linux-kernel - Re: [PATCH 1/2] sched/fair: Use task_groups instead of leaf_cfs_rq_list to walk all cfs

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Date:   Thu, 4 May 2017 13:31:38 -0400
From:   Tejun Heo <tj@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        “linux-kernel@...r.kernel.org” 
        <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mike Galbraith <efault@....de>, Paul Turner <pjt@...gle.com>,
        Chris Mason <clm@...com>,
        “kernel-team@...com” <kernel-team@...com>
Subject: Re: [PATCH 1/2] sched/fair: Use task_groups instead of
 leaf_cfs_rq_list to walk all cfs_rqs

Hello, Peter.

On Thu, May 04, 2017 at 03:31:22PM +0200, Peter Zijlstra wrote:
> Yes we can hit an (almost) dead cfs_rq, but poking the bandwidth
> variables thereof is harmless.
> 
> unthrottle_cfs_rq() also stops doing anything much when it finds the
> cfs_rq is empty, which must be the case if we're removing it.

Yeah, if you're okay with calling the functions on dead cfs_rq's, just
wrapping with rcu_read_lock should be enough.

> I don't know Paul's opinion on RCU GPs happening while stop_machine(),
> but just in case he feels that's fair game, I did add the
> rcu_read_lock() thingies.
> 
> The lockdep assert is mostly documentation, to more easily see
> it is indeed held when we get there.
> 
> I left print_cfs_stats using the leaf list, no point in printing stuff
> that's empty.
> 
> This way we can avoid taking all RQ locks on cgroup destruction.

Looks good to me.

Thanks.

-- 
tejun