Re: [PATCH] sched, fair: Allow a small degree of load imbalance between SD_NUMA domains v2

From: Peter Zijlstra <peterz@infradead.org>
To: Mel Gorman <mgorman@techsingularity.net>
Cc: Vincent Guittot <vincent.guittot@linaro.org>,
	Ingo Molnar <mingo@kernel.org>, Phil Auld <pauld@redhat.com>,
	Valentin Schneider <valentin.schneider@arm.com>,
	Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
	Quentin Perret <quentin.perret@arm.com>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Morten Rasmussen <Morten.Rasmussen@arm.com>,
	Hillf Danton <hdanton@sina.com>, Parth Shah <parth@linux.ibm.com>,
	Rik van Riel <riel@surriel.com>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] sched, fair: Allow a small degree of load imbalance between SD_NUMA domains v2
Date: Tue, 7 Jan 2020 13:28:09 +0100	[thread overview]
Message-ID: <20200107122809.GU2871@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20200107112255.GV2827@hirez.programming.kicks-ass.net>

On Tue, Jan 07, 2020 at 12:22:55PM +0100, Peter Zijlstra wrote:
> On Tue, Jan 07, 2020 at 09:56:55AM +0000, Mel Gorman wrote:

> > +			unsigned int imbalance_adj;
> > +
> > +			/*
> > +			 * Calculate an acceptable degree of imbalance based
> > +			 * on imbalance_adj. However, do not allow a greater
> > +			 * imbalance than the child domains weight to avoid
> > +			 * a case where the allowed imbalance spans multiple
> > +			 * LLCs.
> > +			 */
> 
> That comment is a wee misleading, @child is not an LLC per se. This
> could be the NUMA distance 2 domain, in which case @child is the NUMA
> distance 1 group.
> 
> That said, even then it probably makes sense to ensure you don't idle a
> whole smaller distance group.

Hmm, one more thing. On AMD EPYC, which the multiple LLCs, you'll have
the single NODE domain in between, and that is not marked with SD_NUMA
(iirc).

So specifically the case you want to handle is not in fact handled. The
first SD_NUMA (distance-1) will have all NODE children, which on EPYC
are not LLCs.