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SubjectRe: [PATCH] events: Ensure that timers are updated without requiring read() call
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On Thu, 2011-06-23 at 10:38 -0400, Eric B Munson wrote:
> On Thu, 23 Jun 2011, Peter Zijlstra wrote:
>
> > 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@mgebm.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.
> >
>
> I was afraid that might be the case, is there an easy-ish way to accomplish
> the timer update?

IIRC Stephane did something along these lines, he kept a stamp and then
computed a delta in NMI context using perf_clock(). Let me try and find
where that lives.

See perf_output_read().


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