Re: [PATCH V5 5/7] arm64/perf: Drive BRBE from perf event states

[Mark Rutland]

On Mon, Nov 07, 2022 at 11:55:12AM +0530, Anshuman Khandual wrote:
> Branch stack sampling rides along the normal perf event and all the branch
> records get captured during the PMU interrupt. This just changes perf event
> handling on the arm64 platform to accommodate required BRBE operations that
> will enable branch stack sampling support.
> 
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: linux-perf-users@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> ---
>  arch/arm64/kernel/perf_event.c |  7 ++++++
>  drivers/perf/arm_pmu.c         | 40 ++++++++++++++++++++++++++++++++++
>  2 files changed, 47 insertions(+)
> 
> diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c
> index c97377e28288..97db333d1208 100644
> --- a/arch/arm64/kernel/perf_event.c
> +++ b/arch/arm64/kernel/perf_event.c
> @@ -874,6 +874,13 @@ static irqreturn_t armv8pmu_handle_irq(struct arm_pmu *cpu_pmu)
>  		if (!armpmu_event_set_period(event))
>  			continue;
>  
> +		if (has_branch_stack(event)) {
> +			cpu_pmu->brbe_read(cpuc, event);
> +			data.br_stack = &cpuc->branches->brbe_stack;
> +			data.sample_flags |= PERF_SAMPLE_BRANCH_STACK;
> +			cpu_pmu->brbe_reset(cpuc);
> +		}
> +
>  		/*
>  		 * Perf event overflow will queue the processing of the event as
>  		 * an irq_work which will be taken care of in the handling of
> diff --git a/drivers/perf/arm_pmu.c b/drivers/perf/arm_pmu.c
> index 5048a500441e..1a8dca4e513e 100644
> --- a/drivers/perf/arm_pmu.c
> +++ b/drivers/perf/arm_pmu.c
> @@ -271,12 +271,22 @@ armpmu_stop(struct perf_event *event, int flags)
>  {
>  	struct arm_pmu *armpmu = to_arm_pmu(event->pmu);
>  	struct hw_perf_event *hwc = &event->hw;
> +	struct pmu_hw_events *hw_events = this_cpu_ptr(armpmu->hw_events);
>  
>  	/*
>  	 * ARM pmu always has to update the counter, so ignore
>  	 * PERF_EF_UPDATE, see comments in armpmu_start().
>  	 */
>  	if (!(hwc->state & PERF_HES_STOPPED)) {
> +		if (has_branch_stack(event)) {
> +			WARN_ON_ONCE(!hw_events->brbe_users);
> +			hw_events->brbe_users--;
> +			if (!hw_events->brbe_users) {
> +				hw_events->brbe_context = NULL;
> +				armpmu->brbe_disable(hw_events);
> +			}
> +		}

Can't we do the actual enable/disable we start/stop the PMU as a whole?

If we just counted the numberoof users here we could do the actual
enable/disable in armpmu_{enable,disable}() or armv8pmu_{start,stop}(), like we
do when checking hw_events->used_mask.

[...]

> @@ -349,6 +368,10 @@ armpmu_add(struct perf_event *event, int flags)
>  	hw_events->events[idx] = event;
>  
>  	hwc->state = PERF_HES_STOPPED | PERF_HES_UPTODATE;
> +
> +	if (has_branch_stack(event))
> +		armpmu->brbe_filter(hw_events, event);

What exactly do we need to do here? Since the BRBE is shared, I'm suprised that
there's any pwer-event configuration beyond "yes" or "no". 

> +
>  	if (flags & PERF_EF_START)
>  		armpmu_start(event, PERF_EF_RELOAD);
>  
> @@ -443,6 +466,7 @@ __hw_perf_event_init(struct perf_event *event)
>  {
>  	struct arm_pmu *armpmu = to_arm_pmu(event->pmu);
>  	struct hw_perf_event *hwc = &event->hw;
> +	struct pmu_hw_events *hw_events = this_cpu_ptr(armpmu->hw_events);
>  	int mapping;
>  
>  	hwc->flags = 0;
> @@ -492,6 +516,9 @@ __hw_perf_event_init(struct perf_event *event)
>  		local64_set(&hwc->period_left, hwc->sample_period);
>  	}
>  
> +	if (has_branch_stack(event))
> +		armpmu->brbe_filter(hw_events, event);

I do not understand why we would use hw_events here; at this point the event
has only been created, and not even added yet; it doesn't have a counter index.
isn't even being installed into HW.

What am I missing?

> +
>  	return validate_group(event);
>  }
>  
> @@ -520,6 +547,18 @@ static int armpmu_event_init(struct perf_event *event)
>  	return __hw_perf_event_init(event);
>  }
>  
> +static void armpmu_sched_task(struct perf_event_context *ctx, bool sched_in)
> +{
> +	struct arm_pmu *armpmu = to_arm_pmu(ctx->pmu);
> +	struct pmu_hw_events *hw_events = this_cpu_ptr(armpmu->hw_events);
> +
> +	if (!hw_events->brbe_users)
> +		return;
> +
> +	if (sched_in)
> +		armpmu->brbe_reset(hw_events);

I see that LBR does a save/restore, whereas IIUC here we discard without even
reading the old values. Is that the intent? Shouldn't we snapshot them into the
task context?

Thanks,
Mark.

> +}
> +
>  static void armpmu_enable(struct pmu *pmu)
>  {
>  	struct arm_pmu *armpmu = to_arm_pmu(pmu);
> @@ -877,6 +916,7 @@ static struct arm_pmu *__armpmu_alloc(gfp_t flags)
>  	}
>  
>  	pmu->pmu = (struct pmu) {
> +		.sched_task	= armpmu_sched_task,
>  		.pmu_enable	= armpmu_enable,
>  		.pmu_disable	= armpmu_disable,
>  		.event_init	= armpmu_event_init,
> -- 
> 2.25.1
>