ARM: report present cpus in /proc/cpuinfo

Because arm linux likes to dynamically hotplug cpus, the meaning of online has changed slightly. Previously online meant a cpus is schedulable, and conversely offline means they it is not schedulable. But with the current power management infrastructure there are cpus that can be scheduled (after they are woken up automatically), yet are not considered "online" because the masks and flags for them are not set.

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?arch/arm/kernel/setup.c | ? ?2 +-
?1 files changed, 1 insertions(+), 1 deletions(-)

Because arm linux likes to dynamically hotplug cpus, the meaning of online has changed slightly. Previously online meant a cpus is schedulable, and conversely offline means they it is not schedulable. But with the current power management infrastructure there are cpus that can be scheduled (after they are woken up automatically), yet are not considered "online" because the masks and flags for them are not set.

Quoted 7 lines · Jon Mayo

Because arm linux likes to dynamically hotplug cpus, the meaning of online has changed slightly. Previously online meant a cpus is schedulable, and conversely offline means they it is not schedulable. But with the current power management infrastructure there are cpus that can be scheduled (after they are woken up automatically), yet are not considered "online" because the masks and flags for them are not set.

There be sharks here. glibc can read /proc/cpuinfo to find out how many CPUs are online. glibc can also read /proc/stat to determine that number.

Both files should be using the same test to ensure consistency. That is using the online mask, not the present mask.

If a CPU is hot unplugged (and therefore the bit is clear in the online map) then it is not available for scheduling, and the system will not wake the CPU up without intervention, no matter how high the load will become, without userspace assistance.

So I don't think your change is correct, and the code as it stands is right.

Quoted 9 lines · Russell King - ARM Linux

Quoted 7 lines · Jon Mayo

Because arm linux likes to dynamically hotplug cpus, the meaning of online has changed slightly. Previously online meant a cpus is schedulable, and conversely offline means they it is not schedulable. But with the current power management infrastructure there are cpus that can be scheduled (after they are woken up automatically), yet are not considered "online" because the masks and flags for them are not set.

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There be sharks here. glibc can read /proc/cpuinfo to find out how many CPUs are online. glibc can also read /proc/stat to determine that number.

Yea. that's the issue I had with this patch. I couldn't come up with a way to make /proc/stat behave the same without impact other arches.

also what you described is something we call a race. :) reading and parsing cpuinfo then stat, or vice versa, is not atomic. glibc is just going to have to suck it up and deal with cpu1-3 on my system popping in and out randomly in both cpuinfo and stat with the current implementation.

I don't think the behavior of ARM linux makes sense. Neither change is truly correct in my mind. What I feel is the correct behavior is a list (in both stat and cpuinfo) of all cpus either running a task or ready to run a task.

cpu_possible_mask, cpu_present_mask, and cpu_online_mask don't have semantics on ARM that I feel is right. (I don't understand what cpu_active_mask is, but it's probably not what I want either)

Quoted 15 lines · Jon Mayo

Quoted 9 lines · Russell King - ARM Linux

Quoted 7 lines · Jon Mayo

Because arm linux likes to dynamically hotplug cpus, the meaning of online has changed slightly. Previously online meant a cpus is schedulable, and conversely offline means they it is not schedulable. But with the current power management infrastructure there are cpus that can be scheduled (after they are woken up automatically), yet are not considered "online" because the masks and flags for them are not set.

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There be sharks here. glibc can read /proc/cpuinfo to find out how many CPUs are online. glibc can also read /proc/stat to determine that number.

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Yea. that's the issue I had with this patch. I couldn't come up with a way to make /proc/stat behave the same without impact other arches.

also what you described is something we call a race. :) reading and parsing cpuinfo then stat, or vice versa, is not atomic. glibc is just going to have to suck it up and deal with cpu1-3 on my system popping in and out randomly in both cpuinfo and stat with the current implementation.

Well, that's how it is - Linus tends to be opposed to adding syscalls to allow glibc to get this kind of information, blaming glibc for being dumb instead. If you look at the glibc source, you'll see that it has a hint about a 'syscall' for __get_nprocs - that's quite old and has never happened. I don't hope out much hope of anything changing anytime soon.

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I don't think the behavior of ARM linux makes sense. Neither change is truly correct in my mind. What I feel is the correct behavior is a list (in both stat and cpuinfo) of all cpus either running a task or ready to run a task.

That _is_ what you have listed in /proc/cpuinfo and /proc/stat.

