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

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Date:	Fri, 28 Jun 2013 11:04:47 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
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

On Fri, Jun 28, 2013 at 01:41:20PM +0530, Srikar Dronamraju wrote:

Please trim your replies.

> > +/* Returns true if the destination node has incurred more faults */
> > +static bool migrate_improves_locality(struct task_struct *p, struct lb_env *env)
> > +{
> > +	int src_nid, dst_nid;
> > +
> > +	if (!p->numa_faults || !(env->sd->flags & SD_NUMA))
> > +		return false;
> > +
> > +	src_nid = cpu_to_node(env->src_cpu);
> > +	dst_nid = cpu_to_node(env->dst_cpu);
> > +
> > +	if (src_nid == dst_nid)
> > +		return false;
> > +
> > +	if (p->numa_migrate_seq < sysctl_numa_balancing_settle_count &&
> 
> Lets say even if the numa_migrate_seq is greater than settle_count but running
> on a wrong node, then shouldnt this be taken as a good opportunity to 
> move the task?

I think that's what its doing; so this stmt says; if seq is large and
we're trying to move to the 'right' node; move it noaw.

> > +	    p->numa_preferred_nid == dst_nid)
> > +		return true;
> > +
> > +	return false;
> > +}
> > +
> > +
> >  /*
> >   * 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.
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