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:   Mon, 1 May 2017 15:07:47 -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>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 1/2] sched/fair: Use task_groups instead of
 leaf_cfs_rq_list to walk all cfs_rqs

Hello,

On Mon, May 01, 2017 at 07:02:41PM +0200, Peter Zijlstra wrote:
> n/m, I need to stop staring at a screen. Wrapping those two sites in
> rcu_read_lock() achieves the very same.
> 
> So we want the rcu_read_lock() to serialize against sched_free_group,
> but don't need the new ->online thing and can retain the ->on_list
> stuff. Or I've completely lost the plot (which is entirely possible...)
> 
> I'll stare at this again tomorrow

So, the rcu_read_lock() thing protects against sched_free_group() and
thanks to the order of operations, all online cfs_rq's are guaranteed
to be visbile in the two callbacks; however, nothing prevents the code
paths from seeing already dead cfs_rqs, which *may* be okay if the
code paths are safe to run on dead and unlinked cfs_rqs, but it's
still nasty and fragile.

The new ->online condition which is synchronized by rq->lock
guarantees that both functions only process live ones.  We need
something synchronized by rq->lock to guarantee this whether that's a
new list entry or a flag like ->online.  It'd be better to encapsulate
and document this iteration.

Thanks.

-- 
tejun