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From:   Andy Lutomirski <luto@kernel.org>
Date:   Sat, 27 Jun 2020 14:02:15 -0700
Message-ID: <CALCETrXN5Mm7yvDPsD7Ok=QAVoLH_fnEOgtdU2QCP+-q9u_ALA@mail.gmail.com>
Subject: Re: [PATCH tick-sched] Clarify "NOHZ: local_softirq_pending" warning
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     Frederic Weisbecker <fweisbec@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Andrew Lutomirski <luto@kernel.org>,
        kernel-team <kernel-team@fb.com>
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On Fri, Jun 26, 2020 at 2:05 PM Paul E. McKenney <paulmck@kernel.org> wrote:
>
> Currently, can_stop_idle_tick() prints "NOHZ: local_softirq_pending HH"
> (where "HH" is the hexadecimal softirq vector number) when one or more
> non-RCU softirq handlers are still enablded when checking to stop the
> scheduler-tick interrupt.  This message is not as enlightening as one
> might hope, so this commit changes it to "NOHZ tick-stop error: Non-RCU
> local softirq work is pending, handler #HH.

Thank you!  It would be even better if it would explain *why* the
problem happened, but I suppose this code doesn't actually know.

--Andy

>
> Reported-by: Andy Lutomirski <luto@kernel.org>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>
> ---
>
>  tick-sched.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index f0199a4..349a25a 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -927,7 +927,7 @@ static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)
>
>                 if (ratelimit < 10 &&
>                     (local_softirq_pending() & SOFTIRQ_STOP_IDLE_MASK)) {
> -                       pr_warn("NOHZ: local_softirq_pending %02x\n",
> +                       pr_warn("NOHZ tick-stop error: Non-RCU local softirq work is pending, handler #%02x\n",
>                                 (unsigned int) local_softirq_pending());
>                         ratelimit++;
>                 }