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cpu_possible_mask, cpu_present_mask, and cpu_online_mask don't have semantics on ARM that I feel is right. (I don't understand what cpu_active_mask is, but it's probably not what I want either)

They have their defined meaning.

cpu_possible_mask - the CPU number may be available cpu_present_mask - the CPU number is present and is available to be brought
online upon request by the hotplug code
cpu_online_mask - the CPU is becoming available for scheduling cpu_active_mask - the CPU is fully online and available for scheduling

CPUs only spend a _very_ short time in the online but !active state (about the time it takes the CPU asking for it to be brought up to notice that it has been brought up, and for the scheduler migration code to receive the notification that the CPU is now online.) So you can regard the active mask as a mere copy of the online mask for most purposes.

CPUs may be set in the possible mask but not the present mask - that can happen if you limit the number of CPUs on the kernel command line. However, we have no way to bring those CPUs to "present" status, and so they are not available for bringing online - as far as the software is concerned, they're as good as being physically unplugged.

CPUs in the possible mask indicate CPUs which can be present while this kernel is running.

So, actually, our use of these is correct - it can't be any different to any other architecture, because the code which interprets the state from these masks is all architecture independent.

For instance, the generic hotplug code will refuse to bring a cpu online by doing this test:

         if (cpu_online(cpu) || !cpu_present(cpu))
                 return -EINVAL;

So, we (in arch/arm) can't change that decision. Same for online&& active must both be set in order for any process to be scheduled onto that CPU - if any process is on a CPU which is going offline (and therefore !active, !online) then it will be migrated off that CPU by generic code before the CPU goes offline.

I think what you're getting confused over is that within nvidia, you're probably dynamically hotplugging CPUs, and so offline CPUs are apparantly available to the system if the load on the system rises. That's not something in the generic kernel, and is a custom addition. Such an addition _can_ be seen to change the definition of the above masks, but that's not the fault of the kernel - that's the way you're driving the hotplug system.

So, I don't believe there's anything what so ever wrong here, and I don't believe that we're doing anything different from any other SMP platform wrt these masks.

This issue has had me concerned for a while. Because in userspace it can be advantageous to allocate per-cpu structures on start-up for some threading tricks. but if you use the wrong count, funny things can happen.

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I don't think the behavior of ARM linux makes sense. Neither change is truly correct in my mind. What I feel is the correct behavior is a list (in both stat and cpuinfo) of all cpus either running a task or ready to run a task.

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That _is_ what you have listed in /proc/cpuinfo and /proc/stat.

What I see is my idle cpus are not there because we hot unplug them so their power domains can be turned off. scheduling them can happen, but only if an extra step occurs. From user space it's transparent, from kernel space, there is a whole framework making decisions about when to dynamically turn on what.

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cpu_possible_mask, cpu_present_mask, and cpu_online_mask don't have semantics on ARM that I feel is right. (I don't understand what cpu_active_mask is, but it's probably not what I want either)

They have their defined meaning.

cpu_possible_mask - the CPU number may be available cpu_present_mask - the CPU number is present and is available to be brought
online upon request by the hotplug code
cpu_online_mask - the CPU is becoming available for scheduling cpu_active_mask - the CPU is fully online and available for scheduling

CPUs only spend a _very_ short time in the online but !active state (about the time it takes the CPU asking for it to be brought up to notice that it has been brought up, and for the scheduler migration code to receive the notification that the CPU is now online.) So you can regard the active mask as a mere copy of the online mask for most purposes.

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CPUs may be set in the possible mask but not the present mask - that can happen if you limit the number of CPUs on the kernel command line. However, we have no way to bring those CPUs to "present" status, and so they are not available for bringing online - as far as the software is concerned, they're as good as being physically unplugged.

I don't see a use for that semantic. Why shouldn't we add a couple lines of code to the kernel to scrub out unusable situations?

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So, we (in arch/arm) can't change that decision. Same for online&& active must both be set in order for any process to be scheduled onto that CPU - if any process is on a CPU which is going offline (and therefore !active, !online) then it will be migrated off that CPU by generic code before the CPU goes offline.

I will accept that. But then does that mean we (either arch/arm or mach-tegra) have used the cpu hotplug system incorrectly?

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I think what you're getting confused over is that within nvidia, you're probably dynamically hotplugging CPUs, and so offline CPUs are apparantly available to the system if the load on the system rises. That's not something in the generic kernel, and is a custom addition. Such an addition _can_ be seen to change the definition of the above masks, but that's not the fault of the kernel - that's the way you're driving the hotplug system.

sorry. I thought we weren't the only one in arm driving it this way. if what we've done is strange, I'd like to correct it.

Like if I were to think of a big mainframe or xeon server with hotplug cpus, the way the masks work makes perfect sense. I push a button, all the processes get cleared from the cpu, it is marked ass offline. I pull the card from the cabinet, and then it is !present. and maybe instead a new card at a later date. it's just like any other sort of hotplug thing.

I think my issue with cpuinfo/stat's output is with the semantics for "online" being different for this one architecture (mach-tegra) and possibly others (??) than what I would expect.

