linux-kernel - Re: [RESEND PATCH v7 7/7] cpuidle/poll_state: Poll via smp_cond_load_relaxed

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Message-ID: <87ms5ajp4c.fsf@oracle.com>
Date: Tue, 28 Oct 2025 21:41:55 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: Ankur Arora <ankur.a.arora@...cle.com>, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-pm@...r.kernel.org, bpf@...r.kernel.org, arnd@...db.de,
        catalin.marinas@....com, will@...nel.org, peterz@...radead.org,
        akpm@...ux-foundation.org, mark.rutland@....com, harisokn@...zon.com,
        cl@...two.org, ast@...nel.org, daniel.lezcano@...aro.org,
        memxor@...il.com, zhenglifeng1@...wei.com, xueshuai@...ux.alibaba.com,
        joao.m.martins@...cle.com, boris.ostrovsky@...cle.com,
        konrad.wilk@...cle.com
Subject: Re: [RESEND PATCH v7 7/7] cpuidle/poll_state: Poll via
 smp_cond_load_relaxed_timeout()


Rafael J. Wysocki <rafael@...nel.org> writes:

> On Tue, Oct 28, 2025 at 6:32 AM Ankur Arora <ankur.a.arora@...cle.com> wrote:
>>
>> The inner loop in poll_idle() polls over the thread_info flags,
>> waiting to see if the thread has TIF_NEED_RESCHED set. The loop
>> exits once the condition is met, or if the poll time limit has
>> been exceeded.
>>
>> To minimize the number of instructions executed in each iteration,
>> the time check is done only intermittently (once every
>> POLL_IDLE_RELAX_COUNT iterations). In addition, each loop iteration
>> executes cpu_relax() which on certain platforms provides a hint to
>> the pipeline that the loop busy-waits, allowing the processor to
>> reduce power consumption.
>>
>> This is close to what smp_cond_load_relaxed_timeout() provides. So,
>> restructure the loop and fold the loop condition and the timeout check
>> in smp_cond_load_relaxed_timeout().
>
> Well, it is close, but is it close enough?

I guess that's the question.

>> Cc: "Rafael J. Wysocki" <rafael@...nel.org>
>> Cc: Daniel Lezcano <daniel.lezcano@...aro.org>
>> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
>> ---
>>  drivers/cpuidle/poll_state.c | 29 ++++++++---------------------
>>  1 file changed, 8 insertions(+), 21 deletions(-)
>>
>> diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
>> index 9b6d90a72601..dc7f4b424fec 100644
>> --- a/drivers/cpuidle/poll_state.c
>> +++ b/drivers/cpuidle/poll_state.c
>> @@ -8,35 +8,22 @@
>>  #include <linux/sched/clock.h>
>>  #include <linux/sched/idle.h>
>>
>> -#define POLL_IDLE_RELAX_COUNT  200
>> -
>>  static int __cpuidle poll_idle(struct cpuidle_device *dev,
>>                                struct cpuidle_driver *drv, int index)
>>  {
>> -       u64 time_start;
>> -
>> -       time_start = local_clock_noinstr();
>> +       u64 time_end;
>> +       u32 flags = 0;
>>
>>         dev->poll_time_limit = false;
>>
>> +       time_end = local_clock_noinstr() + cpuidle_poll_time(drv, dev);
>
> Is there any particular reason for doing this unconditionally?  If
> not, then it looks like an arbitrary unrelated change to me.

Agreed. Will fix.

>> +
>>         raw_local_irq_enable();
>>         if (!current_set_polling_and_test()) {
>> -               unsigned int loop_count = 0;
>> -               u64 limit;
>> -
>> -               limit = cpuidle_poll_time(drv, dev);
>> -
>> -               while (!need_resched()) {
>> -                       cpu_relax();
>> -                       if (loop_count++ < POLL_IDLE_RELAX_COUNT)
>> -                               continue;
>> -
>> -                       loop_count = 0;
>> -                       if (local_clock_noinstr() - time_start > limit) {
>> -                               dev->poll_time_limit = true;
>> -                               break;
>> -                       }
>> -               }
>> +               flags = smp_cond_load_relaxed_timeout(&current_thread_info()->flags,
>> +                                                     (VAL & _TIF_NEED_RESCHED),
>> +                                                     (local_clock_noinstr() >= time_end));
>
> So my understanding of this is that it reduces duplication with some
> other places doing similar things.  Fair enough.
>
> However, since there is "timeout" in the name, I'd expect it to take
> the timeout as an argument.

The early versions did have a timeout but that complicated the
implementation significantly. And the current users poll_idle(),
rqspinlock don't need a precise timeout.

smp_cond_load_relaxed_timed(), smp_cond_load_relaxed_timecheck()?

The problem with all suffixes I can think of is that it makes the
interface itself nonobvious.

Possibly something with the sense of bail out might work.

--
ankur