1dd3245b9 ("clocksource: Set cs_watchdog_read() checks based on .uncertainty_margin")

Reported-by Daniel J Blueman <[email protected]>

Signed-off-by Thomas Gleixner <[email protected]>

Link https://lore.kernel.org/lkml/[email protected]/ open here

---
 kernel/time/clocksource.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

kernel/time/clocksource.c +2 -2

@@ -252,7 +252,7 @@ enum wd_read_status {
 
 static enum wd_read_status cs_watchdog_read(struct clocksource *cs, u64 *csnow, u64 *wdnow)
 {
-	int64_t md = 2 * watchdog->uncertainty_margin;
+	int64_t md = watchdog->uncertainty_margin;
 	unsigned int nretries, max_retries;
 	int64_t wd_delay, wd_seq_delay;
 	u64 wd_end, wd_end2;

@@ -285,7 +285,7 @@ static enum wd_read_status cs_watchdog_r
 		 * watchdog test.
 		 */
 		wd_seq_delay = cycles_to_nsec_safe(watchdog, wd_end, wd_end2);
-		if (wd_seq_delay > md)
+		if (wd_seq_delay > 2 * md)
 			goto skip_test;
 	}
 

The "valid" readout delay between the two reads of the watchdog is larger than the valid delta between the resulting watchdog and clocksource intervals, which results in false positive watchdog results.

Assume TSC is the clocksource and HPET is the watchdog and both have a uncertainty margin of 250us (default). The watchdog readout does:

1) wdnow = read(HPET);
2) csnow = read(TSC);
3) wdend = read(HPET);

The valid window for the delta between #1 and #3 is calculated by the uncertainty margins of the watchdog and the clocksource:

    m = 2 * watchdog.uncertainty_margin + cs.uncertainty margin;

which results in 750us for the TSC/HPET case.

The actual interval comparison uses a smaller margin:

    m = watchdog.uncertainty_margin + cs.uncertainty margin;

which results in 500us for the TSC/HPET case.

That means the following scenario will trigger the watchdog:

Watchdog cycle N:

  1)       wdnow[N] = read(HPET);
  2)       csnow[N] = read(TSC);
  3)       wdend[N] = read(HPET);

Assume the delay between #1 and #2 is 100us and the delay between #1 and #3 is within the 750us margin, i.e. the readout is considered valid.

Watchdog cycle N + 1:

  4)       wdnow[N + 1] = read(HPET);
  5)       csnow[N + 1] = read(TSC);
  6)       wdend[N + 1] = read(HPET);

If the delay between #4 and #6 is within the 750us margin then any delay between #4 and #5 which is larger than 600us will fail the interval check and mark the TSC unstable because the intervals are calculated against the previous value:

     wd_int = wdnow[N + 1] - wdnow[N];
     cs_int = csnow[N + 1] - csnow[N];

Putting the above delays in place this results in:

     cs_int = (wdnow[N + 1] + 610us) - (wdnow[N] + 100us);

-> cs_int = wd_int + 510us;

which is obviously larger than the allowed 500us margin and results in marking TSC unstable.

Fix this by using the same margin as the interval comparison. If the delay between two watchdog reads is larger than that, then the readout was either disturbed by interconnect congestion, NMIs or SMIs.

Fixes 4ac1dd3245b9 ("clocksource: Set cs_watchdog_read() checks based on .uncertainty_margin")

Reported-by Daniel J Blueman <[email protected]>

Signed-off-by Thomas Gleixner <[email protected]>

Link https://lore.kernel.org/lkml/[email protected]/ open here

---
  kernel/time/clocksource.c |    4 ++--
  1 file changed, 2 insertions(+), 2 deletions(-)

kernel/time/clocksource.c +1 -1

@@ -252,7 +252,7 @@ enum wd_read_status {
 
  static enum wd_read_status cs_watchdog_read(struct clocksource *cs, u64 *csnow, u64 *wdnow)
  {
-	int64_t md = 2 * watchdog->uncertainty_margin;
+	int64_t md = watchdog->uncertainty_margin;
  	unsigned int nretries, max_retries;
  	int64_t wd_delay, wd_seq_delay;
  	u64 wd_end, wd_end2;