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

From: Tejun Heo
Date: Thu Apr 11 2013 - 15:52:14 EST

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.


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