linux-kernel - Re: [PATCH 00/30] PREEMPT_AUTO: support lazy rescheduling

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Date: Fri, 26 Apr 2024 12:00:00 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
        Ankur Arora
 <ankur.a.arora@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>, peterz@...radead.org,
        paulmck@...nel.org, akpm@...ux-foundation.org, luto@...nel.org,
        bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com,
        mingo@...hat.com, juri.lelli@...hat.com, vincent.guittot@...aro.org,
        willy@...radead.org, mgorman@...e.de, jpoimboe@...nel.org,
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        bristot@...nel.org, mathieu.desnoyers@...icios.com,
        geert@...ux-m68k.org, glaubitz@...sik.fu-berlin.de,
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        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com,
        LKML
 <linux-kernel@...r.kernel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH 00/30] PREEMPT_AUTO: support lazy rescheduling


Shrikanth Hegde <sshegde@...ux.ibm.com> writes:

> On 4/23/24 9:43 PM, Linus Torvalds wrote:
>> On Tue, 23 Apr 2024 at 08:23, Shrikanth Hegde <sshegde@...ux.ibm.com> wrote:
>>>
>>>
>>> Are these the only arch bits that need to be defined? am I missing something very
>>> basic here? will try to debug this further. Any inputs?
>>
>> I don't think powerpc uses the generic *_exit_to_user_mode() helper
>> functions, so you'll need to also add that logic to the low-level
>> powerpc code.
>>
>> IOW, on x86, with this patch series, patch 06/30 did this:
>>
>> -               if (ti_work & _TIF_NEED_RESCHED)
>> +               if (ti_work & (_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY))
>>                         schedule();
>>
>> in kernel/entry/common.c exit_to_user_mode_loop().
>>
>> But that works on x86 because it uses the irqentry_exit_to_user_mode().
>>
>> On PowerPC, I think you need to at least fix up
>>
>>     interrupt_exit_user_prepare_main()
>>
>> similarly (and any other paths like that - I used to know the powerpc
>> code, but that was long long LOOONG ago).
>>
>>                 Linus
>
> Thank you Linus for the pointers. That indeed did the trick.
>
> diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c
> index eca293794a1e..f0f38bf5cea9 100644
> --- a/arch/powerpc/kernel/interrupt.c
> +++ b/arch/powerpc/kernel/interrupt.c
> @@ -185,7 +185,7 @@ interrupt_exit_user_prepare_main(unsigned long ret, struct pt_regs *regs)
>         ti_flags = read_thread_flags();
>         while (unlikely(ti_flags & (_TIF_USER_WORK_MASK & ~_TIF_RESTORE_TM))) {
>                 local_irq_enable();
> -               if (ti_flags & _TIF_NEED_RESCHED) {
> +               if (ti_flags & (_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY) ) {
>                         schedule();
>                 } else {
>
>
> By adding LAZY checks in interrupt_exit_user_prepare_main, softlockup is no longer seen and
> hackbench results are more or less same on smaller system(96CPUS).

Great. I'm guessing these tests are when running in voluntary preemption
mode (under PREEMPT_AUTO).

If you haven't, could you also try full preemption? There you should see
identical results unless something is horribly wrong.

> However, I still see 20-50%
> regression on the larger system(320 CPUS). I will continue to debug why.

Could you try this patch? This is needed because PREEMPT_AUTO turns on
CONFIG_PREEMPTION, but not CONFIG_PREEMPT:

diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c
index eca293794a1e..599410050f6b 100644
--- a/arch/powerpc/kernel/interrupt.c
+++ b/arch/powerpc/kernel/interrupt.c
@@ -396,7 +396,7 @@ notrace unsigned long interrupt_exit_kernel_prepare(struct pt_regs *regs)
                /* Returning to a kernel context with local irqs enabled. */
                WARN_ON_ONCE(!(regs->msr & MSR_EE));
 again:
-               if (IS_ENABLED(CONFIG_PREEMPT)) {
+               if (IS_ENABLED(CONFIG_PREEMPTION)) {
                        /* Return to preemptible kernel context */
                        if (unlikely(read_thread_flags() & _TIF_NEED_RESCHED)) {
                                if (preempt_count() == 0)


--
ankur