linux-kernel - [tip: sched/core] sched/core: Micro-optimize ttwu

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Date:   Sat, 07 Jan 2023 10:53:00 -0000
From:   "tip-bot2 for Chengming Zhou" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Valentin Schneider <vschneid@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Chengming Zhou <zhouchengming@...edance.com>,
        Ingo Molnar <mingo@...nel.org>,
        "Steven Rostedt (Google)" <rostedt@...dmis.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: sched/core] sched/core: Micro-optimize ttwu_runnable()

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     efe09385864f3441c71711f91e621992f9423c01
Gitweb:        https://git.kernel.org/tip/efe09385864f3441c71711f91e621992f9423c01
Author:        Chengming Zhou <zhouchengming@...edance.com>
AuthorDate:    Fri, 23 Dec 2022 18:32:56 +08:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Sat, 07 Jan 2023 10:48:38 +01:00

sched/core: Micro-optimize ttwu_runnable()

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.

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.

Suggested-by: Valentin Schneider <vschneid@...hat.com>
Suggested-by: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Reviewed-by: Valentin Schneider <vschneid@...hat.com>
Reviewed-by: Steven Rostedt (Google) <rostedt@...dmis.org>
Link: https://lore.kernel.org/r/20221223103257.4962-1-zhouchengming@bytedance.com
---
 kernel/sched/core.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index f99ee69..255a318 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3720,9 +3720,16 @@ 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 the task that is current now.
+			 */
+			update_rq_clock(rq);
+			check_preempt_curr(rq, p, wake_flags);
+		}
+		WRITE_ONCE(p->__state, TASK_RUNNING);
+		trace_sched_wakeup(p);
 		ret = 1;
 	}
 	__task_rq_unlock(rq, &rf);