Re: [PATCH v3 4/9] arm: perf: Remove Remove PMU locking

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

On Mon, Jul 08, 2019 at 03:32:52PM +0100, Julien Thierry wrote:
> Since the PMU interrupt saves and restores the value of the selector
> register, there is no need to serialize register accesses against the
> interrupt contexts.
> 
> For operations that can be called with interrupts enabled, preemption
> still needs to be disabled to ensure the programming of the PMU is
> done on the expected CPU and not migrated mid-programming.
> 
> 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: Russell King <linux@armlinux.org.uk>
> ---
>  arch/arm/kernel/perf_event_v7.c | 56 +++--------------------------------------
>  1 file changed, 4 insertions(+), 52 deletions(-)

>  static void armv7pmu_start(struct arm_pmu *cpu_pmu)
>  {
> -	unsigned long flags;
> -	struct pmu_hw_events *events = this_cpu_ptr(cpu_pmu->hw_events);
> -
> -	raw_spin_lock_irqsave(&events->pmu_lock, flags);
> +	preempt_disable();
>  	/* Enable all counters */
>  	armv7_pmnc_write(armv7_pmnc_read() | ARMV7_PMNC_E);
> -	raw_spin_unlock_irqrestore(&events->pmu_lock, flags);
> +	preempt_enable();
>  }
> 
>  static void armv7pmu_stop(struct arm_pmu *cpu_pmu)
>  {
> -	unsigned long flags;
> -	struct pmu_hw_events *events = this_cpu_ptr(cpu_pmu->hw_events);
> -
> -	raw_spin_lock_irqsave(&events->pmu_lock, flags);
> +	preempt_disable();
>  	/* Disable all counters */
>  	armv7_pmnc_write(armv7_pmnc_read() & ~ARMV7_PMNC_E);
> -	raw_spin_unlock_irqrestore(&events->pmu_lock, flags);
> +	preempt_enable();
>  }

As with the arm64 patch, I suspect/hope it's not necessary to disable
preemption here.

Thanks,
Mark.

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel