linux-kernel - Re: [PATCH v3 0/5] Improve newidle lb cost tracking and early abort

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Date:   Wed, 27 Oct 2021 13:53:32 -0700
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...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 v3 0/5] Improve newidle lb cost tracking and early abort

On Wed, 2021-10-27 at 10:49 +0200, Vincent Guittot wrote:
> 
> > Looking at the profile on update_blocked_averages a bit more,
> > the majority of the call to update_blocked_averages
> > happens in run_rebalance_domain.  And we are not
> > including that cost of update_blocked_averages for
> > run_rebalance_domains in our current patch set. I think
> > the patch set should account for that too.
> 
> nohz_newidle_balance keeps using sysctl_sched_migration_cost to
> trigger a _nohz_idle_balance(cpu_rq(cpu), NOHZ_STATS_KICK, CPU_IDLE);
> This would probably benefit to take into account the cost of
> update_blocked_averages instead
> 

For the case where

	this_rq->avg_idle < sysctl_sched_migration_cost

in newidle_balance(), we skip to the out: label

out:
        /* Move the next balance forward */
        if (time_after(this_rq->next_balance, next_balance))
                this_rq->next_balance = next_balance;

        if (pulled_task)
                this_rq->idle_stamp = 0;
        else
                nohz_newidle_balance(this_rq);

and we call nohz_newidle_balance as we don't have a pulled_task.

It seems to make sense to skip the call
to nohz_newidle_balance() for this case?  
We expect a very short idle and a task to wake shortly.  
So we do not have to pull a task
to this idle cpu and incur the migration cost.

Tim