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SubjectRe: [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@kernel.org>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.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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