Softirq aware -rt scheduling

Hey all,

This series is a set of patches that optimize scheduler decisions around realtime tasks and softirqs. This series is a rebased and reworked set of changes that have been shipping on Android devices for a number of years, originally created to resolve audio glitches seen on devices caused by softirqs for network or storage drivers.

Long running softirqs cause issues because they aren’t currently taken into account when a realtime task is woken up, but they will delay realtime tasks from running if the realtime tasks are placed on a cpu currently running a softirq.

This can easily be seen on some devices by running cyclictest* along with some heavy background filesystems noise:

Without the patches:
T: 0 ( 7596) P:99 I:1000 C: 59980 Min: 7 Act: 13 Avg: 29 Max: 4107
T: 1 ( 7597) P:99 I:1500 C: 39990 Min: 14 Act: 26 Avg: 36 Max: 8994
T: 2 ( 7598) P:99 I:2000 C: 29995 Min: 7 Act: 22 Avg: 35 Max: 3616
T: 3 ( 7599) P:99 I:2500 C: 23915 Min: 7 Act: 25 Avg: 49 Max: 40273
T: 4 ( 7600) P:99 I:3000 C: 19995 Min: 8 Act: 22 Avg: 38 Max: 10510
T: 5 ( 7601) P:99 I:3500 C: 17135 Min: 7 Act: 26 Avg: 39 Max: 13194
T: 6 ( 7602) P:99 I:4000 C: 14990 Min: 7 Act: 26 Avg: 40 Max: 9470
T: 7 ( 7603) P:99 I:4500 C: 13318 Min: 8 Act: 29 Avg: 44 Max: 20101

Which you can visually see in the image here:
https://github.com/johnstultz-work/misc/raw/main/images/2022-08-09-softirq-rt-big-latency.png

Which is from the perfetto trace captured here:
https://ui.perfetto.dev/#!/?s=33661aec8ec82c2da0a59263f36f7d72b4a2f4e7a99b28b222bd12ad872f

The first patch adds a bit of generic infrastructure to get the per-cpu softirq_pending flag.

The second patch in the series adds logic to account for when softirqs are running, and then conditionally based on CONFIG_RT_SOFTIRQ_AWARE_SCHED allows rt-task placement to be done in a way that’s aware if a current softirq might be a long-running one, to potentially place the rt task on another free core.

The third patch in the series adds logic in __do_softirq(), also under CONFIG_RT_SOFTIRQ_AWARE_SCHED, to defer some of the potentially long running softirqs to ksoftirqd if a -rt task is currently running on the cpu. This patch also includes a folded down fix that stubbs out ksoftirqd_running() based on CONFIG_RT_SOFTIRQ_AWARE_SCHED, since in changing to more frequently defer long running softirqs, the logic using ksoftirqd_running will end up being too conservative and needlessly delay shorter-running softirqs.

With these patches we see dramatic improvements in the worst case latencies in the cyclictest* + filesystem noise test above:

With the patches
T: 0 ( 7527) P:99 I:1000 C: 59998 Min: 6 Act: 29 Avg: 35 Max: 1734
T: 1 ( 7528) P:99 I:1500 C: 40000 Min: 7 Act: 39 Avg: 35 Max: 1181
T: 2 ( 7529) P:99 I:2000 C: 30000 Min: 7 Act: 25 Avg: 25 Max: 444
T: 3 ( 7530) P:99 I:2500 C: 24000 Min: 7 Act: 34 Avg: 36 Max: 1729
T: 4 ( 7531) P:99 I:3000 C: 20000 Min: 7 Act: 36 Avg: 25 Max: 406
T: 5 ( 7532) P:99 I:3500 C: 17143 Min: 7 Act: 38 Avg: 34 Max: 1264
T: 6 ( 7533) P:99 I:4000 C: 15000 Min: 7 Act: 27 Avg: 33 Max: 2351
T: 7 ( 7534) P:99 I:4500 C: 13334 Min: 7 Act: 41 Avg: 29 Max: 2285

Since these patches have been carried along for years, and have at times badly collided with upstream, I wanted to submit them for some initial review, discussion and feedback so we could hopefully eventually find a reasonable solution that might land upstream.

* Unfortunately cyclictest had a bug that causes it to always affine threads to cpus preventing them from being migrated. So you’ll need to update to the latest version (which includes a fix) to reproduce.

Let me know what you think!

thanks
-john

New in v5:
* Conditionalize active_softirqs handling (suggested by Alexander Gordeev <[email protected]>)
* Reorder rt_task_fits_cpu to have the "fast" function first (Suggested by Alexander Gordeev <[email protected]>)
* Fix bug I introduced in v2 condensing task_thread_info(task)->preempt_count to preempt_count() (Reported-by: Alexander Gordeev <[email protected]>)
* Tweak comment discription to remove the vauge "slow" descriptor of softirqs being run by ksoftirqd (Suggested by Alexander Gordeev <[email protected]>)
* Switch to using CONFIG_RT_SOFTIRQ_AWARE_SCHED (suggested by Joel Fernandes <[email protected]>)
* Simplify cpu_busy_with_softirqs() logic as pointed out by Alexander Gordeev <[email protected]>
* Switch to using IS_ENABLED rather then defining my own macro (suggsted by Joel Fernandes <[email protected]>)

Connor O'Brien (1):
sched: Avoid placing RT threads on cores handling long softirqs

John Stultz (1):
softirq: Add generic accessor to percpu softirq_pending data

Pavankumar Kondeti (1):
softirq: defer softirq processing to ksoftirqd if CPU is busy with RT

 arch/s390/include/asm/hardirq.h |  6 ++++
 include/linux/interrupt.h       | 20 ++++++++++++++
 init/Kconfig                    | 10 +++++++
 kernel/sched/rt.c               | 49 +++++++++++++++++++++++++++------
 kernel/softirq.c                | 46 +++++++++++++++++++++++++++++--
 5 files changed, 120 insertions(+), 11 deletions(-)

2.38.1.431.g37b22c650d-goog