Re: [PATCH v4 6/9] arm64: perf: Do not call irq_work_run in NMI context

[Julien Thierry <julien.thierry@arm.com>]

On 01/08/2019 14:06, Will Deacon wrote:
> On Wed, Jul 17, 2019 at 09:17:09AM +0100, Julien Thierry wrote:
>> Function irq_work_run is not NMI safe and should not be called from NMI
>> context.
>>
>> When PMU interrupt is an NMI do not call irq_work_run. Instead rely on the
>> IRQ work IPI to run the irq_work queue once NMI/IRQ contexts have been
>> exited.
>>
>> Signed-off-by: Julien Thierry <julien.thierry@arm.com>
>> Cc: Will Deacon <will.deacon@arm.com>
>> Cc: Mark Rutland <mark.rutland@arm.com>
>> Cc: Peter Zijlstra <peterz@infradead.org>
>> Cc: Ingo Molnar <mingo@redhat.com>
>> Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
>> Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
>> Cc: Jiri Olsa <jolsa@redhat.com>
>> Cc: Namhyung Kim <namhyung@kernel.org>
>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>> ---
>>  arch/arm64/kernel/perf_event.c | 14 +++++---------
>>  1 file changed, 5 insertions(+), 9 deletions(-)
>>
>> diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c
>> index 0e2cf5d..9c959ad 100644
>> --- a/arch/arm64/kernel/perf_event.c
>> +++ b/arch/arm64/kernel/perf_event.c
>> @@ -776,20 +776,16 @@ static irqreturn_t armv8pmu_handle_irq(struct arm_pmu *cpu_pmu)
>>  		if (!armpmu_event_set_period(event))
>>  			continue;
>>  
>> +		/*
>> +		 * Perf event overflow will queue the processing of the event as
>> +		 * an irq_work which will be taken care of in the handling of
>> +		 * IPI_IRQ_WORK.
>> +		 */
>>  		if (perf_event_overflow(event, &data, regs))
>>  			cpu_pmu->disable(event);
>>  	}
>>  	armv8pmu_start(cpu_pmu);
>>  
>> -	/*
>> -	 * Handle the pending perf events.
>> -	 *
>> -	 * Note: this call *must* be run with interrupts disabled. For
>> -	 * platforms that can have the PMU interrupts raised as an NMI, this
>> -	 * will not work.
>> -	 */
>> -	irq_work_run();
> 
> What about the case where NMIs are not being used (e.g. GICv2)?
> 

As the comment above mentions. The overflow handler will trigger the
IPI_IRQ_WORK which will call the irq_work_run() both for NMI and normal IRQ.

Unless we really need to process the irq_work here, it makes things
simpler to get rid of the call.

It was suggested during the previous version:
https://www.spinics.net/lists/arm-kernel/msg740027.html

Cheers,

-- 
Julien Thierry

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