Messages in this thread | | | Date | Wed, 16 Sep 2015 14:22:49 +0200 | From | Oleg Nesterov <> | Subject | Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace signal_struct->group_rwsem with a global percpu_rwsem) causes regression for libvirt/kvm |
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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.
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