linux-kernel - Re: [PATCH v3 3/7] arm_pmu: Add support for 64bit event counters

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Message-ID: <20180619105730.wvp2t3krqbvyeayu@lakrids.cambridge.arm.com>
Date:   Tue, 19 Jun 2018 11:57:30 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Suzuki K Poulose <suzuki.poulose@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        will.deacon@....com, robin.murphy@....com, julien.thierry@....com
Subject: Re: [PATCH v3 3/7] arm_pmu: Add support for 64bit event counters

On Tue, Jun 19, 2018 at 11:15:38AM +0100, Suzuki K Poulose wrote:
> Each PMU has a set of 32bit event counters. But in some
> special cases, the events could be counted using counters
> which are effectively 64bit wide.
> 
> e.g, Arm V8 PMUv3 has a 64 bit cycle counter which can count
> only the CPU cycles. Also, the PMU can chain the event counters
> to effectively count as a 64bit counter.
> 
> Add support for tracking the events that uses 64bit counters.
> This only affects the periods set for each counter in the core
> driver.
> 
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Will Deacon <will.deacon@....com>
> Reviewed-by: Julien Thierry <julien.thierry@....com>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
> ---
> Changes since v2:
>  - None
> ---
>  drivers/perf/arm_pmu.c       | 15 +++++++++------
>  include/linux/perf/arm_pmu.h |  6 ++++++
>  2 files changed, 15 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/perf/arm_pmu.c b/drivers/perf/arm_pmu.c
> index 6ddc00d..e3766a8 100644
> --- a/drivers/perf/arm_pmu.c
> +++ b/drivers/perf/arm_pmu.c
> @@ -28,9 +28,11 @@
>  static DEFINE_PER_CPU(struct arm_pmu *, cpu_armpmu);
>  static DEFINE_PER_CPU(int, cpu_irq);
>  
> -static inline u64 arm_pmu_max_period(void)
> +static inline u64 arm_pmu_event_max_period(struct perf_event *event)
>  {
> -	return (1ULL << 32) - 1;
> +	return (event->hw.flags & ARMPMU_EVT_64BIT) ?
> +		GENMASK_ULL(63, 0) :
> +		GENMASK_ULL(31, 0);

Please make this:

	if (event->hw.flags & ARMPMU_EVT_64BIT)
		return GENMASK_ULL(63, 0);
	else
		return GENMASK_ULL(31, 0);

... which I think is more legible than a ternary spread across multiple
lines.

The rest looks fine, so with that:

Acked-by: Mark Rutland <mark.rutland@....com>

Mark.

>  }
>  
>  static int
> @@ -122,7 +124,7 @@ int armpmu_event_set_period(struct perf_event *event)
>  	u64 max_period;
>  	int ret = 0;
>  
> -	max_period = arm_pmu_max_period();
> +	max_period = arm_pmu_event_max_period(event);
>  	if (unlikely(left <= -period)) {
>  		left = period;
>  		local64_set(&hwc->period_left, left);
> @@ -148,7 +150,7 @@ int armpmu_event_set_period(struct perf_event *event)
>  
>  	local64_set(&hwc->prev_count, (u64)-left);
>  
> -	armpmu->write_counter(event, (u64)(-left) & 0xffffffff);
> +	armpmu->write_counter(event, (u64)(-left) & max_period);
>  
>  	perf_event_update_userpage(event);
>  
> @@ -160,7 +162,7 @@ u64 armpmu_event_update(struct perf_event *event)
>  	struct arm_pmu *armpmu = to_arm_pmu(event->pmu);
>  	struct hw_perf_event *hwc = &event->hw;
>  	u64 delta, prev_raw_count, new_raw_count;
> -	u64 max_period = arm_pmu_max_period();
> +	u64 max_period = arm_pmu_event_max_period(event);
>  
>  again:
>  	prev_raw_count = local64_read(&hwc->prev_count);
> @@ -368,6 +370,7 @@ __hw_perf_event_init(struct perf_event *event)
>  	struct hw_perf_event *hwc = &event->hw;
>  	int mapping;
>  
> +	hwc->flags = 0;
>  	mapping = armpmu->map_event(event);
>  
>  	if (mapping < 0) {
> @@ -410,7 +413,7 @@ __hw_perf_event_init(struct perf_event *event)
>  		 * is far less likely to overtake the previous one unless
>  		 * you have some serious IRQ latency issues.
>  		 */
> -		hwc->sample_period  = arm_pmu_max_period() >> 1;
> +		hwc->sample_period  = arm_pmu_event_max_period(event) >> 1;
>  		hwc->last_period    = hwc->sample_period;
>  		local64_set(&hwc->period_left, hwc->sample_period);
>  	}
> diff --git a/include/linux/perf/arm_pmu.h b/include/linux/perf/arm_pmu.h
> index f7126a2..10f92e1 100644
> --- a/include/linux/perf/arm_pmu.h
> +++ b/include/linux/perf/arm_pmu.h
> @@ -25,6 +25,12 @@
>   */
>  #define ARMPMU_MAX_HWEVENTS		32
>  
> +/*
> + * ARM PMU hw_event flags
> + */
> +/* Event uses a 64bit counter */
> +#define ARMPMU_EVT_64BIT		1
> +
>  #define HW_OP_UNSUPPORTED		0xFFFF
>  #define C(_x)				PERF_COUNT_HW_CACHE_##_x
>  #define CACHE_OP_UNSUPPORTED		0xFFFF
> -- 
> 2.7.4
>