diff mbox series

[v6,4/7] arm64: perf: Defer irq_work to IPI_IRQ_WORK

Message ID 20200819133419.526889-5-alexandru.elisei@arm.com (mailing list archive)
State New, archived
Headers show
Series arm_pmu: Use NMI for perf interrupt | expand

Commit Message

Alexandru Elisei Aug. 19, 2020, 1:34 p.m. UTC 
From: Julien Thierry <julien.thierry@arm.com>

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
queuing 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.

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(-)

Comments

Peter Zijlstra Aug. 19, 2020, 2:23 p.m. UTC | #1 
On Wed, Aug 19, 2020 at 02:34:16PM +0100, Alexandru Elisei wrote:
> From: Julien Thierry <julien.thierry@arm.com>
> 
> 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
> queuing 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.

Makes sense, IIRC this code was written before ARM grew IPI_IRQ_WORK
support and then it makes sense, but now that you have it and are moving
to NMI-like context this is absolutely the right thing to do.
diff mbox series

Patch

diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c
index 80744c2f1454..5bf283518981 100644
--- a/arch/arm64/kernel/perf_event.c
+++ b/arch/arm64/kernel/perf_event.c
@@ -780,20 +780,16 @@  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;
 }