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

[Laura Abbott <labbott@redhat.com>]

On 04/01/2016 03:15 PM, Laura Abbott wrote:
> On 03/31/2016 07:35 PM, Joonsoo Kim wrote:
>> 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@fedoraproject.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.
>>
>
> with the hackbench benchmark
>
> slub_debug=- 6.834
>
> slub_debug=P 8.059
>
>
> so ~1.2 second difference.
>
>> 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.
>>
>
> Hmmm, I hadn't considered that :-/
>
>> How about allowing cpu partial list for debug cases?
>> It will not hurt fast path and will make less contention on the node
>> lock.
>>
>
> That helps more than this patch! It brings slub_debug=P down to 7.535
> with the same relaxing of restrictions of CMPXCHG (allow the partials
> with poison or redzoning, restrict otherwise).
>
> It still seems unfortunate that deactive_slab takes up so much time
> of __slab_alloc. I'll give some more thought about trying to skip
> the CPU slab activation with the cpu partial list.
>

I realized I was too eager about the number there. That number includes
using the slow path since the CPU partial list activates the fast path.
I'll need to think about how to use the CPU partial list and still
force debugging on the slow path.

Thanks,
Laura

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