On Thu, Jun 28, 2018 at 01:39:42PM +0200, Michal Hocko wrote: > On Wed 27-06-18 07:31:25, Paul E. McKenney wrote: > > On Wed, Jun 27, 2018 at 09:22:07AM +0200, Michal Hocko wrote: > > > On Tue 26-06-18 10:03:45, Paul E. McKenney wrote: > > > [...] > > > > 3. Something else? > > > > > > How hard it would be to use a different API than oom notifiers? E.g. a > > > shrinker which just kicks all the pending callbacks if the reclaim > > > priority reaches low values (e.g. 0)? > > > > Beats me. What is a shrinker? ;-) > > This is a generich mechanism to reclaim memory that is not on standard > LRU lists. Lwn.net surely has some nice coverage (e.g. > https://lwn.net/Articles/548092/). "In addition, there is little agreement over what a call to a shrinker really means or how the called subsystem should respond." ;-) Is this set up using register_shrinker() in mm/vmscan.c? I am guessing that the many mentions of shrinker in DRM are irrelevant. If my guess is correct, the API seems a poor fit for RCU. I can produce an approximate number of RCU callbacks for ->count_objects(), but a given callback might free a lot of memory or none at all. Plus, to actually have ->scan_objects() free them before returning, I would need to use something like rcu_barrier(), which might involve longer delays than desired. Or am I missing something here? > > More seriously, could you please point me at an exemplary shrinker > > use case so I can see what is involved? > > Well, I am not really sure what is the objective of the oom notifier to > point you to the right direction. IIUC you just want to kick callbacks > to be handled sooner under a heavy memory pressure, right? How is that > achieved? Kick a worker? That is achieved by enqueuing a non-lazy callback on each CPU's callback list, but only for those CPUs having non-empty lists. This causes CPUs with lists containing only lazy callbacks to be more aggressive, in particular, it prevents such CPUs from hanging out idle for seconds at a time while they have callbacks on their lists. The enqueuing happens via an IPI to the CPU in question. Thanx, Paul
