Re: [PATCH v11 12/14] cpuidle/poll_state: Wait for need-resched via tif_need_resched_relaxed_wait()

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

Okanovic, Haris <harisokn@amazon.com> writes:

> On Wed, 2026-04-08 at 17:55 +0530, Ankur Arora wrote:
>> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
>>
>>
>>
>> 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, since we want to minimize power consumption in idle, building
>> of cpuidle/poll_state.c continues 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>
>> Acked-by: Rafael J. Wysocki (Intel) <rafael@kernel.org>
>> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
>> ---
>>  drivers/cpuidle/poll_state.c | 21 +--------------------
>>  1 file changed, 1 insertion(+), 20 deletions(-)
>>
>> diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
>> index c7524e4c522a..7443b3e971ba 100644
>> --- a/drivers/cpuidle/poll_state.c
>> +++ b/drivers/cpuidle/poll_state.c
>> @@ -6,41 +6,22 @@
>>  #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;
>>
>>                 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;
>> -                       }
>> -               }
>> +               dev->poll_time_limit = !tif_need_resched_relaxed_wait(limit);
>>         }
>>         raw_local_irq_disable();
>>
>> --
>> 2.31.1
>>
>
> Hi Ankur,
>
> Tested atop latest mainline d60bc1401 with the rest of your haltpoll
> changes from separate thread:
> ~10% improvement in `perf sched bench pipe` micro and ~4-6% throughput
> improvements in mysql,
> postgresql, cassandra, and memcached in under-loaded configurations.
> Tested on AWS Graviton3 and
> Graviton4, ARM Neoverse V1 and V2 cores respectively.
>
> I hope this series can merge soon. It's been stuck in review for more
> than 2 years.
>
> Tested-by: Haris Okanovic <harisokn@amazon.com>

Thanks Haris. Yeah, I don't think there are any open issues left on
this.

--
ankur