Re: [PATCH 7/7] cpuidle: Handle TIF_NR_POLLING on behalf of software polling idle states

[Rafael J. Wysocki]

On Fri, Nov 24, 2023 at 11:33 PM Frederic Weisbecker
<frederic@xxxxxxxxxx> wrote:
>
> Software polling idle states set again TIF_NR_POLLING and clear it upon
> exit. This involves error prone duplicated code and wasted cycles
> performing atomic operations, sometimes RmW fully ordered.
>
> To avoid this, benefit instead from the same generic TIF_NR_POLLING
> handling that is currently in use for hardware polling states.
>
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> ---
>  drivers/cpuidle/cpuidle-powernv.c | 10 ----------
>  drivers/cpuidle/cpuidle-pseries.c | 11 -----------
>  drivers/cpuidle/cpuidle.c         |  4 ++--
>  drivers/cpuidle/poll_state.c      | 30 ++++++++++++------------------
>  4 files changed, 14 insertions(+), 41 deletions(-)
>
> diff --git a/drivers/cpuidle/cpuidle-powernv.c b/drivers/cpuidle/cpuidle-powernv.c
> index 675b8eb81ebd..b88bbf7ead41 100644
> --- a/drivers/cpuidle/cpuidle-powernv.c
> +++ b/drivers/cpuidle/cpuidle-powernv.c
> @@ -71,8 +71,6 @@ static int snooze_loop(struct cpuidle_device *dev,
>  {
>         u64 snooze_exit_time;
>
> -       set_thread_flag(TIF_POLLING_NRFLAG);
> -
>         local_irq_enable();
>
>         snooze_exit_time = get_tb() + get_snooze_timeout(dev, drv, index);
> @@ -81,21 +79,13 @@ static int snooze_loop(struct cpuidle_device *dev,
>         HMT_very_low();
>         while (!need_resched()) {
>                 if (likely(snooze_timeout_en) && get_tb() > snooze_exit_time) {
> -                       /*
> -                        * Task has not woken up but we are exiting the polling
> -                        * loop anyway. Require a barrier after polling is
> -                        * cleared to order subsequent test of need_resched().
> -                        */
> -                       clear_thread_flag(TIF_POLLING_NRFLAG);
>                         dev->poll_time_limit = true;
> -                       smp_mb();
>                         break;
>                 }
>         }
>
>         HMT_medium();
>         ppc64_runlatch_on();
> -       clear_thread_flag(TIF_POLLING_NRFLAG);
>
>         local_irq_disable();
>
> diff --git a/drivers/cpuidle/cpuidle-pseries.c b/drivers/cpuidle/cpuidle-pseries.c
> index 4e08c9a39172..0ae76512b740 100644
> --- a/drivers/cpuidle/cpuidle-pseries.c
> +++ b/drivers/cpuidle/cpuidle-pseries.c
> @@ -39,8 +39,6 @@ int snooze_loop(struct cpuidle_device *dev, struct cpuidle_driver *drv,
>  {
>         u64 snooze_exit_time;
>
> -       set_thread_flag(TIF_POLLING_NRFLAG);
> -
>         pseries_idle_prolog();
>         raw_local_irq_enable();
>         snooze_exit_time = get_tb() + snooze_timeout;
> @@ -50,21 +48,12 @@ int snooze_loop(struct cpuidle_device *dev, struct cpuidle_driver *drv,
>                 HMT_low();
>                 HMT_very_low();
>                 if (likely(snooze_timeout_en) && get_tb() > snooze_exit_time) {
> -                       /*
> -                        * Task has not woken up but we are exiting the polling
> -                        * loop anyway. Require a barrier after polling is
> -                        * cleared to order subsequent test of need_resched().
> -                        */
>                         dev->poll_time_limit = true;
> -                       clear_thread_flag(TIF_POLLING_NRFLAG);
> -                       smp_mb();
>                         break;
>                 }
>         }
>
>         HMT_medium();
> -       clear_thread_flag(TIF_POLLING_NRFLAG);
> -
>         raw_local_irq_disable();
>
>         pseries_idle_epilog();
> diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
> index 49078cc83f4a..9eb811b5d8b6 100644
> --- a/drivers/cpuidle/cpuidle.c
> +++ b/drivers/cpuidle/cpuidle.c
> @@ -236,8 +236,8 @@ noinstr int cpuidle_enter_state(struct cpuidle_device *dev,
>                 broadcast = false;
>         }
>
> -       polling = target_state->flags & CPUIDLE_FLAG_POLLING_HARD;
> -
> +       polling = (target_state->flags & (CPUIDLE_FLAG_POLLING_SOFT |
> +                                         CPUIDLE_FLAG_POLLING_HARD));

The outer parens are not needed on the right-hand side, or apply !! to it.

>         /*
>          * If the target state doesn't poll on need_resched(), this is
>          * the last check after which further TIF_NEED_RESCHED remote setting
> diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
> index a2fe173de117..3bfa251b344a 100644
> --- a/drivers/cpuidle/poll_state.c
> +++ b/drivers/cpuidle/poll_state.c
> @@ -13,35 +13,29 @@
>  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_start = local_clock_noinstr();
> +       unsigned int loop_count = 0;
> +       u64 limit;
>
>         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);
> +       limit = cpuidle_poll_time(drv, dev);
>
> -               while (!need_resched()) {
> -                       cpu_relax();
> -                       if (loop_count++ < POLL_IDLE_RELAX_COUNT)
> -                               continue;
> +       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;
> -                       }
> +               loop_count = 0;
> +               if (local_clock_noinstr() - time_start > limit) {
> +                       dev->poll_time_limit = true;
> +                       break;
>                 }
>         }
>         raw_local_irq_disable();
>
> -       current_clr_polling();
> -
>         return index;
>  }
>
> --