linux-kernel - Re: [PATCH v3 4/7] arm_pmu: Tidy up clear_event

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Date:   Fri, 29 Jun 2018 14:40:50 +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 4/7] arm_pmu: Tidy up clear_event_idx call backs

On Tue, Jun 19, 2018 at 11:15:39AM +0100, Suzuki K Poulose wrote:
> The armpmu uses get_event_idx callback to allocate an event
> counter for a given event, which marks the selected counter
> as "used". Now, when we delete the counter, the arm_pmu goes
> ahead and clears the "used" bit and then invokes the "clear_event_idx"
> call back, which kind of splits the job between the core code
> and the backend. Tidy this up by relying on the clear_event_idx
> to do the book keeping, if available. Otherwise, let the core
> driver do the default "clear" bit operation. This will be useful
> for adding the chained event support, where we leave the event
> idx maintenance to the backend.
> 
> Also, when an event is removed from the PMU, reset the hw.idx
> to indicate that a counter is not allocated for this event,
> to help the backends do better checks. This will be also used
> for the chain counter support.
> 
> 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:
>  - Reset the event counter after an event is removed.
> ---
>  arch/arm/kernel/perf_event_v7.c |  2 ++
>  drivers/perf/arm_pmu.c          | 17 +++++++++++++----
>  2 files changed, 15 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm/kernel/perf_event_v7.c b/arch/arm/kernel/perf_event_v7.c
> index fd7ce01..765d265 100644
> --- a/arch/arm/kernel/perf_event_v7.c
> +++ b/arch/arm/kernel/perf_event_v7.c
> @@ -1637,6 +1637,7 @@ static void krait_pmu_clear_event_idx(struct pmu_hw_events *cpuc,
>  	bool venum_event = EVENT_VENUM(hwc->config_base);
>  	bool krait_event = EVENT_CPU(hwc->config_base);
>  
> +	clear_bit(hwc->idx, cpuc->used_mask);
>  	if (venum_event || krait_event) {
>  		bit = krait_event_to_bit(event, region, group);
>  		clear_bit(bit, cpuc->used_mask);
> @@ -1966,6 +1967,7 @@ static void scorpion_pmu_clear_event_idx(struct pmu_hw_events *cpuc,
>  	bool venum_event = EVENT_VENUM(hwc->config_base);
>  	bool scorpion_event = EVENT_CPU(hwc->config_base);
>  
> +	clear_bit(hwc->idx, cpuc->used_mask);
>  	if (venum_event || scorpion_event) {
>  		bit = scorpion_event_to_bit(event, region, group);
>  		clear_bit(bit, cpuc->used_mask);

As an aside, I think there's an existing problem with krait and
cpu_pm_pmu_setup(), and we'll end up with the same issue when chained
counters use multiple counters for one event.

The krait code sets multiple bits in the PMU's pmu_hw_events::used_mask,
but only one of these will have a corresponding (non-NULL) entry in
pmu_hw_events::events[].

In cpu_pm_pmu_setup(), when we find the auxilliary counter associated
with an event, its bit will be set in used_mask, but the pointer will be
NULL, and that will blow up in start/stop.

We can't just set multiple slots to point at the same counter, or we'd
try to start/stop an event multiple times, which would also be bad.

I guess the best thing to do would be to avoid the test_bit(), and just
skip an idx if hw_events->events[idx] is NULL.

Would you mind spinning a patch to that effect?

Thanks,
Mark.