linux-kernel - Re: [PATCH] perf: arm_pmuv3: Avoid assigning fixed cycle counter with threshold

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Date: Tue, 11 Jun 2024 17:13:49 +0100
From: James Clark <james.clark@....com>
To: "Rob Herring (Arm)" <robh@...nel.org>, Will Deacon <will@...nel.org>,
 Mark Rutland <mark.rutland@....com>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf: arm_pmuv3: Avoid assigning fixed cycle counter with
 threshold



On 11/06/2024 16:50, Rob Herring (Arm) wrote:
> If the user has requested a counting threshold for the CPU cycles event,
> then the fixed cycle counter can't be assigned as it lacks threshold
> support. Currently, the thresholds will work or not randomly depending
> on which counter the event is assigned.
> 
> While using thresholds for CPU cycles doesn't make much sense, it can be
> useful for testing purposes.
> 
> Fixes: 816c26754447 ("arm64: perf: Add support for event counting threshold")
> Signed-off-by: Rob Herring (Arm) <robh@...nel.org>
> ---
>  drivers/perf/arm_pmuv3.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/perf/arm_pmuv3.c b/drivers/perf/arm_pmuv3.c
> index 23fa6c5da82c..2612be29ee23 100644
> --- a/drivers/perf/arm_pmuv3.c
> +++ b/drivers/perf/arm_pmuv3.c
> @@ -939,9 +939,10 @@ static int armv8pmu_get_event_idx(struct pmu_hw_events *cpuc,
>  	struct arm_pmu *cpu_pmu = to_arm_pmu(event->pmu);
>  	struct hw_perf_event *hwc = &event->hw;
>  	unsigned long evtype = hwc->config_base & ARMV8_PMU_EVTYPE_EVENT;
> +	bool has_threshold = !!(hwc->config_base & ARMV8_PMU_EVTYPE_TH);

I was going to say doesn't it need to be (ARMV8_PMU_EVTYPE_TH |
ARMV8_PMU_EVTYPE_TC) for it to give the same results as the hardware.
But then I saw we only enable it if TH != 0, even if TC is set. And now
I'm wondering if I inadvertently disabled a useful combination of options.

The Arm ARM says it's only completely disabled when both TC and TH are 0.

>  
>  	/* Always prefer to place a cycle counter into the cycle counter. */
> -	if (evtype == ARMV8_PMUV3_PERFCTR_CPU_CYCLES) {
> +	if ((evtype == ARMV8_PMUV3_PERFCTR_CPU_CYCLES) && !has_threshold) {
>  		if (!test_and_set_bit(ARMV8_IDX_CYCLE_COUNTER, cpuc->used_mask))
>  			return ARMV8_IDX_CYCLE_COUNTER;
>  		else if (armv8pmu_event_is_64bit(event) &&