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

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Message-ID: <1308840337.1022.132.camel@twins>
Date:	Thu, 23 Jun 2011 16:45:37 +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,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH] events: Ensure that timers are updated without
 requiring read() call

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@...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.
> > 
> 
> 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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