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 16:54:07 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Matt Fleming <matt@...sole-pimps.org>
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 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.

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