Messages in this thread Patch in this message | | | Date | Sun, 18 Apr 2021 22:17:51 -0400 | From | Rik van Riel <> | Subject | [PATCH] sched,fair: skip newidle_balance if a wakeup is pending |
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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@surriel.com> --- 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; + 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) break; } rcu_read_unlock(); -- 2.25.4
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