linux-kernel - Re: [PATCH 5/8] sched: Favour moving tasks towards the preferred node

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Date:	Fri, 28 Jun 2013 15:37:23 +0530
From:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mel Gorman <mgorman@...e.de>, Ingo Molnar <mingo@...nel.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/8] sched: Favour moving tasks towards the preferred node

> > > +
> > > +
> > >  /*
> > >   * can_migrate_task - may task p from runqueue rq be migrated to this_cpu?
> > >   */
> > > @@ -3945,10 +3977,14 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
> > >  
> > >  	/*
> > >  	 * Aggressive migration if:
> > > -	 * 1) task is cache cold, or
> > > -	 * 2) too many balance attempts have failed.
> > > +	 * 1) destination numa is preferred
> > > +	 * 2) task is cache cold, or
> > > +	 * 3) too many balance attempts have failed.
> > >  	 */
> > >  
> > > +	if (migrate_improves_locality(p, env))
> > > +		return 1;
> > 
> > Shouldnt this be under tsk_cache_hot check?
> > 
> > If the task is cache hot, then we would have to update the corresponding  schedstat
> > metrics.
> 
> No; you want migrate_degrades_locality() to be like task_hot(). You want
> to _always_ migrate tasks towards better locality irrespective of local
> cache hotness.
> 

Yes, I understand that numa should have more priority over cache.
But the schedstats will not be updated about whether the task was hot or
cold.

So lets say the task was cache hot but numa wants it to move, then we
should certainly move it but we should update the schedstats to mention that we
moved a cache hot task.

Something akin to this.

	tsk_cache_hot = task_hot(p, env->src_rq->clock_task, env->sd);
	if (tsk_cache_hot) {
		if (migrate_improves_locality(p, env) || 
		 	(env->sd->nr_balance_failed > env->sd->cache_nice_tries)) {
#ifdef CONFIG_SCHEDSTATS
			schedstat_inc(env->sd, lb_hot_gained[env->idle]);
			schedstat_inc(p, se.statistics.nr_forced_migrations);
#endif
			return 1;
		}
		schedstat_inc(p, se.statistics.nr_failed_migrations_hot);
		return 0;
	}
	return 1;

-- 
Thanks and Regards
Srikar Dronamraju

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