Re: [PATCH v19 02/11] perf: arm_pmu: Don't disable counter in armpmu_add()

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 2/3/25 06:12, Rob Herring (Arm) wrote:
> From: Mark Rutland <mark.rutland@arm.com>
> 
> Currently armpmu_add() tries to handle a newly-allocated counter having
> a stale associated event, but this should not be possible, and if this

A stale associated event ? Does that mean hw_events->events[idx] still
points to a valid event even though counter idx has already been freed
up and allocated to a new event.

> were to happen the current mitigation is insufficient and potentially
> expensive. It would be better to warn if we encounter the impossible
> case.

Makes sense.

> 
> Calls to pmu::add() and pmu::del() are serialized by the core perf code,
> and armpmu_del() clears the relevant slot in pmu_hw_events::events[]
> before clearing the bit in pmu_hw_events::used_mask such that the
> counter can be reallocated. Thus when armpmu_add() allocates a counter
> index from pmu_hw_events::used_mask, it should not be possible to observe
> a stale even in pmu_hw_events::events[] unless either
> pmu_hw_events::used_mask or pmu_hw_events::events[] have been corrupted.
> 
> If this were to happen, we'd end up with two events with the same
> event->hw.idx, which would clash with each other during reprogramming,
> deletion, etc, and produce bogus results. Add a WARN_ON_ONCE() for this
> case so that we can detect if this ever occurs in practice.

Agreed.

> 
> That possiblity aside, there's no need to call arm_pmu::disable(event)

s/possiblity/possibility

> for the new event. The PMU reset code initialises the counter in a
> disabled state, and armpmu_del() will disable the counter before it can
> be reused. Remove the redundant disable.
> 
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
> ---
>  drivers/perf/arm_pmu.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/perf/arm_pmu.c b/drivers/perf/arm_pmu.c
> index 398cce3d76fc..2f33e69a8caf 100644
> --- a/drivers/perf/arm_pmu.c
> +++ b/drivers/perf/arm_pmu.c
> @@ -342,12 +342,10 @@ armpmu_add(struct perf_event *event, int flags)
>  	if (idx < 0)
>  		return idx;
>  
> -	/*
> -	 * If there is an event in the counter we are going to use then make
> -	 * sure it is disabled.
> -	 */
> +	/* The newly-allocated counter should be empty */

Should this comment also include what happens when two events some how end
up using the same 'event->hw.idx' as mentioned in the commit message, just
to make things clearer.

> +	WARN_ON_ONCE(hw_events->events[idx]);
> +
>  	event->hw.idx = idx;
> -	armpmu->disable(event);
>  	hw_events->events[idx] = event;
>  
>  	hwc->state = PERF_HES_STOPPED | PERF_HES_UPTODATE;
> 

Otherwise LGTM.

Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>