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Date:   Tue, 24 Apr 2018 16:00:34 +1000
From:   Nicholas Piggin <npiggin@gmail.com>
To:     Shilpasri G Bhat <shilpa.bhat@linux.vnet.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
Subject: Re: [PATCH] cpufreq: powernv: Fix the hardlockup by synchronus
 smp_call in timer interrupt
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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?

Thanks,
Nick