Re: Fix 80d20d35af1e ("nohz: Fix local_timer_softirq_pending()") may have revealed another problem

[Heiner Kallweit]

On 24.08.2018 16:30, Frederic Weisbecker wrote:
> On Fri, Aug 24, 2018 at 10:01:35AM +0200, Thomas Gleixner wrote:
>> On Fri, 24 Aug 2018, Heiner Kallweit wrote:
>>> On 24.08.2018 06:12, Frederic Weisbecker wrote:
>>>> On Thu, Aug 16, 2018 at 08:13:03AM +0200, Heiner Kallweit wrote:
>>>>> Recently I started to get warning "NOHZ: local_softirq_pending 202" and
>>>>> I think it's related to mentioned commit (didn't bisect it yet).
>>>>> See log from suspending.
>>>>>
>>>>> I have no reason to think the fix is wrong, it may just have revealed
>>>>> another issue which existed before and was hidden by the bug.
>>>>>
>>>>> Rgds, Heiner
>>>>>
>>>>> [   75.073353] random: crng init done
>>>>> [   75.073402] random: 7 urandom warning(s) missed due to ratelimiting
>>>>> [   78.619564] PM: suspend entry (deep)
>>>>> [   78.619675] PM: Syncing filesystems ... done.
>>>>> [   78.653684] Freezing user space processes ... (elapsed 0.002 seconds) done.
>>>>> [   78.656094] OOM killer disabled.
>>>>> [   78.656113] Freezing remaining freezable tasks ... (elapsed 0.001 seconds) done.
>>>>> [   78.658177] Suspending console(s) (use no_console_suspend to debug)
>>>>> [   78.663066] nuvoton-cir 00:07: disabled
>>>>> [   78.671817] sd 0:0:0:0: [sda] Synchronizing SCSI cache
>>>>> [   78.672210] sd 0:0:0:0: [sda] Stopping disk
>>>>> [   78.786651] ACPI: Preparing to enter system sleep state S3
>>>>> [   78.789613] PM: Saving platform NVS memory
>>>>> [   78.789759] Disabling non-boot CPUs ...
>>>>> [   78.805154] NOHZ: local_softirq_pending 202
>>>>> [   78.805182] NOHZ: local_softirq_pending 202
>>>>> [   78.807102] smpboot: CPU 1 is now offline
>>>>
>>>> I've tried to reproduce with suspend on disk but got unsuccessful.
>>>>
>>>> A small question as I see someone is having a similar issue with a stable
>>>> release only. On which kernel did you trigger that: upstream or stable?
>>>>
>>>> I'll continue investigating.
>>>>
>>>> Thanks.
>>>>
>>> Affected is recent linux-next, after the commit mentioned in the subject.
>>> I can work around the warning (not sure whether it's a proper fix),
>>> see here:
>>> https://lkml.org/lkml/2018/8/18/272
>>
>> Can you try the one I posted in this thread:
>>
>>  https://lkml.kernel.org/r/alpine.DEB.2.21.1808240851420.1668@xxxxxxxxxxxxxxxxxxxxxxx
>>
>> Also below for reference.
>>
>> Thanks,
>>
>> 	tglx
>>
>> 8<----------------
>> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
>> index 5b33e2f5c0ed..6aab9d54a331 100644
>> --- a/kernel/time/tick-sched.c
>> +++ b/kernel/time/tick-sched.c
>> @@ -888,7 +888,7 @@ static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)
>>  	if (unlikely(local_softirq_pending() && cpu_online(cpu))) {
>>  		static int ratelimit;
>>  
>> -		if (ratelimit < 10 &&
>> +		if (ratelimit < 10 && !in_softirq() &&
>>  		    (local_softirq_pending() & SOFTIRQ_STOP_IDLE_MASK)) {
>>  			pr_warn("NOHZ: local_softirq_pending %02x\n",
>>  				(unsigned int) local_softirq_pending());
> 
> I fear it may not work in his case because it happens in -next and we don't stop
> the idle tick from IRQ tail anymore. So we shouldn't be interrupting a softirq
> in this path. Still it's worth trying, I may well be missing something.
> 
> Thanks.
> 
I tested it and Frederic is right, it doesn't help. Can it be somehow related to
the cpu being brought down during suspend? Because I get the warning only during
suspend when the cpu is inactive already (but still online).