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

[Ankur Arora <ankur.a.arora@oracle.com>]

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

> On 4/27/24 12:30 AM, Ankur Arora wrote:
>>
>
> Hi Ankur,
>
> Sorry for the delay, I was on leave last week.
> There might be a delay in response as I am recovering from fever.

Please take your your time.

>> Great. I'm guessing these tests are when running in voluntary preemption
>> mode (under PREEMPT_AUTO).
>>
>
> It was run under preempt=none.
>
>> If you haven't, could you also try full preemption? There you should see
>> identical results unless something is horribly wrong.
>
> I tried preempt=full with patch you provided below. ran the hackbench for much longer
> with 100000 loops. I don't see any regression on the larger system.
> I see slight improvement in some cases.  I dont see any major regression with 10k ops
> which was tried earlier as well.

Great, so no surprises with preempt=full.

> ==========================================================
> 1L ops.
> ==========================================================
> Process 10 groups          :       9.85,       9.87(-0.20)
> Process 20 groups          :      17.69,      17.32(2.09)
> Process 30 groups          :      25.89,      25.96(-0.27)
> Process 40 groups          :      34.70,      34.61(0.26)
> Process 50 groups          :      44.02,      43.79(0.52)
> Process 60 groups          :      52.72,      52.10(1.18)
> Thread  10 groups          :      10.50,      10.52(-0.19)
> Thread  20 groups          :      18.79,      18.60(1.01)
> Process(Pipe) 10 groups    :      10.39,      10.37(0.19)
> Process(Pipe) 20 groups    :      18.45,      18.54(-0.49)
> Process(Pipe) 30 groups    :      25.63,      25.92(-1.13)
> Process(Pipe) 40 groups    :      33.79,      33.49(0.89)
> Process(Pipe) 50 groups    :      43.15,      41.83(3.06)
> Process(Pipe) 60 groups    :      51.94,      50.32(3.12)
> Thread(Pipe)  10 groups    :      10.73,      10.85(-1.12)
> Thread(Pipe)  20 groups    :      19.24,      19.35(-0.57)

I presume the ones on the left are the baseline numbers with
PREEMPT_AUTO on the right?

Also, these are with preempt=none/voluntary/full?

> ==========================================================
> 10k ops.
>
> Process 10 groups          :       1.10,       1.10(0.00)
> Process 20 groups          :       1.89,       1.88(0.53)
> Process 30 groups          :       2.82,       2.80(0.71)
> Process 40 groups          :       3.76,       3.76(0.00)
> Process 50 groups          :       4.66,       4.79(-2.79)
> Process 60 groups          :       5.74,       5.92(-3.14)
> thread  10 groups          :       1.22,       1.20(1.64)
> thread  20 groups          :       2.05,       2.05(0.00)
> Process(Pipe) 10 groups    :       1.13,       1.13(0.00)
> Process(Pipe) 20 groups    :       1.98,       1.93(2.53)
> Process(Pipe) 30 groups    :       2.91,       2.75(5.50)
> Process(Pipe) 40 groups    :       3.85,       3.65(5.19)
> Process(Pipe) 50 groups    :       4.91,       4.91(0.00)
> Process(Pipe) 60 groups    :       5.56,       5.90(-6.12)
> thread(Pipe)  10 groups    :       1.23,       1.23(0.00)
> thread(Pipe)  20 groups    :       1.99,       1.99(0.00)
> ==========================================================
>
> Other than hackbench, I see slight improvement in unixbench and stress-ng --cpu workloads.
>>
>>> 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:

Just wanted to check if the regression you were seeing with preempt=none
is fixed?

> This patch can be considered as the enablement patch for Powerpc for preempt_auto.
> Michael, Nick, Do you see any concerns?
>
> Ankur, Could you please add this patch, if there are no objections.

Of course. Thanks for the patch.

The patch overall looks good. A minor comment below.

 Will add it to v2.

> ---
> From 878a5a7c990e3459758a5d19d7697b07d8d27d0f Mon Sep 17 00:00:00 2001
> From: Shrikanth Hegde <sshegde@linux.ibm.com>
> Date: Tue, 7 May 2024 04:42:04 -0500
> Subject: [PATCH] powerpc: add support for preempt_auto
>
> Add PowerPC arch support for PREEMPT_AUTO by defining LAZY bits.
>
> Since PowerPC doesn't use generic exit to functions, Add
> NR_LAZY check in exit to user and exit to kernel from interrupt
> routines.
>
> Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
> ---
>  arch/powerpc/Kconfig                   |  1 +
>  arch/powerpc/include/asm/thread_info.h | 11 ++++++++++-
>  arch/powerpc/kernel/interrupt.c        |  6 ++++--
>  3 files changed, 15 insertions(+), 3 deletions(-)

...

> diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c
> index eca293794a1e..0c0b7010995a 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 {
>  			/*
> @@ -396,7 +396,9 @@ 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_PREEMPT_AUTO) && IS_ENABLED(CONFIG_PREEMPTION)) ||
> +		    (!IS_ENABLED(CONFIG_PREEMPT_AUTO) && IS_ENABLED(CONFIG_PREEMPT))) {

Seems to me, this could just be:

        if (IS_EANBLED(CONFIG_PREEMPTION))

But, maybe the intent is to exclude support for, say
(PREEMPT_DYNAMIC && PREEMPT_NONE), then maybe it is best to explicitly
encode that?

Alternately, a comment on the disparate treatment for PREEMPT_AUTO and
!PREEMPT_AUTO might be helpful.


Thanks
Ankur

>  			/* Return to preemptible kernel context */
>  			if (unlikely(read_thread_flags() & _TIF_NEED_RESCHED)) {
>  				if (preempt_count() == 0)