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Subject: Re: [PATCH v5 4/7] arm64: perf: Defer irq_work to IPI_IRQ_WORK
To:     Mark Rutland <mark.rutland@arm.com>
Cc:     linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        maz@kernel.org, will@kernel.org, catalin.marinas@arm.com,
        Julien Thierry <julien.thierry@arm.com>,
        Julien Thierry <julien.thierry.kdev@gmail.com>,
        Will Deacon <will.deacon@arm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>
References: <20200617113851.607706-1-alexandru.elisei@arm.com>
 <20200617113851.607706-5-alexandru.elisei@arm.com>
 <20200622141918.GF88608@C02TD0UTHF1T.local>
From:   Alexandru Elisei <alexandru.elisei@arm.com>
Message-ID: <c044fbd2-238b-2309-f08d-56f20e845fc1@arm.com>
Date:   Tue, 23 Jun 2020 10:47:32 +0100
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Hi Mark,

On 6/22/20 3:19 PM, Mark Rutland wrote:
> On Wed, Jun 17, 2020 at 12:38:48PM +0100, Alexandru Elisei wrote:
>> From: Julien Thierry <julien.thierry@arm.com>
>>
>> perf_event_overflow() can queue an irq_work on the current PE, which is
>> executed via an IPI. Move the processing of the irq_work from the PMU IRQ
>> handler to the IPI handler, which gets executed immediately afterwards.
>>
>> This also makes the IRQ handler NMI safe, because it removes the call to
>> irq_work_run().
> It wasn't entirely clear to me what the situation was today, and why
> this was sound. How about the following to spell that out more
> explicitly:
>
> | When handling events armv8pmu_handle_irq() calls
> | perf_event_overflow(), and subsequently calls irq_work_run() to handle
> | any work queued by perf_event_overflow(). As perf_event_overflow()
> | raises IPI_IRQ_WORK when queing the work, this isn't strictly
> | necessary and the work could be handled as part of the IPI_IRQ_WORK
> | handler.
> |
> | In the common case the IPI handler will run immediately after the PMU
> | IRQ handler, and where the PE is heavily loaded with interrupts other
> | handlers may run first, widening the window where some counters are
> | disabled.
> |
> | In practice this window is unlikely to be a significant issue, and
> | removing the call to irq_work_run() would make the PMU IRQ handler NMI
> | safe in addition to making it simpler, so let's do that.
>
> Thanks,
> Mark.

That is much better than my commit message, I will definitely update it with your
suggestion.

Thanks,
Alex
>
>> Cc: Julien Thierry <julien.thierry.kdev@gmail.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>
>> Signed-off-by: Julien Thierry <julien.thierry@arm.com>
>> [Reworded commit]
>> Signed-off-by: Alexandru Elisei <alexandru.elisei@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 a6195022be7d..cf1d92030790 100644
>> --- a/arch/arm64/kernel/perf_event.c
>> +++ b/arch/arm64/kernel/perf_event.c
>> @@ -750,20 +750,16 @@ 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();
>> -
>>  	return IRQ_HANDLED;
>>  }
>>  
>> -- 
>> 2.27.0
>>