linux-kernel - Re: [PATCH] kernel/sched: Update schedstats when migrating threads

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Message-ID: <YhYKL4hxx4TNKHGD@hirez.programming.kicks-ass.net>
Date:   Wed, 23 Feb 2022 11:19:27 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Carlos Bilbao <carlos.bilbao@....com>
Cc:     juri.lelli@...hat.com, vincent.guittot@...aro.org,
        mingo@...hat.com, dietmar.eggemann@....com, rostedt@...dmis.org,
        bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kernel/sched: Update schedstats when migrating threads

On Wed, Jan 26, 2022 at 09:22:23AM -0600, Carlos Bilbao wrote:
> The kernel manages per-task scheduler statistics or schedstats. Such
> counters should be reinitialized when the thread is migrated to a
> different core rq, except for the values recording number of migrations.

I'm confused, why should we reset schedstats on migrate? I'm thinking
this breaks per-task, since tasks tend to bounce around quite a lot.

> Signed-off-by: Carlos Bilbao <carlos.bilbao@....com>
> ---
>  kernel/sched/core.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index fe53e510e711..d64c2a290176 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -8757,6 +8757,7 @@ bool sched_smp_initialized __read_mostly;
>  int migrate_task_to(struct task_struct *p, int target_cpu)
>  {
>  	struct migration_arg arg = { p, target_cpu };
> +	uint64_t forced_migrations, migrations_cold;
>  	int curr_cpu = task_cpu(p);
>  
>  	if (curr_cpu == target_cpu)
> @@ -8765,7 +8766,14 @@ int migrate_task_to(struct task_struct *p, int target_cpu)
>  	if (!cpumask_test_cpu(target_cpu, p->cpus_ptr))
>  		return -EINVAL;
>  
> -	/* TODO: This is not properly updating schedstats */
> +	if (schedstat_enabled()) {
> +		forced_migrations = schedstat_val(p->stats.nr_forced_migrations);
> +		migrations_cold = schedstat_val(p->stats.nr_migrations_cold);
> +		memset(&p->stats, 0, sizeof(p->stats));
> +		schedstat_set(p->stats.nr_forced_migrations, forced_migrations);
> +		schedstat_set(p->stats.nr_migrations_cold, migrations_cold);
> +		schedstat_inc(p->stats.nr_migrations_cold);
> +	}
>  
>  	trace_sched_move_numa(p, curr_cpu, target_cpu);
>  	return stop_one_cpu(curr_cpu, migration_cpu_stop, &arg);
> -- 
> 2.27.0
>