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Date:   Mon, 6 Jul 2020 12:48:16 -0400
From:   Qian Cai <cai@....pw>
To:     "Paul E. McKenney" <paulmck@...nel.org>
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 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.

[  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.

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

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