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Message-ID: <20220412085845.GA14088@vingu-book>
Date: Tue, 12 Apr 2022 10:58:45 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Kuyo Chang <kuyo.chang@...iatek.com>
Cc: Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Matthias Brugger <matthias.bgg@...il.com>,
wsd_upstream@...iatek.com, linux-kernel@...r.kernel.org,
linux-arm-kernel@...ts.infradead.org,
linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH 1/1] sched/pelt: Refine the enqueue_load_avg calculate
method
Le mardi 12 avril 2022 à 10:51:23 (+0800), Kuyo Chang a écrit :
> On Mon, 2022-04-11 at 10:39 +0200, Vincent Guittot wrote:
> > On Mon, 11 Apr 2022 at 08:17, Kuyo Chang <kuyo.chang@...iatek.com>
> > wrote:
> > >
> > > From: kuyo chang <kuyo.chang@...iatek.com>
> > >
> > > I meet the warning message at cfs_rq_is_decayed at below code.
> > >
> > > SCHED_WARN_ON(cfs_rq->avg.load_avg ||
> > > cfs_rq->avg.util_avg ||
> > > cfs_rq->avg.runnable_avg)
> > >
> > > Following is the calltrace.
> > >
> > > Call trace:
> > > __update_blocked_fair
> > > update_blocked_averages
> > > newidle_balance
> > > pick_next_task_fair
> > > __schedule
> > > schedule
> > > pipe_read
> > > vfs_read
> > > ksys_read
> > >
> > > After code analyzing and some debug messages, I found it exits a
> > > corner
> > > case at attach_entity_load_avg which will cause load_sum is zero
> > > and
> > > load_avg is not.
> > > Consider se_weight is 88761 according by sched_prio_to_weight
> > > table.
> > > And assume the get_pelt_divider() is 47742, se->avg.load_avg is 1.
> > > By the calculating for se->avg.load_sum as following will become
> > > zero
> > > as following.
> > > se->avg.load_sum =
> > > div_u64(se->avg.load_avg * se->avg.load_sum,
> > > se_weight(se));
> > > se->avg.load_sum = 1*47742/88761 = 0.
> >
> > The root problem is there, se->avg.load_sum must not be null if
> > se->avg.load_avg is not null because the correct relation between
> > _avg
> > and _sum is:
> >
> > load_avg = weight * load_sum / divider.
> >
> > so the fix should be attach_entity_load_avg() and probably the below
> > is enough
> >
> > se->avg.load_sum = div_u64(se->avg.load_avg * se->avg.load_sum,
> > se_weight(se)) + 1;
>
> Thanks for your kindly suggestion.
> +1 would make the calcuation for load_sum may be overestimate?
> How about the below code make sense for fix the corner case?
>
> ---
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3832,7 +3832,8 @@ static void attach_entity_load_avg(struct cfs_rq
> *cfs_rq, struct sched_entity *s
> se->avg.load_sum = divider;
> if (se_weight(se)) {
> se->avg.load_sum =
> - div_u64(se->avg.load_avg * se->avg.load_sum,
> se_weight(se));
> + (se->avg.load_avg * se->avg.load_sum >
> se_weight(se)) ?
> + div_u64(se->avg.load_avg * se->avg.load_sum,
> se_weight(se)) : 1;
> }
>
> enqueue_load_avg(cfs_rq, se);
> --
> 2.18.0
In this case, the below is easier to read
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 1658a9428d96..2c685474db23 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3836,10 +3836,12 @@ static void attach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s
se->avg.runnable_sum = se->avg.runnable_avg * divider;
- se->avg.load_sum = divider;
- if (se_weight(se)) {
+ se->avg.load_sum = se->avg.load_avg * divider;
+ if (se_weight(se) < se->avg.load_sum) {
se->avg.load_sum =
- div_u64(se->avg.load_avg * se->avg.load_sum, se_weight(se));
+ div_u64(se->avg.load_sum, se_weight(se));
+ } else {
+ se->avg.load_sum = 1;
}
enqueue_load_avg(cfs_rq, se);
>
>
> > >
> > > After enqueue_load_avg code as below.
> > > cfs_rq->avg.load_avg += se->avg.load_avg;
> > > cfs_rq->avg.load_sum += se_weight(se) * se->avg.load_sum;
> > >
> > > Then the load_sum for cfs_rq will be 1 while the load_sum for
> > > cfs_rq is 0.
> > > So it will hit the warning message.
> > >
> > > After all, I refer the following commit patch to do the similar
> > > thing at
> > > enqueue_load_avg.
> > > sched/pelt: Relax the sync of load_sum with load_avg
> > >
> > > After long time testing, the kernel warning was gone and the system
> > > runs
> > > as well as before.
> > >
> > > Signed-off-by: kuyo chang <kuyo.chang@...iatek.com>
> > > ---
> > > kernel/sched/fair.c | 6 ++++--
> > > 1 file changed, 4 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index d4bd299d67ab..30d8b6dba249 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -3074,8 +3074,10 @@ account_entity_dequeue(struct cfs_rq
> > > *cfs_rq, struct sched_entity *se)
> > > static inline void
> > > enqueue_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se)
> > > {
> > > - cfs_rq->avg.load_avg += se->avg.load_avg;
> > > - cfs_rq->avg.load_sum += se_weight(se) * se->avg.load_sum;
> > > + add_positive(&cfs_rq->avg.load_avg, se->avg.load_avg);
> > > + add_positive(&cfs_rq->avg.load_sum, se_weight(se) * se-
> > > >avg.load_sum);
> > > + cfs_rq->avg.load_sum = max_t(u32, cfs_rq->avg.load_sum,
> > > + cfs_rq->avg.load_avg *
> > > PELT_MIN_DIVIDER);
> > > }
> > >
> > > static inline void
> > > --
> > > 2.18.0
> > >
>
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