Low-res tick handler device not going to ONESHOT_STOPPED when tick is stopped (was: rcu_sched self-detected stall on CPU)

[Nicholas Piggin]

Oops, fixed subject...

Excerpts from Nicholas Piggin's message of April 13, 2022 3:11 pm:
> +Daniel, Thomas, Viresh
> 
> Subject: Re: rcu_sched self-detected stall on CPU
> 
> Excerpts from Michael Ellerman's message of April 9, 2022 12:42 am:
>> Michael Ellerman <mpe@xxxxxxxxxxxxxx> writes:
>>> "Paul E. McKenney" <paulmck@xxxxxxxxxx> writes:
>>>> On Wed, Apr 06, 2022 at 05:31:10PM +0800, Zhouyi Zhou wrote:
>>>>> Hi
>>>>> 
>>>>> I can reproduce it in a ppc virtual cloud server provided by Oregon
>>>>> State University.  Following is what I do:
>>>>> 1) curl -l https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/snapshot/linux-5.18-rc1.tar.gz
>>>>> -o linux-5.18-rc1.tar.gz
>>>>> 2) tar zxf linux-5.18-rc1.tar.gz
>>>>> 3) cp config linux-5.18-rc1/.config
>>>>> 4) cd linux-5.18-rc1
>>>>> 5) make vmlinux -j 8
>>>>> 6) qemu-system-ppc64 -kernel vmlinux -nographic -vga none -no-reboot
>>>>> -smp 2 (QEMU 4.2.1)
>>>>> 7) after 12 rounds, the bug got reproduced:
>>>>> (http://154.223.142.244/logs/20220406/qemu.log.txt)
>>>>
>>>> Just to make sure, are you both seeing the same thing?  Last I knew,
>>>> Zhouyi was chasing an RCU-tasks issue that appears only in kernels
>>>> built with CONFIG_PROVE_RCU=y, which Miguel does not have set.  Or did
>>>> I miss something?
>>>>
>>>> Miguel is instead seeing an RCU CPU stall warning where RCU's grace-period
>>>> kthread slept for three milliseconds, but did not wake up for more than
>>>> 20 seconds.  This kthread would normally have awakened on CPU 1, but
>>>> CPU 1 looks to me to be very unhealthy, as can be seen in your console
>>>> output below (but maybe my idea of what is healthy for powerpc systems
>>>> is outdated).  Please see also the inline annotations.
>>>>
>>>> Thoughts from the PPC guys?
>>>
>>> I haven't seen it in my testing. But using Miguel's config I can
>>> reproduce it seemingly on every boot.
>>>
>>> For me it bisects to:
>>>
>>>   35de589cb879 ("powerpc/time: improve decrementer clockevent processing")
>>>
>>> Which seems plausible.
>>>
>>> Reverting that on mainline makes the bug go away.
>>>
>>> I don't see an obvious bug in the diff, but I could be wrong, or the old
>>> code was papering over an existing bug?
>>>
>>> I'll try and work out what it is about Miguel's config that exposes
>>> this vs our defconfig, that might give us a clue.
>> 
>> It's CONFIG_HIGH_RES_TIMERS=n which triggers the stall.
>> 
>> I can reproduce just with:
>> 
>>   $ make ppc64le_guest_defconfig
>>   $ ./scripts/config -d HIGH_RES_TIMERS
>> 
>> We have no defconfigs that disable HIGH_RES_TIMERS, I didn't even
>> realise you could disable it TBH :)
>> 
>> The Rust CI has it disabled because I copied that from the x86 defconfig
>> they were using back when I added the Rust support. I think that was
>> meant to be a stripped down fast config for CI, but the result is it's
>> just using a badly tested combination which is not helpful.
>> 
>> So I'll send a patch to turn HIGH_RES_TIMERS on for the Rust CI, and we
>> can debug this further without blocking them.
> 
> So we traced the problem down to possibly a misunderstanding between 
> decrementer clock event device and core code.
> 
> The decrementer is only oneshot*ish*. It actually needs to either be 
> reprogrammed or shut down otherwise it just continues to cause 
> interrupts.
> 
> Before commit 35de589cb879, it was sort of two-shot. The initial 
> interrupt at the programmed time would set its internal next_tb variable 
> to ~0 and call the ->event_handler(). If that did not set_next_event or 
> stop the timer, the interrupt will fire again immediately, notice 
> next_tb is ~0, and only then stop the decrementer interrupt.
> 
> So that was already kind of ugly, this patch just turned it into a hang.
> 
> The problem happens when the tick is stopped with an event still 
> pending, then tick_nohz_handler() is called, but it bails out because 
> tick_stopped == 1 so the device never gets programmed again, and so it 
> keeps firing.
> 
> How to fix it? Before commit a7cba02deced, powerpc's decrementer was 
> really oneshot, but we would like to avoid doing that because it requires 
> additional programming of the hardware on each timer interrupt. We have 
> the ONESHOT_STOPPED state which seems to be just about what we want.
> 
> Did the ONESHOT_STOPPED patch just miss this case, or is there a reason 
> we don't stop it here? This patch seems to fix the hang (not heavily
> tested though).
>  
> Thanks,
> Nick
> 
> ---
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 2d76c91b85de..7e13a55b6b71 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -1364,9 +1364,11 @@ static void tick_nohz_handler(struct clock_event_device *dev)
>  	tick_sched_do_timer(ts, now);
>  	tick_sched_handle(ts, regs);
>  
> -	/* No need to reprogram if we are running tickless  */
> -	if (unlikely(ts->tick_stopped))
> +	if (unlikely(ts->tick_stopped)) {
> +		/* If we are tickless, change the clock event to stopped */
> +		tick_program_event(KTIME_MAX, 1);
>  		return;
> +	}
>  
>  	hrtimer_forward(&ts->sched_timer, now, TICK_NSEC);
>  	tick_program_event(hrtimer_get_expires(&ts->sched_timer), 1);
>