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Date: Thu, 20 Dec 2012 12:27:46 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Michal Hocko <mhocko@suse.cz>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Mel Gorman <mgorman@suse.de>, Rik van Riel <riel@redhat.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [PATCH v2] mm: limit mmu_gather batching to fix soft lockups on
 !CONFIG_PREEMPT
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On Thu, 20 Dec 2012 13:47:10 +0100
Michal Hocko <mhocko@suse.cz> wrote:

> > > + */
> > > +#if defined(CONFIG_PREEMPT_COUNT)
> > > +#define MAX_GATHER_BATCH_COUNT	(UINT_MAX)
> > > +#else
> > > +#define MAX_GATHER_BATCH_COUNT	(((1UL<<(30-PAGE_SHIFT))/MAX_GATHER_BATCH))
> > 
> > Geeze.  I spent waaaaay too long staring at that expression trying to
> > work out "how many pages is in a batch" and gave up.
> > 
> > Realistically, I don't think we need to worry about CONFIG_PREEMPT here
> > - if we just limit the thing to, say, 64k pages per batch then that
> > will be OK for preemptible and non-preemptible kernels. 
> 
> I wanted the fix to be as non-intrusive as possible so I didn't want to
> touch PREEMPT (which is default in many configs) at all. I am OK to a
> single limit of course.

non-intrusive is nice, but best-implementation is nicer.

> > The performance difference between "64k" and "infinite" will be
> > miniscule and unmeasurable.
> > 
> > Also, the batch count should be independent of PAGE_SIZE.  Because
> > PAGE_SIZE can vary by a factor of 16 and you don't want to fix the
> > problem on 4k page size but leave it broken on 64k page size.
> 
> MAX_GATHER_BATCH depends on the page size so I didn't want to differ
> without a good reason.

There's a good reason!  PAGE_SIZE can vary by a factor of 16, and if
this results in the unpreemptible-CPU-effort varying by a factor of 16
then that's bad, and we should change things so the
unpreemptible-CPU-effort is independent of PAGE_SIZE.