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

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Date:   Fri, 29 Jun 2018 15:18:23 +0100
From:   Suzuki K Poulose <Suzuki.Poulose@....com>
To:     Mark Rutland <mark.rutland@....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


Hi Mark,

On 29/06/18 14:40, Mark Rutland wrote:
> 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[].

The Krait pmu allocates the "krait" specific event bit beyond the armpmu->num_events.
See krait_event_to_bit(). And we don't go beyond armpmu->num_events for the "events"
check. So we should be safe. And it passes on the idx within the num_events back
to armpmu. So whatever krait pmu does is invisible to the generic driver.

> 
> 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.

No, we don't. Since we cap the loops at armpmu->num_events.

> 
> 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?

Eitherway, I could split that change to a new one.


Cheers
Suzuki