Messages in this thread | | | From | Vincent Guittot <> | Date | Wed, 6 Oct 2021 10:12:53 +0200 | Subject | Re: [PATCH 2/2] sched/fair: Skip update_blocked_averages if we are defering load balance |
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On Tue, 5 Oct 2021 at 22:49, Peter Zijlstra <peterz@infradead.org> 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@linaro.org> > > Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com> > > --- > > 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 :/
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