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Message-ID: <Y4sA5xDe+sBfQUnG@chenyu5-mobl1>
Date: Sat, 3 Dec 2022 15:55:19 +0800
From: Chen Yu <yu.c.chen@...el.com>
To: Chengming Zhou <zhouchengming@...edance.com>
CC: <mingo@...hat.com>, <peterz@...radead.org>, <vschneid@...hat.com>,
<juri.lelli@...hat.com>, <vincent.guittot@...aro.org>,
<dietmar.eggemann@....com>, <rostedt@...dmis.org>,
<bsegall@...gle.com>, <mgorman@...e.de>, <bristot@...hat.com>,
<linux-kernel@...r.kernel.org>, <bagasdotme@...il.com>
Subject: Re: [PATCH v4] sched/core: Minor optimize ttwu_runnable()
On 2022-12-02 at 16:06:44 +0800, Chengming Zhou wrote:
> ttwu_runnable() is used as a fast wakeup path when the wakee task
> is running on CPU or runnable on RQ, in both cases we can just
> set its state to TASK_RUNNING to prevent a sleep.
>
> If the wakee task is on_cpu running, we don't need to update_rq_clock()
> or check_preempt_curr().
>
> But if the wakee task is on_rq && !on_cpu (e.g. an IRQ hit before
> the task got to schedule() and the task been preempted), we should
> check_preempt_curr() to see if it can preempt the current running.
>
> Reorganize ttwu_do_wakeup() and ttwu_do_activate() to make
> ttwu_do_wakeup() only mark the task runnable, so it can be used
> in ttwu_runnable() and try_to_wake_up() fast paths.
>
> This also removes the class->task_woken() callback from ttwu_runnable(),
> which wasn't required per the RT/DL implementations: any required push
> operation would have been queued during class->set_next_task() when p
> got preempted.
>
> ttwu_runnable() also loses the update to rq->idle_stamp, as by definition
> the rq cannot be idle in this scenario.
>
> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
> Suggested-by: Valentin Schneider <vschneid@...hat.com>
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Reviewed-by: Valentin Schneider <vschneid@...hat.com>
> ---
> v4:
> - s/This patch reorg/Reorganize/ per Bagas Sanjaya. Thanks!
>
> v3:
> - Improve the changelog per Valentin Schneider. Thanks!
>
> v2:
> - keep check_preempt_curr() for on_rq && !on_cpu case in ttwu_runnable(),
> per Valentin Schneider.
> - reorg ttwu_do_wakeup() and ttwu_do_activate() code, so ttwu_do_wakeup()
> can be reused in ttwu_runnable(), per Peter Zijlstra.
> - reuse ttwu_do_wakeup() in try_to_wake_up() (p == current) fast path too,
> so ttwu_do_wakeup() become the only place we mark task runnable.
> ---
> kernel/sched/core.c | 73 ++++++++++++++++++++++++---------------------
> 1 file changed, 39 insertions(+), 34 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 314c2c0219d9..d8216485b0ad 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3623,14 +3623,39 @@ ttwu_stat(struct task_struct *p, int cpu, int wake_flags)
> }
>
> /*
> - * Mark the task runnable and perform wakeup-preemption.
> + * Mark the task runnable.
> */
> -static void ttwu_do_wakeup(struct rq *rq, struct task_struct *p, int wake_flags,
> - struct rq_flags *rf)
> +static inline void ttwu_do_wakeup(struct task_struct *p)
> {
> - check_preempt_curr(rq, p, wake_flags);
> WRITE_ONCE(p->__state, TASK_RUNNING);
> trace_sched_wakeup(p);
> +}
> +
> +static void
> +ttwu_do_activate(struct rq *rq, struct task_struct *p, int wake_flags,
> + struct rq_flags *rf)
> +{
> + int en_flags = ENQUEUE_WAKEUP | ENQUEUE_NOCLOCK;
> +
> + lockdep_assert_rq_held(rq);
> +
> + if (p->sched_contributes_to_load)
> + rq->nr_uninterruptible--;
> +
> +#ifdef CONFIG_SMP
> + if (wake_flags & WF_MIGRATED)
> + en_flags |= ENQUEUE_MIGRATED;
> + else
> +#endif
> + if (p->in_iowait) {
> + delayacct_blkio_end(p);
> + atomic_dec(&task_rq(p)->nr_iowait);
> + }
> +
> + activate_task(rq, p, en_flags);
> + check_preempt_curr(rq, p, wake_flags);
> +
> + ttwu_do_wakeup(p);
>
> #ifdef CONFIG_SMP
> if (p->sched_class->task_woken) {
> @@ -3660,31 +3685,6 @@ static void ttwu_do_wakeup(struct rq *rq, struct task_struct *p, int wake_flags,
> #endif
> }
>
> -static void
> -ttwu_do_activate(struct rq *rq, struct task_struct *p, int wake_flags,
> - struct rq_flags *rf)
> -{
> - int en_flags = ENQUEUE_WAKEUP | ENQUEUE_NOCLOCK;
> -
> - lockdep_assert_rq_held(rq);
> -
> - if (p->sched_contributes_to_load)
> - rq->nr_uninterruptible--;
> -
> -#ifdef CONFIG_SMP
> - if (wake_flags & WF_MIGRATED)
> - en_flags |= ENQUEUE_MIGRATED;
> - else
> -#endif
> - if (p->in_iowait) {
> - delayacct_blkio_end(p);
> - atomic_dec(&task_rq(p)->nr_iowait);
> - }
> -
> - activate_task(rq, p, en_flags);
> - ttwu_do_wakeup(rq, p, wake_flags, rf);
> -}
> -
> /*
> * Consider @p being inside a wait loop:
> *
> @@ -3718,9 +3718,15 @@ static int ttwu_runnable(struct task_struct *p, int wake_flags)
>
> rq = __task_rq_lock(p, &rf);
> if (task_on_rq_queued(p)) {
> - /* check_preempt_curr() may use rq clock */
> - update_rq_clock(rq);
> - ttwu_do_wakeup(rq, p, wake_flags, &rf);
> + if (!task_on_cpu(rq, p)) {
> + /*
> + * When on_rq && !on_cpu the task is preempted, see if
> + * it should preempt whatever is current there now.
> + */
> + update_rq_clock(rq);
> + check_preempt_curr(rq, p, wake_flags);
> + }
> + ttwu_do_wakeup(p);
> ret = 1;
> }
> __task_rq_unlock(rq, &rf);
> @@ -4086,8 +4092,7 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
> goto out;
>
> trace_sched_waking(p);
> - WRITE_ONCE(p->__state, TASK_RUNNING);
> - trace_sched_wakeup(p);
> + ttwu_do_wakeup(p);
> goto out;
> }
>
Just wonder if we could split the reorganization and optimization into two patches,
so we can track the code change a little easier in the future?
thanks,
Chenyu
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