linux-kernel - Re: [PATCH tick-sched] Clarify "NOHZ: local_softirq

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Message-ID: <20200627214629.GH9247@paulmck-ThinkPad-P72>
Date:   Sat, 27 Jun 2020 14:46:29 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...com>
Subject: Re: [PATCH tick-sched] Clarify "NOHZ: local_softirq_pending" warning

On Sat, Jun 27, 2020 at 02:02:15PM -0700, Andy Lutomirski wrote:
> On Fri, Jun 26, 2020 at 2:05 PM Paul E. McKenney <paulmck@...nel.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.

Glad to help!

To your point, is it possible to bisect the appearance of this message,
or is it as usual non-reproducible?  (Hey, had to ask!)

							Thanx, Paul

> --Andy
> 
> >
> > Reported-by: Andy Lutomirski <luto@...nel.org>
> > Signed-off-by: Paul E. McKenney <paulmck@...nel.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++;
> >                 }