linux-kernel - Re: [PATCH] cpuset: mm: Remove memory barrier damage from the page allocator

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Date:	Fri, 2 Mar 2012 13:53:45 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Mel Gorman <mgorman@...e.de>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Miao Xie <miaox@...fujitsu.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cpuset: mm: Remove memory barrier damage from the page
 allocator

On Fri, 2 Mar 2012, Mel Gorman wrote:

> I considered using a seqlock but it isn't cheap. The read side is heavy
> with the possibility that it starts spinning and incurs a read barrier
> (looking at read_seqbegin()) here. The retry block incurs another read
> barrier so basically it would not be no better than what is there currently
> (which at a 4% performance hit, sucks)

Oh. You dont have a read barrier? So your approach is buggy? We could have
read a state before someone else incremented the seq counter, then cached
it, then we read the counter, did the processing and found that the
sequid was not changed?

> In the case of seqlocks, a reader will backoff if a writer is in progress
> but the page allocator doesn't need that which is why I felt it was ok

You can just not use the writer section if you think that is ok. Doubt it
but lets at least start using a known serialization construct that would
allow us to fix it up if we find that we need to update multiple variables
protected by the seqlock.

> Allocation failure is an unusual situation that can trigger application
> exit or an OOM so it's ok to treat it as a slow path. A normal seqlock
> would retry unconditionally and potentially have to handle the case
> where it needs to free the page before retrying which is pointless.

It will only retry as long as the writer hold the "lock". Like a spinlock
the holdoff times depends on the size of the critical section and
initially you could just avoid having write sections.

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