[PATCH v7 4/7] arm64: perf: Defer irq_work to IPI_IRQ_WORK

[Alexandru Elisei]

From: Julien Thierry <julien.thierry at 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 at gmail.com>
Cc: Will Deacon <will.deacon at arm.com>
Cc: Mark Rutland <mark.rutland at arm.com>
Cc: Peter Zijlstra <peterz at infradead.org>
Cc: Ingo Molnar <mingo at redhat.com>
Cc: Arnaldo Carvalho de Melo <acme at kernel.org>
Cc: Alexander Shishkin <alexander.shishkin at linux.intel.com>
Cc: Jiri Olsa <jolsa at redhat.com>
Cc: Namhyung Kim <namhyung at kernel.org>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Signed-off-by: Julien Thierry <julien.thierry at arm.com>
[Alexandru E.: Reworded commit message]
Signed-off-by: Alexandru Elisei <alexandru.elisei at 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 610500166f90..ef206fb146b9 100644
--- a/arch/arm64/kernel/perf_event.c
+++ b/arch/arm64/kernel/perf_event.c
@@ -783,20 +783,16 @@ 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;
 }
 
-- 
2.28.0