Re: [PATCH 2/2] sched/fair: Skip update_blocked_averages if we are defering load balance

[Vincent Guittot]

On Tue, 5 Oct 2021 at 22:49, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Mon, Oct 04, 2021 at 07:14:51PM +0200, Vincent Guittot wrote:
> > In newidle_balance(), the scheduler skips load balance to the new idle cpu
> > when the 1st sd of this_rq is:
> >
> >    this_rq->avg_idle < sd->max_newidle_lb_cost
> >
> > Doing a costly call to update_blocked_averages() will not be useful and
> > simply adds overhead when this condition is true.
> >
> > Check the condition early in newidle_balance() to skip
> > update_blocked_averages() when possible.
> >
> > Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> > Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> > ---
> >  kernel/sched/fair.c | 9 ++++++---
> >  1 file changed, 6 insertions(+), 3 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 1f78b2e3b71c..1294b78503d9 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -10841,17 +10841,20 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
> >        */
> >       rq_unpin_lock(this_rq, rf);
> >
> > +     rcu_read_lock();
> > +     sd = rcu_dereference_check_sched_domain(this_rq->sd);
> > +
> >       if (this_rq->avg_idle < sysctl_sched_migration_cost ||
> > -         !READ_ONCE(this_rq->rd->overload)) {
> > +             !READ_ONCE(this_rq->rd->overload) ||
> > +             (sd && this_rq->avg_idle < sd->max_newidle_lb_cost)) {
>
> set cino=(0:0, please.
>
> Also, people have, in the past, tried to get rid of the first clause
> here, perhaps this can replace it instead of augment it?

Yes, that's a good point.
either sd->max_newidle_lb_cost >= sysctl_sched_migration_cost and the
current condition is covered by the new condition
or sd->max_newidle_lb_cost < sysctl_sched_migration_cost and we will
run newidle balance. But this also means that we have time to run the
newly idle LB

>
> >
> > -             rcu_read_lock();
> > -             sd = rcu_dereference_check_sched_domain(this_rq->sd);
> >               if (sd)
> >                       update_next_balance(sd, &next_balance);
> >               rcu_read_unlock();
> >
> >               goto out;
> >       }
> > +     rcu_read_unlock();
>
> There's another rcu_read_lock section right below this, at the very
> least we can merge them.

we release the rq lock in between which adds more conditions to check
to all situations

>
> Also, IIRC we're running all this with premption disabled, and since
> rcu-sched got folded into rcu, all that rcu_read_*lock() stuff isn't
> strictly required anymore.
>
> (we're full circle there, back in the day RCU implied RCU-sched and the
> scheduler relied on preempt-disable for lots of this stuff, then Paul
> split them, and I spend a fair amount of time adding all this
> rcu_read_*lock() crud, and now he's merge them again, and it can go
> again).
>
> Except of course, I think we need to make rcu_dereference_check happy
> first :/