linux-kernel - Re: [PATCH v2] make dm and dm-crypt forward cgroup context (was: dm-crypt parallelization patches)

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Date:	Thu, 11 Apr 2013 12:52:03 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	Vivek Goyal <vgoyal@...hat.com>, Jens Axboe <axboe@...nel.dk>,
	Mike Snitzer <snitzer@...hat.com>,
	Milan Broz <gmazyland@...il.com>, dm-devel@...hat.com,
	Andi Kleen <andi@...stfloor.org>, dm-crypt@...ut.de,
	linux-kernel@...r.kernel.org,
	Christoph Hellwig <hch@...radead.org>,
	Christian Schmidt <schmidt@...add.de>,
	"Alasdair G. Kergon" <agk@...hat.com>
Subject: Re: [PATCH v2] make dm and dm-crypt forward cgroup context (was:
 dm-crypt parallelization patches)

On Thu, Apr 11, 2013 at 03:49:20PM -0400, Mikulas Patocka wrote:
> If the bi_css pointer points to a structure that is shared between 
> processes, using atomic instruction causes cache line boucing - it doesn't 
> cost a few instructions, it costs 2-3 hundreds cycles.
> 
> I modified the patch to use new flag BIO_DROP_CGROUP_REFCOUNT to note that 
> the refcount must be decremented - if the flag is set, refcounts must be 
> decremented when bio is destroyed, if it is not set, references are 
> borrowed from upper layer bio.
> 
> It is less bug-prone than the previous patch.

If this becomes an actual bottleneck, the right thing to do is making
css ref per-cpu.  Please stop messing around with refcounting.

NACK.

-- 
tejun
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