linux-kernel - Re: [PATCH v7 01/11] rcu: Wake up nocb gp thread on rcu_barrier

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Date:   Wed, 5 Oct 2022 12:39:20 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        rushikesh.s.kadam@...el.com, urezki@...il.com,
        neeraj.iitr10@...il.com, paulmck@...nel.org, rostedt@...dmis.org,
        youssefesmat@...gle.com, surenb@...gle.com
Subject: Re: [PATCH v7 01/11] rcu: Wake up nocb gp thread on
 rcu_barrier_entrain()

On Tue, Oct 04, 2022 at 06:57:59PM -0400, Joel Fernandes wrote:
> >> needed after an entrain. Otherwise, the RCU barrier callback can wait in
> >> the queue for several seconds before the lazy callbacks in front of it
> >> are serviced.
> > 
> > It's not about lazy callbacks here (but you can mention the fact that
> > waking nocb_gp if necessary after flushing bypass is a beneficial side
> > effect for further lazy implementation).
> > 
> > So here is the possible bad scenario:
> > 
> > 1) CPU 0 is nocb, it queues a callback
> > 2) CPU 0 goes idle (or userspace with nohz_full) forever
> > 3) The grace period related to that callback elapses
> > 4) The callback is moved to the done list (but is not invoked yet), there are no more pending for CPU 0
> > 5) CPU 1 calls rcu_barrier() and entrains to CPU 0 cblist
> 
> CPU 1 can only entrain into CPU 0 if the CPU is offline:
> 
> 		if (!rcu_rdp_cpu_online(rdp)) {
> 			rcu_barrier_entrain(rdp);
> 			WARN_ON_ONCE(READ_ONCE(rdp->barrier_seq_snap) != gseq);
> 			raw_spin_unlock_irqrestore(&rcu_state.barrier_lock,
> 			...
> 			continue;
> 		}

Ah good point. So CPU 1 sends an IPI to CPU 0 which entrains itself.
And then looks like the result is the same.


> 
> Otherwise an IPI does the entraining. So I do not see how CPU 0 being idle
> causes the cross-CPU entraining.

It doesn't but it shows that the CPU isn't going to enqueue any further
callback before a while. Though even if it did, it may not even solve the
situation, not until an RCU_NOCB_WAKE_FORCE is issued...

> 
> > 6) CPU 1 waits forever
> 
> But, I agree it can still wait forever, once the IPI handler does the
> entraining, since nothing will do the GP thread wakeup.
> 
> >>
> >> Reported-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > 
> > Fixes: 5d6742b37727 ("rcu/nocb: Use rcu_segcblist for no-CBs CPUs")
> 
> So, do you mind writing a proper patch with a proper commit message and Fixes
> tag then? It can independent of this series and add my Reported-by tag,
> thanks!

Ok will do.

Thanks!

> 
> Thanks!
> 
>  - Joel
>