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Sender: Tejun Heo <htejun@gmail.com>
Date:   Wed, 26 Apr 2023 09:55:55 -1000
From:   Tejun Heo <tj@kernel.org>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@meta.com, rostedt@goodmis.org, riel@surriel.com
Subject: Re: [PATCH RFC rcu] Stop rcu_tasks_invoke_cbs() from using
 never-online CPUs
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References: <83d037d1-ef12-4b31-a7b9-7b1ed6c3ae42@paulmck-laptop>
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Hello, Paul.

On Wed, Apr 26, 2023 at 10:26:38AM -0700, Paul E. McKenney wrote:
> The rcu_tasks_invoke_cbs() relies on queue_work_on() to silently fall
> back to WORK_CPU_UNBOUND when the specified CPU is offline.  However,
> the queue_work_on() function's silent fallback mechanism relies on that
> CPU having been online at some time in the past.  When queue_work_on()
> is passed a CPU that has never been online, workqueue lockups ensue,
> which can be bad for your kernel's general health and well-being.
> 
> This commit therefore checks whether a given CPU is currently online,
> and, if not substitutes WORK_CPU_UNBOUND in the subsequent call to
> queue_work_on().  Why not simply omit the queue_work_on() call entirely?
> Because this function is flooding callback-invocation notifications
> to all CPUs, and must deal with possibilities that include a sparse
> cpu_possible_mask.
> 
> Fixes: d363f833c6d88 rcu-tasks: Use workqueues for multiple rcu_tasks_invoke_cbs() invocations
> Reported-by: Tejun Heo <tj@kernel.org>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

I don't understand the code at all but wonder whether it can do sth similar
to cpumask_any_distribute() which RR's through the specified cpumask. Would
it make sense to change rcu_tasks_invoke_cbs() to do something similar
against cpu_online_mask?

Thanks.

-- 
tejun