linux-kernel - Re: [RFC][PATCH 1/5] perf: Check if we should exclude idle thread in perf_exclude

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Date:	Tue, 31 Aug 2010 16:20:12 +0100
From:	Matt Fleming <matt@...sole-pimps.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Zhang Rui <rui.zhang@...el.com>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Robert Richter <robert.richter@....com>,
	Lin Ming <ming.m.lin@...el.com>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Don Zickus <dzickus@...hat.com>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Len Brown <lenb@...nel.org>,
	Matthew Garrett <mjg59@...f.ucam.org>
Subject: Re: [RFC][PATCH 1/5] perf: Check if we should exclude idle thread
	in perf_exclude_event()

On Tue, Aug 31, 2010 at 04:54:07PM +0200, Frederic Weisbecker wrote:
> On Mon, Aug 30, 2010 at 01:13:43PM +0100, Matt Fleming wrote:
> > Don't open code the event check for excluding the idle thread. Instead
> > include the check in perf_exclude_event().
> > 
> > Signed-off-by: Matt Fleming <matt@...sole-pimps.org>
> > ---
> >  kernel/perf_event.c |    8 +++++---
> >  1 files changed, 5 insertions(+), 3 deletions(-)
> > 
> > diff --git a/kernel/perf_event.c b/kernel/perf_event.c
> > index 0d38f27..16b0476 100644
> > --- a/kernel/perf_event.c
> > +++ b/kernel/perf_event.c
> > @@ -4310,6 +4310,9 @@ static int perf_exclude_event(struct perf_event *event,
> >  
> >  		if (event->attr.exclude_kernel && !user_mode(regs))
> >  			return 1;
> > +
> > +		if (event->attr.exclude_idle && current->pid == 0)
> > +			return 1;
> 
> 
> 
> Right.
> 
> But one of the problems people have reported is that they can miss
> interrupts samples if they happen in idle. Hence we have decided
> that exclude_idle should exclude events that happen in idle process
> context but not in interrupts interrupting idle.
> 
> So adding an in_interrupt() check would perhaps be better.
> 
> I plan to do this exclusion using the per context exclusion, which is
> a patchset I have in queue. But until then, having this patch is better.
> 
> 
> 
> >  	}
> >  
> >  	return 0;
> > @@ -4512,9 +4515,8 @@ static enum hrtimer_restart perf_swevent_hrtimer(struct hrtimer *hrtimer)
> >  	regs = get_irq_regs();
> >  
> >  	if (regs && !perf_exclude_event(event, regs)) {
> > -		if (!(event->attr.exclude_idle && current->pid == 0))
> > -			if (perf_event_overflow(event, 0, &data, regs))
> > -				ret = HRTIMER_NORESTART;
> > +		if (perf_event_overflow(event, 0, &data, regs))
> > +			ret = HRTIMER_NORESTART;
> 
> 
> 
> But yeah if we add an in_interrupt() check in perf_exclude_event(), it
> won't work here. This one needs to check if irqs are nesting :)
> 
> Bah, checking we interrupted softirqs is probably enough. I guess we
> don't care about nesting hardirqs.

This patch isn't really worth it on its own, I only grouped the idle
check into perf_exclude_event() because patch 3/5 introduced a new
caller. As you've said, the semantics at the various callsites are
quite complex. It's probably best to wait for your patchset :)
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