linux-kernel - Re: [PATCH v3 7/7] cpuidle/poll_state: replace cpu_relax with smp_cond_load

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Date: Mon, 29 Jan 2024 21:20:57 +0200
From: Mihai Carabas <mihai.carabas@...cle.com>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: linux-arm-kernel@...ts.infradead.org, kvm@...r.kernel.org,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        catalin.marinas@....com, will@...nel.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, x86@...nel.org, hpa@...or.com,
        pbonzini@...hat.com, wanpengli@...cent.com, vkuznets@...hat.com,
        daniel.lezcano@...aro.org, akpm@...ux-foundation.org, pmladek@...e.com,
        peterz@...radead.org, dianders@...omium.org, npiggin@...il.com,
        rick.p.edgecombe@...el.com, joao.m.martins@...cle.com,
        juerg.haefliger@...onical.com, mic@...ikod.net, arnd@...db.de,
        ankur.a.arora@...cle.com
Subject: Re: [PATCH v3 7/7] cpuidle/poll_state: replace cpu_relax with
 smp_cond_load_relaxed

La 29.01.2024 16:52, Rafael J. Wysocki a scris:
> On Mon, Jan 29, 2024 at 12:56 PM Mihai Carabas <mihai.carabas@...cle.com> wrote:
>> cpu_relax on ARM64 does a simple "yield". Thus we replace it with
>> smp_cond_load_relaxed which basically does a "wfe".
>>
>> Suggested-by: Peter Zijlstra <peterz@...radead.org>
>> Signed-off-by: Mihai Carabas <mihai.carabas@...cle.com>
>> ---
>>   drivers/cpuidle/poll_state.c | 14 +++++++++-----
>>   1 file changed, 9 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
>> index 9b6d90a72601..440cd713e39a 100644
>> --- a/drivers/cpuidle/poll_state.c
>> +++ b/drivers/cpuidle/poll_state.c
>> @@ -26,12 +26,16 @@ static int __cpuidle poll_idle(struct cpuidle_device *dev,
>>
>>                  limit = cpuidle_poll_time(drv, dev);
>>
>> -               while (!need_resched()) {
>> -                       cpu_relax();
>> -                       if (loop_count++ < POLL_IDLE_RELAX_COUNT)
>> -                               continue;
>> -
>> +               for (;;) {
>>                          loop_count = 0;
>> +
>> +                       smp_cond_load_relaxed(&current_thread_info()->flags,
>> +                                             (VAL & _TIF_NEED_RESCHED) ||
>> +                                             (loop_count++ >= POLL_IDLE_RELAX_COUNT));
> The inner parens are not necessary AFAICS.

Provides better reading. Do you want to remove these?

> Also, doesn't this return a value which can be used for checking if
> _TIF_NEED_RESCHED is set instead of the condition below?

Yes, indeed - should I modify this check? (somehow I wanted to preserve 
the original check)

Thank you,
Mihai

>> +
>> +                       if (loop_count < POLL_IDLE_RELAX_COUNT)
>> +                               break;
>> +
>>                          if (local_clock_noinstr() - time_start > limit) {
>>                                  dev->poll_time_limit = true;
>>                                  break;
>> --