linux-kernel - Re: [PATCH] sched: Avoid sched domain rebuilding when hotplugging an isolate cpu

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Message-ID: <OFADFAFD55.AFAF2BD6-ON48257E92.0041AF59-48257E92.0042627C@zte.com.cn>
Date:	Thu, 30 Jul 2015 20:05:30 +0800
From:	pang.xunlei@....com.cn
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Xunlei Pang <pang.xunlei@...aro.org>
Subject: Re: [PATCH] sched: Avoid sched domain rebuilding when hotplugging an
 isolate cpu

Peter Zijlstra <peterz@...radead.org> wrote 2015-07-30 PM 07:51:39:
> 
> Re: [PATCH] sched: Avoid sched domain rebuilding when hotplugging an
> isolate cpu
> 
> On Thu, Jul 30, 2015 at 06:54:47PM +0800, pang.xunlei@....com.cn wrote:
> > Hi Peter,
> > 
> > Peter Zijlstra <peterz@...radead.org> wrote 2015-07-30 PM 06:30:37:
> > > Re: [PATCH] sched: Avoid sched domain rebuilding when hotplugging an
> > > isolate cpu
> > > 
> > > On Fri, Jul 17, 2015 at 04:03:45PM +0800, Xunlei Pang wrote:
> > > > From: Xunlei Pang <pang.xunlei@...aro.org>
> > > > 
> > > > The sched domain is always rebuilt when hotplugging an isolate 
cpu,
> > > > as we can see in partition_sched_domains() with the NULL doms_new,
> > > > current sched domain is destroyed, then rebuilt. Worse still, the
> > > > rebuilding will reset all the parameters of the sched_domain. This
> > > > makes no sense and contradicts the isloate concept.
> > > 
> > > What parameters? isolcpus should have fairly trivial domains to 
begin
> > > with.
> > 
> > I mean the parameters under 
> > /proc/sys/kernel/sched_domain/cpu<m>/domain<n>,
> > they will be reset to the default value once sched domain is rebuilt, 
if 
> > users have tuned them before, it's very annoying.
> > 
> > So we should avoid the rebuilding if can, like the case in this patch.
> 
> No, those are _DEBUG_ knobs, they're not guaranteed to have any
> persistence what so ever.

Yea, but that's not the point of the patch, the point is the isolate cpu 
is 
not attached to sched domain, so when it is hotplugged, it should also not 

touch the sched domain. Maybe I should remove this from the changelog :-) 

Thanks,
-Xunlei

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