Re: [RFC][PATCH] slub: Introduce 'alternate' per cpu partial lists

[Laura Abbott]

On 06/13/2017 09:45 PM, Joonsoo Kim wrote:
> On Thu, Jun 08, 2017 at 04:53:04PM -0700, Laura Abbott wrote:
>> SLUB debugging features (poisoning, red zoning etc.) skip the fast path
>> completely. This ensures there is a single place to do all checks and
>> take any locks that may be necessary for debugging. The overhead of some
>> of the debugging features (e.g. poisoning) ends up being comparatively
>> small vs the overhead of not using the fast path.
>>
>> We don't want to impose any kind of overhead on the fast path so
>> introduce the notion of an alternate fast path. This is essentially the
>> same idea as the existing fast path (store partially used pages on the
>> per-cpu list) but it happens after the real fast path. Debugging that
>> doesn't require locks (poisoning/red zoning) can happen on this path to
>> avoid the penalty of always needing to go for the slow path.
>>
>> Signed-off-by: Laura Abbott <labbott@xxxxxxxxxx>
>> ---
>> This is a follow up to my previous proposal to speed up slub_debug=P
>> https://marc.info/?l=linux-mm&m=145920558822958&w=2 . The current approach
>> is hopelessly slow and can't really be used outside of limited debugging.
>> The goal is to make slub_debug=P more usable for general use.
>>
>> Joonsoo Kim pointed out that my previous attempt still wouldn't scale
>> as it still involved taking the list_lock for every allocation. He suggested
>> adding per-cpu support, as did Christoph Lameter in a separate thread. This
>> proposal adds a separate per-cpu list for use when poisoning is enabled.
>> For this version, I'm mostly looking for general feedback about how reasonable
>> this approach is before trying to clean it up more.
>>
>> - Some of this code is redundant and can probably be combined.
>> - The fast path is very sensitive and it was suggested I leave it alone. The
>> approach I took means the fastpath cmpxchg always fails before trying the
>> alternate cmpxchg. From some of my profiling, the cmpxchg seemed to be fairly
>> expensive.
> 
> It looks better to modify the fastpath for non-debuging poisoning. If
> we use the jump label, it doesn't cause any overhead to the fastpath
> for the user who doesn't use this feature. It really makes thing
> simpler. Only a few more lines will be needed in the fastpath.
> 

When I initially tried something like that with the first version
I still saw an increase in the fast path even with slub_debug=-.
I might not have been testing with jump labels though and I didn't
spend that much time trying to narrow down the issue.

Thanks,
Laura

> Christoph, any opinion?
> 
> Thanks.
>