linux-kernel - Re: 4.2.2: NR_CPUS effectively being 1 bug

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Date:	Sat, 03 Oct 2015 14:49:01 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Christoph Lameter <cl@...ux.com>
Subject: Re: 4.2.2: NR_CPUS effectively being 1 bug

On Sat, 2015-10-03 at 14:31 +0200, Frederic Weisbecker wrote:
> On Sat, Oct 03, 2015 at 02:04:44PM +0200, Mike Galbraith wrote:
> > On Sat, 2015-10-03 at 10:52 +0200, Ingo Molnar wrote:
> > > Ok, looks like a patch with good intentions but bad effects. Mind sending a revert 
> > > patch, changelogged, signed off?
> > 
> > No.  They know.
> 
> The reason for this patch is that NO_HZ_FULL is only useful on a CPU if no task
> other than the desired one can be scheduled on it. Hence the cpu_isolated_map.

Yes, but makes it needlessly static.

> Only those who enable NO_HZ_FULL_ALL by accident do complain, not those who
> really use it so far. At least it makes people realize their mistake.
> 
> That said I never liked that cpu_isolated_map. And some regular non-isolation
> work may be needed to be done even on NO_HZ_FULL_ALL machines and it that
> case we get screwed.

ATM, using nohz_full CPUs for generic work has a high price, but those
CPUs work just fine.  Andy is allegedly gonna make that overhead go
away, at which time dynamic sets become a much more attractive, but you
can do that now.

> So I should revert that and defer that isolation work to explicit affinity
> setting or cpusets.

Yay.

	-Mike


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