linux-kernel - Re: [PATCH 1/2] kernel/events: Add option to notify through signals on wakeup

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Date:   Wed, 12 Jul 2017 13:46:27 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>, Ingo Molnar <mingo@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] kernel/events: Add option to notify through signals
 on wakeup

On Mon, Jun 19, 2017 at 08:01:07PM +0530, Naveen N. Rao wrote:

> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 6c4e523dc1e2..812fcfc767f4 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -2679,7 +2679,8 @@ static int _perf_event_refresh(struct perf_event *event, int refresh)
>  	/*
>  	 * not supported on inherited events

That comment wants updating to explain the signal crud..

>  	 */
> -	if (event->attr.inherit || !is_sampling_event(event))
> +	if (event->attr.inherit || event->attr.signal_on_wakeup ||
> +			!is_sampling_event(event))
>  		return -EINVAL;
>  
>  	atomic_add(refresh, &event->event_limit);

> @@ -7362,12 +7362,15 @@ static int __perf_event_overflow(struct perf_event *event,
>  	 * events
>  	 */
>  
> +	if (!event->attr.signal_on_wakeup) {
> +		int events = atomic_read(&event->event_limit);
> +		event->pending_kill = POLL_IN;
> +		if (events && atomic_dec_and_test(&event->event_limit)) {
> +			ret = 1;
> +			event->pending_kill = POLL_HUP;
>  
> +			perf_event_disable_inatomic(event);
> +		}
>  	}

So even without event_limit (IOC_REFRESH) this would've generated
SIGIO:POLL_IN.

>  
>  	READ_ONCE(event->overflow_handler)(event, data, regs);
> @@ -10408,6 +10411,7 @@ perf_event_exit_event(struct perf_event *child_event,
>  		perf_group_detach(child_event);
>  	list_del_event(child_event, child_ctx);
>  	child_event->state = PERF_EVENT_STATE_EXIT; /* is_event_hup() */
> +	child_event->pending_kill = POLL_HUP;

This looks like an undocumented change..

>  	raw_spin_unlock_irq(&child_ctx->lock);
>  
>  	/*
> diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
> index 2831480c63a2..4e7c728569a8 100644
> --- a/kernel/events/ring_buffer.c
> +++ b/kernel/events/ring_buffer.c
> @@ -21,6 +21,9 @@ static void perf_output_wakeup(struct perf_output_handle *handle)
>  {
>  	atomic_set(&handle->rb->poll, POLLIN);
>  
> +	if (handle->event->attr.signal_on_wakeup)
> +		handle->event->pending_kill = POLL_IN;
> +

And this is the bit that generates SIGIO:POLL_IN on wakeup.

>  	handle->event->pending_wakeup = 1;
>  	irq_work_queue(&handle->event->pending);
>  }
> -- 
> 2.13.1
>