Re: [PATCH 3/3] events: Ensure that timers are updated without requiring read() call

Peter Zijlstra
2011-06-24T09:45:12Z

On Thu, 2011-06-23 at 16:34 -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@mgebm.net>
> ---
> kernel/events/core.c | 12 ++++++++++++
> 1 files changed, 12 insertions(+), 0 deletions(-)
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 9e9a7fa..e3be175 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3382,6 +3382,18 @@ void perf_event_update_userpage(struct perf_event *event)
> struct perf_buffer *buffer;
>
> rcu_read_lock();
> + /*
> + * compute total_time_enabled, total_time_running
> + * based on snapshot values taken when the event
> + * was last scheduled in.
> + *
> + * we cannot simply called update_context_time()
> + * because of locking issue as we are called in

s/are/can be/

> + * NMI context
> + */
> + calc_timer_values(event,
> + &event->total_time_enabled,
> + &event->total_time_running);

I'm not sure writing those from NMI context is a sane thing to do, best
is to compute the values into a local variable and use that variable
below.

Took the first two patches.

> buffer = rcu_dereference(event->buffer);
> if (!buffer)
> goto unlock;