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Re: [dm-devel] [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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