Re: [RESEND PATCH v7 7/7] cpuidle/poll_state: Poll via smp_cond_load_relaxed_timeout()

Ankur Arora
2025-10-29T04:42:30Z

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

> On Tue, Oct 28, 2025 at 6:32 AM Ankur Arora <ankur.a.arora@oracle.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@kernel.org>
>> Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
>> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.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