Messages in this thread | | | From | Vincent Guittot <> | Date | Wed, 21 Apr 2021 19:27:00 +0200 | Subject | Re: [PATCH v3] sched,fair: skip newidle_balance if a wakeup is pending |
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Hi Rik,
On Tue, 20 Apr 2021 at 18:07, Rik van Riel <riel@surriel.com> 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 10% on average. > The schedstats run_delay number shows a similar improvement. > > Signed-off-by: Rik van Riel <riel@surriel.com> > --- > kernel/sched/fair.c | 18 ++++++++++++++++-- > 1 file changed, 16 insertions(+), 2 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 69680158963f..fd80175c3b3e 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -10594,6 +10594,14 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf) > u64 curr_cost = 0; > > update_misfit_status(NULL, this_rq); > + > + /* > + * There is a task waiting to run. No need to search for one. > + * Return 0; the task will be enqueued when switching to idle. > + */ > + if (this_rq->ttwu_pending) > + return 0; > + > /* > * We must set idle_stamp _before_ calling idle_balance(), such that we > * measure the duration of idle_balance() as idle time. > @@ -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(); > @@ -10688,7 +10697,12 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf) > if (this_rq->nr_running != this_rq->cfs.h_nr_running) > pulled_task = -1; > > - if (pulled_task) > + /* > + * If we are no longer idle, do not let the time spent here pull > + * down this_rq->avg_idle. That could lead to newidle_balance not > + * doing enough work, and the CPU actually going idle. > + */ > + if (pulled_task || this_rq->ttwu_pending)
I'm still running some benchmarks to evaluate the impact of your patch and more especially the line above which clears this_rq->idle_stamp and skips the time spent in newidle_balance from being accounted for in avg_idle. I have some results which show some regression because of this test especially with hackbench. On large system, the time spent in newidle_balance can be significant and we can't ignore it just because this_rq->ttwu_pending is set while looping the domains because without newidle_balance the idle time would have been large and we end up screwing up the metric
> this_rq->idle_stamp = 0; > > rq_repin_lock(this_rq, rf); > -- > 2.25.4 > >
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