On Tue, Aug 18, 2020 at 06:55:11PM +0200, Thomas Gleixner wrote: > On Tue, Aug 18 2020 at 09:13, Paul E. McKenney wrote: > > On Tue, Aug 18, 2020 at 04:43:14PM +0200, Thomas Gleixner wrote: > >> On Tue, Aug 18 2020 at 06:53, Paul E. McKenney wrote: > >> > On Tue, Aug 18, 2020 at 09:43:44AM +0200, Michal Hocko wrote: > >> >> Thomas had a good point that it doesn't really make much sense to > >> >> optimize for flooders because that just makes them more effective. > >> > > >> > The point is not to make the flooders go faster, but rather for the > >> > system to be robust in the face of flooders. Robust as in harder for > >> > a flooder to OOM the system. > >> > > >> > And reducing the number of post-grace-period cache misses makes it > >> > easier for the callback-invocation-time memory freeing to keep up with > >> > the flooder, thus avoiding (or at least delaying) the OOM. > >> > >> Throttling the flooder is incresing robustness far more than reducing > >> cache misses. > > > > True, but it takes time to identify a flooding event that needs to be > > throttled (as in milliseconds). This time cannot be made up. > > Not really. A flooding event will deplete your preallocated pages very > fast, so you have to go into the allocator and get new ones which > naturally throttles the offender. Should it turn out that we can in fact go into the allocator, completely agreed. > So if your open/close thing uses the new single argument free which has > to be called from sleepable context then the allocation either gives you > a page or that thing has to wait. No fancy extras. In the single-argument kvfree_rcu() case, completely agreed. > You still can have a page reserved for your other regular things and > once that it gone, you have to fall back to the linked list for > those. But when that happens the extra cache misses are not your main > problem anymore. The extra cache misses are a problem in that case because they throttle the reclamation, which anti-throttles the producer, especially in the case where callback invocation is offloaded. Thanx, Paul
