sched,fair: Skip newidle_balance if a wakeup is pending
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 10% on average. The schedstats run_delay number shows a similar improvement. Signed-off-by: Rik van Riel <riel@surriel.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org> Acked-by: Mel Gorman <mgorman@suse.de> Link: https://lkml.kernel.org/r/20210422130236.0bb353df@imladris.surriel.com
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@ -10592,6 +10592,14 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
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u64 curr_cost = 0;
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update_misfit_status(NULL, this_rq);
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/*
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* There is a task waiting to run. No need to search for one.
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* Return 0; the task will be enqueued when switching to idle.
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*/
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if (this_rq->ttwu_pending)
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return 0;
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/*
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* We must set idle_stamp _before_ calling idle_balance(), such that we
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* measure the duration of idle_balance() as idle time.
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@ -10657,7 +10665,8 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
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* Stop searching for tasks to pull if there are
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* now runnable tasks on this rq.
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*/
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if (pulled_task || this_rq->nr_running > 0)
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if (pulled_task || this_rq->nr_running > 0 ||
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this_rq->ttwu_pending)
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break;
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}
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rcu_read_unlock();
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