linux-kernel - Re: [PATCH 04/10] sched/fair: Dynamically update cfs_overload

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Date:   Mon, 22 Oct 2018 18:56:31 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steve Sistare <steven.sistare@...cle.com>
Cc:     mingo@...hat.com, subhra.mazumdar@...cle.com,
        dhaval.giani@...cle.com, rohit.k.jain@...cle.com,
        daniel.m.jordan@...cle.com, pavel.tatashin@...rosoft.com,
        matt@...eblueprint.co.uk, umgwanakikbuti@...il.com,
        riel@...hat.com, jbacik@...com, juri.lelli@...hat.com,
        linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 04/10] sched/fair: Dynamically update cfs_overload_cpus

On Mon, Oct 22, 2018 at 07:59:35AM -0700, Steve Sistare wrote:
> An overloaded CPU has more than 1 runnable task.  When a CFS task wakes
> on a CPU, if h_nr_running transitions from 1 to more, then set the CPU in
> the cfs_overload_cpus bitmap.  When a CFS task sleeps, if h_nr_running
> transitions from 2 to less, then clear the CPU in cfs_overload_cpus.

> @@ -5111,8 +5147,12 @@ static inline void hrtick_update(struct rq *rq)
>  		update_cfs_group(se);
>  	}
>  
> -	if (!se)
> +	if (!se) {
>  		add_nr_running(rq, 1);
> +		if (prev_nr == 1)
> +			overload_set(rq);
> +
> +	}

This is very similar to what RT already has, except they also track if
the tasks in question is in fact migratable (p->nr_cpus_allowed > 1).

And similarly, it might make sense to use the sparse mask thing over
there.