linux-kernel - Re: [RFC] workqueue: avoiding unbounded wq on isolated CPUs by default

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Message-ID: <20150718133602.GA3041@lerouge>
Date:	Sat, 18 Jul 2015 15:36:03 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Mike Galbraith <umgwanakikbuti@...il.com>
Cc:	Tejun Heo <tj@...nel.org>,
	Daniel Bristot de Oliveira <bristot@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Rik van Riel <riel@...hat.com>,
	"Luis Claudio R. Goncalves" <lclaudio@...g.org>
Subject: Re: [RFC] workqueue: avoiding unbounded wq on isolated CPUs by
 default

On Fri, Jul 17, 2015 at 07:15:48PM +0200, Mike Galbraith wrote:
> On Fri, 2015-07-17 at 11:27 -0400, Tejun Heo wrote:
> 
> > I'm just curious whether there was any specific reason we didn't do
> > this before (ISTR people discussing it back then too).
> 
> I'm dead set against all this auto-presume nonsense fwtw  Allocating a
> pool of no_hz_full _capable_ CPUs should not entice the kernel to make
> any rash assumptions.  Let users do the button poking, they know what
> they want, and when they want it.

We need to make a choice then. Either we do all the affinity tuning from
userspace with a common tool, which is what I had wished before everybody
asked for pre-settings.

Or we do it in the kernel, now we should define some kind of CONFIG_ISOLATION
to make that proper and rule the various kinds of isolation people are
interested in.

But we can't leave it half-way like it is currently with everything preset on
top of nohz: rcu nocb mask, watchdog mask, cpu_isolation_map and exclude workqueue.
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