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

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Message-ID: <02391a39-b785-6056-49c4-c0ff34b3b780@arm.com>
Date:   Fri, 19 Aug 2016 16:00:57 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Steve Muckle <steve.muckle@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>
Cc:     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>,
        Steve Muckle <smuckle@...aro.org>
Subject: Re: [PATCH] sched: fix incorrect PELT values on SMT

Hi Steve,

On 19/08/16 02:55, Steve Muckle wrote:
> PELT scales its util_sum and util_avg values via
> arch_scale_cpu_capacity(). If that function is passed the CPU's sched
> domain then it will reduce the scaling capacity if SD_SHARE_CPUCAPACITY
> is set. PELT does not pass in the sd however. The other caller of
> arch_scale_cpu_capacity, update_cpu_capacity(), does. This means
> util_sum and util_avg scale beyond the CPU capacity on SMT.
> 
> On an Intel i7-3630QM for example rq->cpu_capacity_orig is 589 but
> util_avg scales up to 1024.
> 
> Fix this by passing in the sd in __update_load_avg() as well.
> 
> Signed-off-by: Steve Muckle <smuckle@...aro.org>
> ---
>  kernel/sched/fair.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> 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.

>  
>  	/* delta_w is the amount already accumulated against our next period */
>  	delta_w = sa->period_contrib;
>