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
Date: Thu, 20 Dec 2012 12:27:46 -0800 [thread overview]
Message-ID: <20121220122746.72d889fd.akpm@linux-foundation.org> (raw)
In-Reply-To: <20121220124710.GA31912@dhcp22.suse.cz>
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.
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WARNING: multiple messages have this Message-ID (diff)
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
Date: Thu, 20 Dec 2012 12:27:46 -0800 [thread overview]
Message-ID: <20121220122746.72d889fd.akpm@linux-foundation.org> (raw)
In-Reply-To: <20121220124710.GA31912@dhcp22.suse.cz>
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.
next prev parent reply other threads:[~2012-12-20 20:27 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-12-18 16:11 [PATCH] mm: cond_resched in tlb_flush_mmu to fix soft lockups on !CONFIG_PREEMPT Michal Hocko
2012-12-18 16:11 ` Michal Hocko
2012-12-18 18:01 ` Rik van Riel
2012-12-18 18:01 ` Rik van Riel
2012-12-18 22:02 ` Andrew Morton
2012-12-18 22:02 ` Andrew Morton
2012-12-18 23:50 ` Michal Hocko
2012-12-18 23:50 ` Michal Hocko
2012-12-19 0:00 ` Andrew Morton
2012-12-19 0:00 ` Andrew Morton
2012-12-19 15:04 ` [PATCH v2] mm: limit mmu_gather batching " Michal Hocko
2012-12-19 15:04 ` Michal Hocko
2012-12-19 21:13 ` Andrew Morton
2012-12-19 21:13 ` Andrew Morton
2012-12-20 10:24 ` Mel Gorman
2012-12-20 10:24 ` Mel Gorman
2012-12-20 12:47 ` Michal Hocko
2012-12-20 12:47 ` Michal Hocko
2012-12-20 20:27 ` Andrew Morton [this message]
2012-12-20 20:27 ` Andrew Morton
2012-12-20 22:36 ` [PATCH v3] " Michal Hocko
2012-12-20 22:36 ` Michal Hocko
2012-12-21 8:09 ` Michal Hocko
2012-12-21 8:09 ` Michal Hocko
2013-04-27 7:50 ` [PATCH] mm: cond_resched in tlb_flush_mmu " Simon Jeons
2013-04-27 7:50 ` Simon Jeons
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