linux-kernel - Re: [PATCH] events: Ensure that timers are updated without requiring read() call

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Date:	Thu, 23 Jun 2011 16:25:50 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Eric B Munson <emunson@...bm.net>
Cc:	mingo@...e.hu, borislav.petkov@....com, bblum@...rew.cmu.edu,
	linux-kernel@...r.kernel.org, mhack@...ibm.com
Subject: Re: [PATCH] events: Ensure that timers are updated without
 requiring read() call

On Thu, 2011-06-23 at 10:15 -0400, Eric B Munson wrote:
> The event tracing infrastructure exposes two timers which should be updated
> each time the value of the counter is updated.  Currently, these counters are
> only updated when userspace calls read() on the fd associated with an event.
> This means that counters which are read via the mmap'd page exclusively never
> have their timers updated.  This patch adds ensures that the timers are updated
> each time the values in the mmap'd page are updated.
> 
> Signed-off-by: Eric B Munson <emunson@...bm.net>
> ---
>  kernel/events/core.c |    1 +
>  1 files changed, 1 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 9efe710..3dd4ebe 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3369,6 +3369,7 @@ void perf_event_update_userpage(struct perf_event *event)
>  	struct perf_buffer *buffer;
>  
>  	rcu_read_lock();
> +	update_event_times(event);
>  	buffer = rcu_dereference(event->buffer);
>  	if (!buffer)
>  		goto unlock;

Not that easy, perf_event_update_userpage() can happen from NMI context,
and update_event_times() needs to be done under ctx->lock.
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