> Subject: [PATCH v6 01/10] cpuidle/poll_state: poll via smp_cond_load_relaxed()
>
> From: Mihai Carabas <mihai.carabas@xxxxxxxxxx>
>
> The inner loop in poll_idle() polls up to POLL_IDLE_RELAX_COUNT times,
> checking to see if the thread has the TIF_NEED_RESCHED bit 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 each iteration, the
> time check is done only infrequently (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 is busy-waiting, thus allowing the processor to reduce power
> consumption.
>
> However, cpu_relax() is defined optimally only on x86. On arm64, for
> instance, it is implemented as a YIELD which only serves a hint to the
> CPU that it prioritize a different hardware thread if one is available.
> arm64, however, does expose a more optimal polling mechanism via
> smp_cond_load_relaxed() which uses LDXR, WFE to wait until a store
> to a specified region.
>
> So restructure the loop, folding both checks in smp_cond_load_relaxed().
> Also, move the time check to the head of the loop allowing it to exit
> straight-away once TIF_NEED_RESCHED is set.
>
> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Mihai Carabas <mihai.carabas@xxxxxxxxxx>
> Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
> ---
> drivers/cpuidle/poll_state.c | 10 +++++-----
> 1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
> index 9b6d90a72601..532e4ed19e0f 100644
> --- a/drivers/cpuidle/poll_state.c
> +++ b/drivers/cpuidle/poll_state.c
> @@ -21,21 +21,21 @@ static int __cpuidle poll_idle(struct cpuidle_device *dev,
>
> raw_local_irq_enable();
> if (!current_set_polling_and_test()) {
> - unsigned int loop_count = 0;
> + unsigned int loop_count;
> 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;
> }
> +
> + smp_cond_load_relaxed(¤t_thread_info()->flags,
> + VAL & _TIF_NEED_RESCHED ||
> + loop_count++ >= POLL_IDLE_RELAX_COUNT);
> }
> }
> raw_local_irq_disable();
> --
> 2.43.5
Reviewed-by: Misono Tomohiro <misono.tomohiro@xxxxxxxxxxx>
Regards,
Tomohiro
