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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
