Re: [PATCH 1/9] perf: Remove redundant parent context check from context_equiv

[Peter Zijlstra <peterz@infradead.org>]

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On Mon, Sep 08, 2014 at 02:19:52PM +0200, Jiri Olsa wrote:
> > I have another 'problem' with 179033b3e064. What if you 'want' to
> > continue monitoring after the initial task died? Eg. if you want to
> > monitor crap that unconditionally daemonizes.
> 
> right.. did not think of that.. need to check more, but
> seems like just the check for children should be enough
> 

Indeed, that should work.

> ---
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index bf482ccbdbe1..341d0b47ca14 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3568,6 +3568,19 @@ static int perf_event_read_one(struct perf_event *event,
>  	return n * sizeof(u64);
>  }
>  
> +static bool is_event_hup(struct perf_event *event)
> +{
> +	bool no_children;
> +
> +	if (event->state != PERF_EVENT_STATE_EXIT)
> +		return false;
> +
> +	mutex_lock(&event->child_mutex);
> +	no_children = list_empty(&event->child_list);
> +	mutex_unlock(&event->child_mutex);
> +	return no_children;
> +}
> +
>  /*
>   * Read the performance event - simple non blocking version for now
>   */
> @@ -3582,8 +3595,7 @@ perf_read_hw(struct perf_event *event, char __user *buf, size_t count)
>  	 * error state (i.e. because it was pinned but it couldn't be
>  	 * scheduled on to the CPU at some point).
>  	 */
> -	if ((event->state == PERF_EVENT_STATE_ERROR) ||
> -	    (event->state == PERF_EVENT_STATE_EXIT))
> +	if ((event->state == PERF_EVENT_STATE_ERROR) || (is_event_hup(event)))
>  		return 0;

Do we want this? It seems like a fairly sensible thing to start a
counter and wait for the thing to die, only to then read the total
count. But with this on we get 0s.

I suppose Stephane's email got to you after you did this and we should
be dropping this thing entirely?

>  	if (count < event->read_size)
> @@ -3614,7 +3626,7 @@ static unsigned int perf_poll(struct file *file, poll_table *wait)
>  
>  	poll_wait(file, &event->waitq, wait);
>  
> -	if (event->state == PERF_EVENT_STATE_EXIT)
> +	if (is_event_hup(event))
>  		return events;
>  
>  	/*

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