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

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Message-ID: <63314d92-9226-4005-a166-37c8648c5351@linux.ibm.com>
Date: Fri, 26 Apr 2024 13:16:10 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>,
        Ankur Arora <ankur.a.arora@...cle.com>
Cc: 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,
        mark.rutland@....com, jgross@...e.com, andrew.cooper3@...rix.com,
        bristot@...nel.org, mathieu.desnoyers@...icios.com,
        geert@...ux-m68k.org, glaubitz@...sik.fu-berlin.de,
        anton.ivanov@...bridgegreys.com, mattst88@...il.com,
        krypton@...ich-teichert.org, rostedt@...dmis.org,
        David.Laight@...lab.com, richard@....at, mjguzik@...il.com,
        jon.grimm@....com, bharata@....com, raghavendra.kt@....com,
        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



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