linux-kernel - Re: [PATCH] mm,oom: Bring OOM notifier callbacks to outside of OOM killer.

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Message-Id: <20180626235014.GS3593@linux.vnet.ibm.com>
Date:   Tue, 26 Jun 2018 16:50:14 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Michal Hocko <mhocko@...nel.org>,
        David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm,oom: Bring OOM notifier callbacks to outside of OOM
 killer.

On Wed, Jun 27, 2018 at 05:10:48AM +0900, Tetsuo Handa wrote:
> On 2018/06/27 2:03, Paul E. McKenney wrote:
> > There are a lot of ways it could be made concurrency safe.  If you need
> > me to do this, please do let me know.
> > 
> > That said, the way it is now written, if you invoke rcu_oom_notify()
> > twice in a row, the second invocation will wait until the memory from
> > the first invocation is freed.  What do you want me to do if you invoke
> > me concurrently?
> > 
> > 1.	One invocation "wins", waits for the earlier callbacks to
> > 	complete, then encourages any subsequent callbacks to be
> > 	processed more quickly.  The other invocations return
> > 	immediately without doing anything.
> > 
> > 2.	The invocations serialize, with each invocation waiting for
> > 	the callbacks from previous invocation (in mutex_lock() order
> > 	or some such), and then starting a new round.
> > 
> > 3.	Something else?
> > 
> > 							Thanx, Paul
> 
> As far as I can see,
> 
> -	atomic_set(&oom_callback_count, 1);
> +	atomic_inc(&oom_callback_count);
> 
> should be sufficient.

I don't see how that helps.  For example, suppose that two tasks
invoked rcu_oom_notify() at about the same time.  Then they could
both see oom_callback_count equal to zero, both atomically increment
oom_callback_count, then both do the IPI invoking rcu_oom_notify_cpu()
on each online CPU.

So far, so good.  But rcu_oom_notify_cpu() enqueues a per-CPU RCU
callback, and enqueuing the same callback twice in quick succession
would fatally tangle RCU's callback lists.

What am I missing here?

							Thanx, Paul