On Wed 30-11-16 09:23:55, Paul E. McKenney wrote: > On Wed, Nov 30, 2016 at 06:05:57PM +0100, Michal Hocko wrote: > > On Wed 30-11-16 17:38:20, Peter Zijlstra wrote: > > > On Wed, Nov 30, 2016 at 06:29:55AM -0800, Paul E. McKenney wrote: > > > > We can, and you are correct that cond_resched() does not unconditionally > > > > supply RCU quiescent states, and never has. Last time I tried to add > > > > cond_resched_rcu_qs() semantics to cond_resched(), I got told "no", > > > > but perhaps it is time to try again. > > > > > > Well, you got told: "ARRGH my benchmark goes all regress", or something > > > along those lines. Didn't we recently dig out those commits for some > > > reason or other? > > > > > > Finding out what benchmark that was and running it against this patch > > > would make sense. > > > > > > Also, I seem to have missed, why are we going through this again? > > > > Well, the point I've brought that up is because having basically two > > APIs for cond_resched is more than confusing. Basically all longer in > > kernel loops do cond_resched() but it seems that this will not help the > > silence RCU lockup detector in rare cases where nothing really wants to > > schedule. I am really not sure whether we want to sprinkle > > cond_resched_rcu_qs at random places just to silence RCU detector... > > Just in case there is any doubt on this point, any patch of mine adding > cond_resched_rcu_qs() functionality to cond_resched() cannot go upstream > without Peter's Acked-by. Yeah, that is clear to me. I just wanted to clarify the "why are we going through this again" part ;) > Or did you have some other solution in mind? Not really. The fact that cond_resched() cannot silence RCU stall detector under some circumstances is sad. I believe we shouldn't have two different APIs to control scheduling and RCU latencies because that just asks for whack a mole games and some level of confusion... -- Michal Hocko SUSE Labs -- 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>