Re: [PATCH v8 12/12] cpuidle/poll_state: Wait for need-resched via tif_need_resched_relaxed_wait()

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

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

> On Mon, Dec 15, 2025 at 5:55 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 rate-limited. 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.
>>
>> Switch over to tif_need_resched_relaxed_wait() instead, since that
>> provides exactly that.
>>
>> However, given that when running in idle we want to minimize our power
>> consumption, continue to depend on CONFIG_ARCH_HAS_CPU_RELAX as that
>> serves as an indicator that the platform supports an optimized version
>> of tif_need_resched_relaxed_wait() (via
>> smp_cond_load_acquire_timeout()).
>>
>> Cc: "Rafael J. Wysocki" <rafael@kernel.org>
>> Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
>> Cc: linux-pm@vger.kernel.org
>> Suggested-by: "Rafael J. Wysocki" <rafael@kernel.org>
>> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
>> ---
>>
>> Notes:
>>   - use tif_need_resched_relaxed_wait() instead of
>>     smp_cond_load_relaxed_timeout()
>>
>>  drivers/cpuidle/poll_state.c | 27 +++++----------------------
>>  1 file changed, 5 insertions(+), 22 deletions(-)
>>
>> diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
>> index c7524e4c522a..20136b3a08c2 100644
>> --- a/drivers/cpuidle/poll_state.c
>> +++ b/drivers/cpuidle/poll_state.c
>> @@ -6,41 +6,24 @@
>>  #include <linux/cpuidle.h>
>>  #include <linux/export.h>
>>  #include <linux/irqflags.h>
>> -#include <linux/sched.h>
>> -#include <linux/sched/clock.h>
>>  #include <linux/sched/idle.h>
>>  #include <linux/sprintf.h>
>>  #include <linux/types.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();
>> -
>>         dev->poll_time_limit = false;
>>
>>         raw_local_irq_enable();
>>         if (!current_set_polling_and_test()) {
>> -               unsigned int loop_count = 0;
>> -               u64 limit;
>> +               s64 limit;
>> +               bool nr_set;
>
> It doesn't look like the nr_set variable is really needed.
>
>>
>> -               limit = cpuidle_poll_time(drv, dev);
>> +               limit = (s64)cpuidle_poll_time(drv, dev);
>
> Is the explicit cast needed to suppress a warning?  If not, I'd drop it.

Ack.

>>
>> -               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;
>> -                       }
>> -               }
>> +               nr_set = tif_need_resched_relaxed_wait(limit);
>> +               dev->poll_time_limit = !nr_set;
>
> This can be
>
> dev->poll_time_limit = !tif_need_resched_relaxed_wait(limit);

Yeah that looks pretty clear.


Thanks!
--
ankur