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Date: Thu, 30 Jul 2015 13:51:39 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: pang.xunlei@zte.com.cn
Cc: linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        Xunlei Pang <pang.xunlei@linaro.org>
Subject: Re: [PATCH] sched: Avoid sched domain rebuilding when hotplugging an
 isolate cpu
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On Thu, Jul 30, 2015 at 06:54:47PM +0800, pang.xunlei@zte.com.cn wrote:
> Hi Peter,
> 
> Peter Zijlstra <peterz@infradead.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@linaro.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.