Re: [PATCH] sched,fair: skip newidle_balance if a wakeup is pending
From: Peter Zijlstra
Date: Mon Apr 19 2021 - 06:04:08 EST
On Sun, Apr 18, 2021 at 10:17:51PM -0400, Rik van Riel wrote:
> The try_to_wake_up function has an optimization where it can queue
> a task for wakeup on its previous CPU, if the task is still in the
> middle of going to sleep inside schedule().
> Once schedule() re-enables IRQs, the task will be woken up with an
> IPI, and placed back on the runqueue.
> If we have such a wakeup pending, there is no need to search other
> CPUs for runnable tasks. Just skip (or bail out early from) newidle
> balancing, and run the just woken up task.
> For a memcache like workload test, this reduces total CPU use by
> about 2%, proportionally split between user and system time,
> and p99 and p95 application response time by 2-3% on average.
> The schedstats run_delay number shows a similar improvement.
> Signed-off-by: Rik van Riel <riel@xxxxxxxxxxx>
> kernel/sched/fair.c | 11 ++++++++++-
> 1 file changed, 10 insertions(+), 1 deletion(-)
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 69680158963f..19a92c48939f 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7163,6 +7163,14 @@ done: __maybe_unused;
> if (!rf)
> return NULL;
> + /*
> + * We have a woken up task pending here. No need to search for ones
> + * elsewhere. This task will be enqueued the moment we unblock irqs
> + * upon exiting the scheduler.
> + */
> + if (rq->ttwu_pending)
> + return NULL;
As reported by the robot, that needs an CONFIG_SMP guard of sorts,
#ifdef might work I suppose.
> new_tasks = newidle_balance(rq, rf);
> @@ -10661,7 +10669,8 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
> * Stop searching for tasks to pull if there are
> * now runnable tasks on this rq.
> - if (pulled_task || this_rq->nr_running > 0)
> + if (pulled_task || this_rq->nr_running > 0 ||
> + this_rq->ttwu_pending)
Either cino=(0:0 or just bust the limit and make it 84 chars.