Messages in this thread | | | From | bsegall@google ... | Subject | Re: [RFC] sched/fair: hard lockup in sched_cfs_period_timer | Date | Wed, 06 Mar 2019 11:25:02 -0800 |
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Phil Auld <pauld@redhat.com> writes:
> On Tue, Mar 05, 2019 at 12:45:34PM -0800 bsegall@google.com wrote: >> Phil Auld <pauld@redhat.com> writes: >> >> > Interestingly, if I limit the number of child cgroups to the number of >> > them I'm actually putting processes into (16 down from 2500) the problem >> > does not reproduce. >> >> That is indeed interesting, and definitely not something we'd want to >> matter. (Particularly if it's not root->a->b->c...->throttled_cgroup or >> root->throttled->a->...->thread vs root->throttled_cgroup, which is what >> I was originally thinking of) >> > > The locking may be a red herring. > > The setup is root->throttled->a where a is 1-2500. There are 4 threads in > each of the first 16 a groups. The parent, throttled, is where the > cfs_period/quota_us are set. > > I wonder if the problem is the walk_tg_tree_from() call in unthrottle_cfs_rq(). > > The distribute_cfg_runtime looks to be O(n * m) where n is number of > throttled cfs_rqs and m is the number of child cgroups. But I'm not > completely clear on how the hierarchical cgroups play together here. > > I'll pull on this thread some. > > Thanks for your input. > > > Cheers, > Phil
Yeah, that isn't under the cfs_b lock, but is still part of distribute (and under rq lock, which might also matter). I was thinking too much about just the cfs_b regions. I'm not sure there's any good general optimization there.
I suppose cfs_rqs (tgs/cfs_bs?) could have "nearest ancestor with a quota" pointer and ones with quota could have "descendants with quota" list, parallel to the children/parent lists of tgs. Then throttle/unthrottle would only have to visit these lists, and child cgroups/cfs_rqs without their own quotas would just check cfs_rq->nearest_quota_cfs_rq->throttle_count. throttled_clock_task_time can also probably be tracked there.
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