Re: [PATCH 4/4] sched/numa: Do not move imbalanced load purely on the basis of an idle CPU

From: Mel Gorman <mgorman@techsingularity.net>
To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>, Rik van Riel <riel@surriel.com>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 4/4] sched/numa: Do not move imbalanced load purely on the basis of an idle CPU
Date: Fri, 7 Sep 2018 15:28:46 +0100	[thread overview]
Message-ID: <20180907142846.GG1719@techsingularity.net> (raw)
In-Reply-To: <20180907134201.GC3995@linux.vnet.ibm.com>

On Fri, Sep 07, 2018 at 07:12:01PM +0530, Srikar Dronamraju wrote:
> > Yeah, I was afraid it would.. Srikar, can you also evaluate, I suspect
> > we'll have to pick one of these two patches.
> 
> I can surely run some benchmarks between the two patches.
> However comparing  Mel's patch with
> http://lkml.kernel.org/r/1533276841-16341-4-git-send-email-srikar@linux.vnet.ibm.com
> 
> Mel's patch
> 
>   	if (!cur) {
>  -		if (maymove || imp > env->best_imp)
>  +		if (maymove)
>   			goto assign;
>   		else
> http://lkml.kernel.org/r/1533276841-16341-4-git-send-email-srikar@linux.vnet.ibm.com
> 
> 
>  	if (!cur) {
> -		if (maymove || imp > env->best_imp)
> +		if (maymove && moveimp >= env->best_imp)
>  			goto assign;
>  		else
> 
> In Mel's fix, if we already found a candidate task to swap and then encounter a
> idle cpu, we are going ahead and overwriting the swap candidate. There is
> always a chance that swap candidate is a better fit than moving to idle cpu.
> 

There is a chance but to find out, the task has to be dequeued and requeued
on a maybe. An idle CPU is less disruptive and the only task affected is
migrating to the preferred node where, based on previous fault behaviour,
should have better locality. It's also a simplier patch but I'm going to
be biased towards my own patch, the tests will decide one way or the other.

-- 
Mel Gorman
SUSE Labs