linux-kernel - Re: [PATCH] ARM64: Enable multi-core scheduler support by default

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Message-ID: <20151030153543.GY8899@e104818-lin.cambridge.arm.com>
Date:	Fri, 30 Oct 2015 15:35:43 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Dietmar Eggemann <dietmar.eggemann@....com>
Cc:	Will Deacon <will.deacon@....com>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] ARM64: Enable multi-core scheduler support by default

On Fri, Oct 30, 2015 at 12:29:00PM +0000, Dietmar Eggemann wrote:
> On 10/29/2015 05:19 PM, Catalin Marinas wrote:
> >On Mon, Oct 19, 2015 at 05:55:49PM +0100, Dietmar Eggemann wrote:
> >>Make sure that the task scheduler domain hierarchy is set-up correctly
> >>on systems with single or multi-cluster topology.
> >>
> >>Signed-off-by: Dietmar Eggemann <dietmar.eggemann@....com>
> >>---
> >>  arch/arm64/configs/defconfig | 1 +
> >>  1 file changed, 1 insertion(+)
> >>
> >>diff --git a/arch/arm64/configs/defconfig b/arch/arm64/configs/defconfig
> >>index 34d71dd86781..dc70d715bc38 100644
> >>--- a/arch/arm64/configs/defconfig
> >>+++ b/arch/arm64/configs/defconfig
> >>@@ -51,6 +51,7 @@ CONFIG_PCI=y
> >>  CONFIG_PCI_MSI=y
> >>  CONFIG_PCI_XGENE=y
> >>  CONFIG_SMP=y
> >>+CONFIG_SCHED_MC=y
> >>  CONFIG_PREEMPT=y
> >>  CONFIG_KSM=y
> >>  CONFIG_TRANSPARENT_HUGEPAGE=y
> >
> >Question: does enabling this option have any (positive) effect on the
> >vanilla kernel or it requires additional patches like EAS?
> 
> It improves the task scheduler decisions on multi-cluster systems. You have
> to iterate over all cpu's less frequently when gathering the sched group
> statistics e.g. during CFS load-balancing.
> 
> You essentially replace a flat (all cpus) with an hierarchical approach
> (lowest level: sched group eq. cpu, next level: sched group eq. cluster) and
> do CFS load balancing frequently between two cpu's of the same cluster
> (lowest level) and less frequently between two cpus from different cluster.
> 
> Not sure that I would see an effect on the performance score on a JUNO board
> when running hackbench. Could give it a try ... Probably easier to spot on a
> machine with >32 logical cpus.

I did some tests on Juno last week with and without this option and I
haven't noticed any difference.

Anyway, thanks for the explanation. Patch applied.

-- 
Catalin
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