linux-kernel - Re: [PATCH] sched: fix incorrect PELT values on SMT

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Date:   Fri, 19 Aug 2016 13:13:59 -0700
From:   Steve Muckle <steve.muckle@...aro.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Steve Muckle <steve.muckle@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, "Rafael J . Wysocki" <rafael@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <Juri.Lelli@....com>,
        Patrick Bellasi <patrick.bellasi@....com>
Subject: Re: [PATCH] sched: fix incorrect PELT values on SMT

On Fri, Aug 19, 2016 at 04:00:57PM +0100, Dietmar Eggemann wrote:
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 61d485421bed..95d34b337152 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -2731,7 +2731,7 @@ __update_load_avg(u64 now, int cpu, struct sched_avg *sa,
> >  	sa->last_update_time = now;
> >  
> >  	scale_freq = arch_scale_freq_capacity(NULL, cpu);
> > -	scale_cpu = arch_scale_cpu_capacity(NULL, cpu);
> > +	scale_cpu = arch_scale_cpu_capacity(cpu_rq(cpu)->sd, cpu);
> 
> Wouldn't you have to subscribe to this rcu pointer rq->sd w/ something
> like 'rcu_dereference(cpu_rq(cpu)->sd)'?
> 
> IMHO, __update_load_avg() is called outside existing RCU read-side
> critical sections as well so there would be a pair of
> rcu_read_lock()/rcu_read_unlock() required in this case.

Thanks Dietmar for the review.

Yeah I didn't consider that this was protected with rcu. It looks like
I'm abandoning this approach anyway though and doing something limited
just to schedutil.

thanks,
Steve