Quoted 4 lines · Jon Mayo

This issue has had me concerned for a while. Because in userspace it can be advantageous to allocate per-cpu structures on start-up for some threading tricks. but if you use the wrong count, funny things can happen.

Again, if you look at the glibc sources, you'll find that they (in theory) provide two different calls - getconf(_SC_NPROCESSORS_CONF) and getconf(_SC_NPROCESSORS_ONLN). See sysdeps/posix/sysconf.c.

That suggests you you should be using getconf(_SC_NPROCESSORS_CONF).

However, these are provided by __get_nprocs_conf() and __get_nprocs() respectively. See sysdeps/unix/sysv/linux/getsysstats.c. Notice this comment:

/* As far as I know Linux has no separate numbers for configured and
available processors. So make the `get_nprocs_conf' function an alias. */
strong_alias (__get_nprocs, __get_nprocs_conf)

So, getconf(_SC_NPROCESSORS_CONF) and getconf(_SC_NPROCESSORS_ONLN) will probably return the same thing - and it seems to me that you require that to be fixed. That's not for us in ARM to sort out - that's a _generic_ kernel and glibc issue, and needs to be discussed elsewhere.

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I don't think the behavior of ARM linux makes sense. Neither change is truly correct in my mind. What I feel is the correct behavior is a list (in both stat and cpuinfo) of all cpus either running a task or ready to run a task.

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That _is_ what you have listed in /proc/cpuinfo and /proc/stat.

What I see is my idle cpus are not there because we hot unplug them so their power domains can be turned off. scheduling them can happen, but only if an extra step occurs. From user space it's transparent, from kernel space, there is a whole framework making decisions about when to dynamically turn on what.

Exactly. You're complaining that the kernels interpretation of the masks is not correct because you're driving it with a user program which effectively changes that behaviour.

So, if we change the interpretation of the masks, we'll then have people who aren't using your user program complaining that the masks are wrong for them. It's a no-win situation - there is no overall benefit to changing the kernel.

The fact that you're using a user program which dynamically hot-plugs CPUs means that _you're_ changing the system behaviour by running that program, and _you're_ changing the meaning of those masks.

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cpu_possible_mask, cpu_present_mask, and cpu_online_mask don't have semantics on ARM that I feel is right. (I don't understand what cpu_active_mask is, but it's probably not what I want either)

They have their defined meaning.

cpu_possible_mask - the CPU number may be available cpu_present_mask - the CPU number is present and is available to be brought
online upon request by the hotplug code
cpu_online_mask - the CPU is becoming available for scheduling cpu_active_mask - the CPU is fully online and available for scheduling

CPUs only spend a _very_ short time in the online but !active state (about the time it takes the CPU asking for it to be brought up to notice that it has been brought up, and for the scheduler migration code to receive the notification that the CPU is now online.) So you can regard the active mask as a mere copy of the online mask for most purposes.

Quoted 6 lines

CPUs may be set in the possible mask but not the present mask - that can happen if you limit the number of CPUs on the kernel command line. However, we have no way to bring those CPUs to "present" status, and so they are not available for bringing online - as far as the software is concerned, they're as good as being physically unplugged.

Quoted 2 lines

I don't see a use for that semantic. Why shouldn't we add a couple lines of code to the kernel to scrub out unusable situations?

Think about it - if you have real hot-pluggable CPUs (servers do), do you _really_ want to try to bring online a possible CPU (iow, there's a socket on the board) but one which isn't present (iow, the socket is empty.)

That's what the possible + !present case caters for. Possible tells the kernel how many CPUs to allocate per-cpu data structures for. present tells it whether a CPU can be onlined or not.

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So, we (in arch/arm) can't change that decision. Same for online&& active must both be set in order for any process to be scheduled onto that CPU - if any process is on a CPU which is going offline (and therefore !active, !online) then it will be migrated off that CPU by generic code before the CPU goes offline.

I will accept that. But then does that mean we (either arch/arm or mach-tegra) have used the cpu hotplug system incorrectly?

It means you're using it in ways that it was not originally designed to be used - which is for the physical act of hot-plugging CPUs in servers. CPU hotplug was never designed from the outset for this kind of dynamic CPU power management.

Yes, you _can_ use the CPU hotplug interfaces to do this, but as you're finding, there are problems with doing this.

We can't go around making ARM use CPU hotplug differently from everyone else because that'll make things extremely fragile. As you've already found out, glibc getconf() ultimately uses /proc/stat to return the number of CPUs. So in order to allow dynamic hotplugging _and_ return the sensible 'online CPUs' where 'online' means both those which are currently running and dormant, you need to change generic code.

Plus, there's the issue of CPU affinity for processes and IRQs. With current CPU hotplug, a process which has chosen to bind to a particular CPU will have that binding destroyed when the CPU is hot unplugged, and its affinity will be broken. It will be run on a different CPU. This is probably not the semantics you desire.

