On 09/16, Paolo Bonzini wrote: > > > On 16/09/2015 10:57, Christian Borntraeger wrote: > > Am 16.09.2015 um 10:32 schrieb Paolo Bonzini: > >> > >> > >> On 15/09/2015 19:38, Paul E. McKenney wrote: > >>> Excellent points! > >>> > >>> Other options in such situations include the following: > >>> > >>> o Rework so that the code uses call_rcu*() instead of *_expedited(). > >>> > >>> o Maintain a per-task or per-CPU counter so that every so many > >>> *_expedited() invocations instead uses the non-expedited > >>> counterpart. (For example, synchronize_rcu instead of > >>> synchronize_rcu_expedited().) > >> > >> Or just use ratelimit (untested): > > > > One of my tests was to always replace synchronize_sched_expedited with > > synchronize_sched and things turned out to be even worse. Not sure if > > it makes sense to test yopur in-the-middle approach? > > I don't think it applies here, since down_write/up_write is a > synchronous API. > > If the revert isn't easy, I think backporting rcu_sync is the best bet. I leave this to Paul and Tejun... at least I think this is not v4.2 material. > The issue is that rcu_sync doesn't eliminate synchronize_sched, Yes, but it eliminates _expedited(). This is good, but otoh this means that (say) individual __cgroup_procs_write() can take much more time. However, it won't block the readers and/or disturb the whole system. And percpu_up_write() doesn't do synchronize_sched() at all. > it only > makes it more rare. Yes, so we can hope that multiple __cgroup_procs_write()'s can "share" a single synchronize_sched(). > So it's possible that it isn't eliminating the root > cause of the problem. We will see... Just in case, currently the usage of percpu_down_write() is suboptimal. We do not need to do ->sync() under cgroup_mutex. But this needs some WIP changes in rcu_sync. Plus we can do more improvements, but this is off-topic right now. Oleg. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
