On Wed 14-12-16 08:48:27, Paul E. McKenney wrote: > On Wed, Dec 14, 2016 at 05:15:41PM +0100, Michal Hocko wrote: > > On Wed 14-12-16 03:06:09, Paul E. McKenney wrote: > > > On Wed, Dec 14, 2016 at 10:54:25AM +0100, Michal Hocko wrote: > > > > On Tue 13-12-16 07:14:08, Paul E. McKenney wrote: > > > > > Just FYI for the moment... > > > > > > > > > > So even with the slowed-down checking, making cond_resched() do what > > > > > cond_resched_rcu_qs() does results in a smallish but quite measurable > > > > > degradation according to 0day. > > > > > > > > So if I understand those results properly, the reason seems to be the > > > > increased involuntary context switches, right? Or am I misreading the > > > > data? > > > > I am looking at your "sched,rcu: Make cond_resched() provide RCU > > > > quiescent state" in linux-next and I am wondering whether rcu_all_qs has > > > > to be called unconditionally and not only when should_resched failed few > > > > times? I guess you have discussed that with Peter already but do not > > > > remember the outcome. > > > > > > My first thought is to wait for the grace period to age further before > > > checking, the idea being to avoid increasing cond_resched() overhead > > > any further. But if that doesn't work, then yes, I may have to look at > > > adding more checks to cond_resched(). > > > > This might be really naive but would something like the following work? > > The overhead should be pretty much negligible, I guess. Ideally the pcp > > variable could be set somewhere from check_cpu_stall() but I couldn't > > wrap my head around that code to see how exactly. > > My concern (perhaps misplaced) with this approach is that there are > quite a few tight loops containing cond_resched(). So I would still > need to throttle the resulting grace-period acceleration to keep the > context switches down to a dull roar. Yes, I see your point. Something based on the stall timeout would be much better of course. I just failed to come up with something that would make sense. This was more my lack of familiarity with the code so I hope you will be more successful ;) -- 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>