Re: [PATCH v5 4/7] arm64: perf: Defer irq_work to IPI_IRQ_WORK

[Mark Rutland <mark.rutland@arm.com>]

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.

> 
> 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
>