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Subject: Re: [PATCH] cpufreq: powernv: Fix the hardlockup by synchronus
 smp_call in timer interrupt
To:     Nicholas Piggin <npiggin@gmail.com>
References: <1524544906-31512-1-git-send-email-shilpa.bhat@linux.vnet.ibm.com>
 <20180424160034.6e9d2274@roar.ozlabs.ibm.com>
 <c88d51af-9eb8-6bb3-4201-8909cda0241b@linux.vnet.ibm.com>
 <20180424173147.7bcd86c5@roar.ozlabs.ibm.com>
Cc:     rjw@rjwysocki.net, viresh.kumar@linaro.org,
        benh@kernel.crashing.org, mpe@ellerman.id.au,
        linux-pm@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        linux-kernel@vger.kernel.org, ppaidipe@linux.vnet.ibm.com,
        svaidy@linux.vnet.ibm.com
From:   Shilpasri G Bhat <shilpa.bhat@linux.vnet.ibm.com>
Date:   Tue, 24 Apr 2018 16:17:29 +0530
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On 04/24/2018 01:01 PM, Nicholas Piggin wrote:
> On Tue, 24 Apr 2018 12:47:32 +0530
> Shilpasri G Bhat <shilpa.bhat@linux.vnet.ibm.com> wrote:
> 
>> Hi,
>>
>> On 04/24/2018 11:30 AM, Nicholas Piggin wrote:
>>> On Tue, 24 Apr 2018 10:11:46 +0530
>>> Shilpasri G Bhat <shilpa.bhat@linux.vnet.ibm.com> wrote:
>>>   
>>>> gpstate_timer_handler() uses synchronous smp_call to set the pstate
>>>> on the requested core. This causes the below hard lockup:
>>>>
>>>> [c000003fe566b320] [c0000000001d5340] smp_call_function_single+0x110/0x180 (unreliable)
>>>> [c000003fe566b390] [c0000000001d55e0] smp_call_function_any+0x180/0x250
>>>> [c000003fe566b3f0] [c000000000acd3e8] gpstate_timer_handler+0x1e8/0x580
>>>> [c000003fe566b4a0] [c0000000001b46b0] call_timer_fn+0x50/0x1c0
>>>> [c000003fe566b520] [c0000000001b4958] expire_timers+0x138/0x1f0
>>>> [c000003fe566b590] [c0000000001b4bf8] run_timer_softirq+0x1e8/0x270
>>>> [c000003fe566b630] [c000000000d0d6c8] __do_softirq+0x158/0x3e4
>>>> [c000003fe566b710] [c000000000114be8] irq_exit+0xe8/0x120
>>>> [c000003fe566b730] [c000000000024d0c] timer_interrupt+0x9c/0xe0
>>>> [c000003fe566b760] [c000000000009014] decrementer_common+0x114/0x120
>>>> --- interrupt: 901 at doorbell_global_ipi+0x34/0x50
>>>> LR = arch_send_call_function_ipi_mask+0x120/0x130
>>>> [c000003fe566ba50] [c00000000004876c] arch_send_call_function_ipi_mask+0x4c/0x130 (unreliable)
>>>> [c000003fe566ba90] [c0000000001d59f0] smp_call_function_many+0x340/0x450
>>>> [c000003fe566bb00] [c000000000075f18] pmdp_invalidate+0x98/0xe0
>>>> [c000003fe566bb30] [c0000000003a1120] change_huge_pmd+0xe0/0x270
>>>> [c000003fe566bba0] [c000000000349278] change_protection_range+0xb88/0xe40
>>>> [c000003fe566bcf0] [c0000000003496c0] mprotect_fixup+0x140/0x340
>>>> [c000003fe566bdb0] [c000000000349a74] SyS_mprotect+0x1b4/0x350
>>>> [c000003fe566be30] [c00000000000b184] system_call+0x58/0x6c
>>>>
>>>> Fix this by using the asynchronus smp_call in the timer interrupt handler.
>>>> We don't have to wait in this handler until the pstates are changed on
>>>> the core. This change will not have any impact on the global pstate
>>>> ramp-down algorithm.
>>>>
>>>> Reported-by: Nicholas Piggin <npiggin@gmail.com>
>>>> Reported-by: Pridhiviraj Paidipeddi <ppaidipe@linux.vnet.ibm.com>
>>>> Signed-off-by: Shilpasri G Bhat <shilpa.bhat@linux.vnet.ibm.com>
>>>> ---
>>>>  drivers/cpufreq/powernv-cpufreq.c | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/cpufreq/powernv-cpufreq.c b/drivers/cpufreq/powernv-cpufreq.c
>>>> index 0591874..7e0c752 100644
>>>> --- a/drivers/cpufreq/powernv-cpufreq.c
>>>> +++ b/drivers/cpufreq/powernv-cpufreq.c
>>>> @@ -721,7 +721,7 @@ void gpstate_timer_handler(struct timer_list *t)
>>>>  	spin_unlock(&gpstates->gpstate_lock);
>>>>  
>>>>  	/* Timer may get migrated to a different cpu on cpu hot unplug */
>>>> -	smp_call_function_any(policy->cpus, set_pstate, &freq_data, 1);
>>>> +	smp_call_function_any(policy->cpus, set_pstate, &freq_data, 0);
>>>>  }
>>>>  
>>>>  /*  
>>>
>>> This can still deadlock because !wait case still ends up having to wait
>>> if another !wait smp_call_function caller had previously used the
>>> call single data for this cpu.
>>>
>>> If you go this way you would have to use smp_call_function_async, which
>>> is more work.
>>>
>>> As a rule it would be better to avoid smp_call_function entirely if
>>> possible. Can you ensure the timer is running on the right CPU? Use
>>> add_timer_on and try again if the timer is on the wrong CPU, perhaps?
>>>   
>>
>> Yeah that is doable we can check for the cpu and re-queue it. We will only
>> ramp-down slower in that case which is no harm.
> 
> Great, I'd be much happier avoiding that IPI. I guess it should happen
> quite rarely that we have to queue on a different CPU. I would say just
> do add_timer unless we have migrated to the wrong CPU, then do add_timer_on
> in that case (it's a bit slower).

(The gpstates->timer is initialized with TIMER_PINNED and is a timer per cpufreq
policy / or per core)

We are currently using mod_timer() and this gets triggered in the code-path of
the cpufreq's governor timer which is per-policy (i.e per core in our case).
This ensures the timer is always fired on one of the policy->cpus as the
deferred kworker is also scheduled on one of the policy->cpus.

We were good until this patch 7bc54b652f13119f64e87dd96bb792efbfc5a786
where after we could leave a migrated timer and subsequent re-queues from the
timer context on the wrong cpu. For this I agree we need a add_timer_on() to
correct it.

> 
>> (If the targeted core turns out to be offline then we will not queue the timer
>> again as we would have already set the pstate to min in the cpu-down path.)
> 
> Something I noticed is that if we can not get the lock (trylock fails),
> then the timer does not get queued again. Should it?
> 

Since the gpstates->timer is per-core I am assuming that it should not fail in
the trylock. (which sounds like an unlikely case to me where we have two expiry
on the same timer)

> Thanks,
> Nick
>