linux-kernel - Re: [RFC][PATCH] mm/slub: Skip CPU slab activation when debugging

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Date:	Fri, 1 Apr 2016 11:35:33 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Laura Abbott <labbott@...hat.com>
Cc:	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Laura Abbott <labbott@...oraproject.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>
Subject: Re: [RFC][PATCH] mm/slub: Skip CPU slab activation when debugging

On Mon, Mar 28, 2016 at 03:53:01PM -0700, Laura Abbott wrote:
> The per-cpu slab is designed to be the primary path for allocation in SLUB
> since it assumed allocations will go through the fast path if possible.
> When debugging is enabled, the fast path is disabled and per-cpu
> allocations are not used. The current debugging code path still activates
> the cpu slab for allocations and then immediately deactivates it. This
> is useless work. When a slab is enabled for debugging, skip cpu
> activation.
> 
> Signed-off-by: Laura Abbott <labbott@...oraproject.org>
> ---
> This is a follow on to the optimization of the debug paths for poisoning
> With this I get ~2 second drop on hackbench -g 20 -l 1000 with slub_debug=P
> and no noticable change with slub_debug=- .

I'd like to know the performance difference between slub_debug=P and
slub_debug=- with this change.

Although this patch increases hackbench performance, I'm not sure it's
sufficient for the production system. Concurrent slab allocation request
will contend the node lock in every allocation attempt. So, there would be
other ues-cases that performance drop due to slub_debug=P cannot be
accepted even if it is security feature.

How about allowing cpu partial list for debug cases?
It will not hurt fast path and will make less contention on the node
lock.

Thanks.