Date: Tue, 7 Jul 2015 19:36:44 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>, Christoph Lameter <cl@linux.com>,
        Rik van Riel <riel@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 3/3] kmod: Remove unecessary explicit wide CPU affinity
	setting
Message-ID: <20150707173644.GA20422@redhat.com>
References: <1436196821-13962-1-git-send-email-fweisbec@gmail.com> <1436196821-13962-4-git-send-email-fweisbec@gmail.com> <20150707163030.GA16647@redhat.com> <20150707171014.GH20498@lerouge>
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On 07/07, Frederic Weisbecker wrote:
>
> On Tue, Jul 07, 2015 at 06:30:30PM +0200, Oleg Nesterov wrote:
> >
> > > Not only useless it even breaks nohz full. The housekeeping work
> > > (general kernel internal code that user doesn't care much about) is
> > > handled by a reduced set of CPUs in nohz full, precisely those that are
> > > not included by nohz_full= kernel parameters. For example unbound
> > > workqueues are handled by housekeeping CPUs.
> >
> > Confused... I do not see how workqueue_attrs->cpumask can depend on
> > tick_nohz_full_mask or housekeeping_mask. Could you explain?
>
> People who want CPU isolation will likely write
> /sys/devices/virtual/workqueue/cpumask to a reduced set of CPUs, typically
> CPU 0 that is used for housekeeping in nohz full.

Well, khelper_wq is not WQ_SYSFS, so I am not sure this is possible.

But this doesn't really matter, people can change cpu affinity. But
"workqueues are handled by housekeeping CPUs" doesn't look right.

> In fact we should add the code which initialize wq_unbound_cpumask
> to housekeeping_mask automatically.

Perhaps, but until then the changelog above looks really confusing,
as if workqueue.c already does this automagically ;)

Oleg.

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