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

[Joonsoo Kim]

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.

Christoph, any opinion?

Thanks.