Re: [PATCH 1/2] perf: fix PERF_EVENT_IOC_PERIOD to force-reset the period

[Will Deacon <will.deacon@arm.com>]

Hi Peter,

Do you have any thoughts on the below? It would be good to add your
signed-off to the patch, since you wrote it originally.

Cheers,

Will

On Wed, Nov 27, 2013 at 01:54:38PM +0000, Will Deacon wrote:
> From: Peter Zijlstra <peterz@infradead.org>
> 
> Vince Weaver reports that, on all architectures apart from ARM,
> PERF_EVENT_IOC_PERIOD doesn't actually update the period until the next
> event fires. This is counter-intuitive behaviour and is better dealt
> with in the core code.
> 
> This patch ensures that the period is forcefully reset when dealing with
> such a request in the core code. A subsequent patch removes the
> equivalent hack from the ARM back-end.
> 
> Cc: Vince Weaver <vincent.weaver@maine.edu>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Will Deacon <will.deacon@arm.com>
> ---
> 
> Peter -- I didn't want this to get lost, so I picked it up, fixed a typo
>          and wrote a commit message. Could I add your SoB please?
> 
> 	 Cheers,
> 		Will
> 
>  kernel/events/core.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index d724e7757cd1..1c626b923ddf 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3527,7 +3527,7 @@ static void perf_event_for_each(struct perf_event *event,
>  static int perf_event_period(struct perf_event *event, u64 __user *arg)
>  {
>  	struct perf_event_context *ctx = event->ctx;
> -	int ret = 0;
> +	int ret = 0, active;
>  	u64 value;
>  
>  	if (!is_sampling_event(event))
> @@ -3551,6 +3551,20 @@ static int perf_event_period(struct perf_event *event, u64 __user *arg)
>  		event->attr.sample_period = value;
>  		event->hw.sample_period = value;
>  	}
> +
> +	active = (event->state == PERF_EVENT_STATE_ACTIVE);
> +	if (active) {
> +		perf_pmu_disable(ctx->pmu);
> +		event->pmu->stop(event, PERF_EF_UPDATE);
> +	}
> +
> +	local64_set(&event->hw.period_left, 0);
> +
> +	if (active) {
> +		event->pmu->start(event, PERF_EF_RELOAD);
> +		perf_pmu_enable(ctx->pmu);
> +	}
> +
>  unlock:
>  	raw_spin_unlock_irq(&ctx->lock);
>  
> -- 
> 1.8.2.2
>