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. > -- 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>
