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Date:   Fri, 14 Dec 2018 12:31:11 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc:     Petr Mladek <pmladek@suse.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] kernel/hung_task.c: Break RCU locks based on jiffies.
Message-Id: <20181214123111.266cae10f71ea6b277d634c6@linux-foundation.org>
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On Sat, 15 Dec 2018 00:17:38 +0900 Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> wrote:

> check_hung_uninterruptible_tasks() is currently calling rcu_lock_break()
> for every 1024 threads. But check_hung_task() is very slow if printk()
> was called, and is very fast otherwise. If many threads within some 1024
> threads called printk(), the RCU grace period might be extended enough
> to trigger RCU stall warnings. Therefore, calling rcu_lock_break() for
> every some fixed jiffies will be safer.
> 
> --- a/kernel/hung_task.c
> +++ b/kernel/hung_task.c
> @@ -34,7 +34,7 @@
>   * is disabled during the critical section. It also controls the size of
>   * the RCU grace period. So it needs to be upper-bound.
>   */
> -#define HUNG_TASK_BATCHING 1024
> +#define HUNG_TASK_LOCK_BREAK (HZ / 10)

This won't work correctly if rcu_cpu_stall_timeout is set to something
stupidly small.  Perhaps is would be better to make this code aware of
the current rcu_cpu_stall_timeout setting?