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Date:   Wed, 06 Mar 2019 11:25:02 -0800
From:   bsegall@...gle.com
To:     Phil Auld <pauld@...hat.com>
Cc:     mingo@...hat.com, peterz@...radead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC]  sched/fair: hard lockup in sched_cfs_period_timer

Phil Auld <pauld@...hat.com> writes:

> On Tue, Mar 05, 2019 at 12:45:34PM -0800 bsegall@...gle.com wrote:
>> Phil Auld <pauld@...hat.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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