On Mon, 19 Apr 2021 at 18:51, Rik van Riel <r...@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 2-3% on average. > The schedstats run_delay number shows a similar improvement. > > Signed-off-by: Rik van Riel <r...@surriel.com> > --- > v2: > - fix !SMP build error and prev-not-CFS case by moving check into > newidle_balance > - fix formatting of if condition > - audit newidle_balance return value use to make sure we get that right > - reset idle_stamp when breaking out of the loop due to ->ttwu_pending > > kernel/sched/fair.c | 13 +++++++++++-- > 1 file changed, 11 insertions(+), 2 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 69680158963f..5e26f013e182 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 +10697,7 @@ 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 (pulled_task || this_rq->ttwu_pending)
This needs at least a comment to explain why we must clear this_rq->idle_stamp when this_rq->ttwu_pending is set whereas it is also done during sched_ttwu_pending() > this_rq->idle_stamp = 0; > > rq_repin_lock(this_rq, rf); > -- > 2.25.4 > >