From: "Paul E. McKenney" <paulmck@kernel.org>
To: kernel-team@fb.com, linux-kernel@vger.kernel.org,
quic_mojha@quicinc.com, rcu@vger.kernel.org, rostedt@goodmis.org,
tj@kernel.org, Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH rcu 3/3] rcu: Allow expedited RCU grace periods on incoming CPUs
Date: Mon, 14 Feb 2022 08:44:35 -0800 [thread overview]
Message-ID: <20220214164435.GA2805255@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <20220209233811.GC557593@lothringen>
On Thu, Feb 10, 2022 at 12:38:11AM +0100, Frederic Weisbecker wrote:
> On Fri, Feb 04, 2022 at 02:55:07PM -0800, Paul E. McKenney wrote:
> > Although it is usually safe to invoke synchronize_rcu_expedited() from a
> > preemption-enabled CPU-hotplug notifier, if it is invoked from a notifier
> > between CPUHP_AP_RCUTREE_ONLINE and CPUHP_AP_ACTIVE, its attempts to
> > invoke a workqueue handler will hang due to RCU waiting on a CPU that
> > the scheduler is not paying attention to. This commit therefore expands
> > use of the existing workqueue-independent synchronize_rcu_expedited()
> > from early boot to also include CPUs that are being hotplugged.
> >
> > Link: https://lore.kernel.org/lkml/7359f994-8aaf-3cea-f5cf-c0d3929689d6@quicinc.com/
> > Reported-by: Mukesh Ojha <quic_mojha@quicinc.com>
> > Cc: Tejun Heo <tj@kernel.org>
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>
> I'm surprised by this scheduler behaviour.
>
> Since sched_cpu_activate() hasn't been called yet,
> rq->balance_callback = balance_push_callback. As a result, balance_push() should
> be called at the end of schedule() when the workqueue is picked as the next task.
> Then eventually the workqueue should be immediately preempted by the stop task to
> be migrated elsewhere.
>
> So I must be missing something. For the fun, I booted the following and it
> didn't produce any issue:
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 80faf2273ce9..b1e74a508881 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -4234,6 +4234,8 @@ int rcutree_online_cpu(unsigned int cpu)
>
> // Stop-machine done, so allow nohz_full to disable tick.
> tick_dep_clear(TICK_DEP_BIT_RCU);
> + if (cpu != 0)
> + synchronize_rcu_expedited();
> return 0;
> }
That does seem compelling. And others have argued that the workqueue
system's handling of offline CPUs should deal with this.
Mukesh, was this a theoretical bug, or did you actually make it happen?
If you made it happen, as seems to have been the case given your original
email [1], could you please post your reproducer?
Thanx, Paul
[1] https://lore.kernel.org/lkml/7359f994-8aaf-3cea-f5cf-c0d3929689d6@quicinc.com/
next prev parent reply other threads:[~2022-02-14 16:44 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-09 23:38 [PATCH rcu 3/3] rcu: Allow expedited RCU grace periods on incoming CPUs Frederic Weisbecker
2022-02-14 16:44 ` Paul E. McKenney [this message]
2022-02-15 14:28 ` Mukesh Ojha
[not found] ` <f8cff19c-5e8f-a7ed-c2ff-49a264b4e342@quicinc.com>
2022-02-15 17:39 ` Paul E. McKenney
2022-02-17 16:13 ` Mukesh Ojha
2022-02-18 17:33 ` Mukesh Ojha
2022-02-18 18:09 ` Paul E. McKenney
-- strict thread matches above, loose matches on Subject: below --
2022-02-04 22:54 [PATCH rcu 0/3] Expedited-grace-period updates for v5.18 Paul E. McKenney
2022-02-04 22:55 ` [PATCH rcu 3/3] rcu: Allow expedited RCU grace periods on incoming CPUs Paul E. McKenney
2022-02-08 18:56 ` Tejun Heo
2022-02-09 18:23 ` Mukesh Ojha
2022-02-09 22:06 ` Paul E. McKenney
2022-02-11 18:44 ` Mukesh Ojha
2022-02-11 22:14 ` Paul E. McKenney
2022-02-12 8:47 ` Mukesh Ojha
2022-02-12 11:28 ` Neeraj Upadhyay
2022-02-12 13:56 ` Mukesh Ojha
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