I'd argue that trying to do dynamic hotplugging is the wrong approach, especially as there is CPUidle (see below.)

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I think what you're getting confused over is that within nvidia, you're probably dynamically hotplugging CPUs, and so offline CPUs are apparantly available to the system if the load on the system rises. That's not something in the generic kernel, and is a custom addition. Such an addition _can_ be seen to change the definition of the above masks, but that's not the fault of the kernel - that's the way you're driving the hotplug system.

sorry. I thought we weren't the only one in arm driving it this way. if what we've done is strange, I'd like to correct it.

I'm aware of ARM having done something like this in their SMP group in the early days of SMP support, but I wasn't aware that it was still being actively persued. I guess that it never really got out of the prototyping stage (or maybe it did but they chose not to address these issues.)

Quoted 6 lines

Like if I were to think of a big mainframe or xeon server with hotplug cpus, the way the masks work makes perfect sense. I push a button, all the processes get cleared from the cpu, it is marked ass offline. I pull the card from the cabinet, and then it is !present. and maybe instead a new card at a later date. it's just like any other sort of hotplug thing.

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I think my issue with cpuinfo/stat's output is with the semantics for "online" being different for this one architecture (mach-tegra) and possibly others (??) than what I would expect.

No, it's no different. As I've explained above, the difference is that you're running a userspace program which automatically does the hotplugging depending on the system load.

That _can_ be viewed as fundamentally changing the system behaviour because CPUs which are offlined are still available for scheduling should the system load become high enough to trigger it.

I think there's an easier way to solve this problem: there is the CPU idle infrastructure, which allows idle CPUs to remain online while allowing them to power down when they're not required. Because they remain online, the scheduler will migrate tasks to them if they're not doing anything, and maybe that's something else to look at.

The other advantage of CPUidle is that you're not breaking the affinity of anything when the CPU is powered down, unlike hotplug.

So, I think you really should be looking at CPUidle, rather than trying to do dynamic hotplugging based on system load.

Quoted 5 lines · Russell King - ARM Linux

Think about it - if you have real hot-pluggable CPUs (servers do), do you _really_ want to try to bring online a possible CPU (iow, there's a socket on the board) but one which isn't present (iow, the socket is empty.)

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That's what the possible + !present case caters for. Possible tells the kernel how many CPUs to allocate per-cpu data structures for. present tells it whether a CPU can be onlined or not.

Yes, that's the difference between present and possible. I'm not suggesting we report cpus that do not exist. I'm suggesting we report cpus that are present, online or not.

Quoted 11 lines · Jon Mayo

Quoted 9 lines · Russell King - ARM Linux

Think about it - if you have real hot-pluggable CPUs (servers do), do you _really_ want to try to bring online a possible CPU (iow, there's a socket on the board) but one which isn't present (iow, the socket is empty.)

That's what the possible + !present case caters for. Possible tells the kernel how many CPUs to allocate per-cpu data structures for. present tells it whether a CPU can be onlined or not.

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Yes, that's the difference between present and possible. I'm not suggesting we report cpus that do not exist. I'm suggesting we report cpus that are present, online or not.

Which is _what_ we do. The problem is that mach-tegra is causing the established well defined APIs to mean something else, and then you're complaining that those APIs don't mean what they were defined to be.

You're really shooting yourself in the foot here, and at this point there is nothing left to discuss.

I can't help you. You need to discuss this with folk who look after the hotplug CPU stuff.

You no longer have an ARM architecture problem, your problem is that you're abusing stuff to get what you want and then complaining that stuff doesn't work as you want it.

Quoted 12 lines · Russell King - ARM Linux

Quoted 11 lines · Jon Mayo

Quoted 9 lines · Russell King - ARM Linux

Think about it - if you have real hot-pluggable CPUs (servers do), do you _really_ want to try to bring online a possible CPU (iow, there's a socket on the board) but one which isn't present (iow, the socket is empty.)

That's what the possible + !present case caters for. Possible tells the kernel how many CPUs to allocate per-cpu data structures for. present tells it whether a CPU can be onlined or not.

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Yes, that's the difference between present and possible. I'm not suggesting we report cpus that do not exist. I'm suggesting we report cpus that are present, online or not.

Which is _what_ we do. The problem is that mach-tegra is causing the established well defined APIs to mean something else, and then you're complaining that those APIs don't mean what they were defined to be.

in arch/arm/kernel/setup.c :

#if defined(CONFIG_SMP)
        for_each_online_cpu(i) {

no, the arm kernel reports online cpus, not present cpus. I now agree that this is the correct behavior, and is consistent with all other platforms. but your responses about present versus possible don't match the code.