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

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Date:   Mon, 6 Jul 2020 10:06:49 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Qian Cai <cai@....pw>
Cc:     fweisbec@...il.com, tglx@...utronix.de, mingo@...nel.org,
        linux-kernel@...r.kernel.org, luto@...nel.org, kernel-team@...com
Subject: Re: [PATCH tick-sched] Clarify "NOHZ: local_softirq_pending" warning

On Mon, Jul 06, 2020 at 12:48:16PM -0400, Qian Cai wrote:
> On Fri, Jun 26, 2020 at 02:05:06PM -0700, Paul E. McKenney 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.
> > 
> > Reported-by: Andy Lutomirski <luto@...nel.org>
> > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> 
> CPU hotplug will sometimes trigger this warning on linux-next from NUMA
> bare-metal which the commit makes it feel more like errors.

One point of the change was to make it clear that this is an error.
So success!  ;-)

> [  267.734981] smpboot: CPU 42 is now offline
> [  267.991940] smpboot: Booting Node 2 Processor 42 APIC 0x29
> [  267.998370] numa_add_cpu cpu 42 node 2: mask now 8-11,40-43
> [  268.092380] ACPI: \_SB_.SCK0.C015: Found 2 idle states
> [  268.181917] NOHZ tick-stop error: Non-RCU local softirq work is pending, handler #282
> [  268.190585] numa_remove_cpu cpu 1 node 0: mask now 0,2-3,32-35
> [  268.194075] smpboot: CPU 1 is now offline
> [  268.415320] x86: Booting SMP configuration:
> [  268.420245] smpboot: Booting Node 0 Processor 1 APIC 0x2
> 
> # git clone https://github.com/cailca/linux-mm
> # cd linux-mm; make
> # ./random 4 (it just soft offline and online each CPU)
> 
> The x86.config is also included there. This is only reproducible on x86
> so far.

I must defer to Frederic and Andy on this one.  But now that you mention
it, I do see this on some of my runs, which are x86, but not bare metal.
I clearly need to upgrade my console-log parsing.

							Thanx, Paul

> > ---
> > 
> >  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++;
> >  		}