linux-kernel - Re: [PATCH 1/3] perf tools: Put proper period for for samples without PERIOD sample

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Date:	Wed, 5 Feb 2014 15:33:29 +0100
From:	Jiri Olsa <jolsa@...hat.com>
To:	Namhyung Kim <namhyung@...nel.org>
Cc:	linux-kernel@...r.kernel.org,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	David Ahern <dsahern@...il.com>, Ingo Molnar <mingo@...e.hu>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Adrian Hunter <adrian.hunter@...el.com>
Subject: Re: [PATCH 1/3] perf tools: Put proper period for for samples
 without PERIOD sample_type

On Wed, Feb 05, 2014 at 10:27:30AM +0900, Namhyung Kim wrote:
> Hi Jiri,
> 
> On Mon,  3 Feb 2014 12:44:41 +0100, Jiri Olsa wrote:
> > We use PERF_SAMPLE_PERIOD sample type only for frequency
> > setup -F (default) option. The -c does not need store period,
> > because it's always the same.
> >
> > In -c case the report code uses '1' as  period. Fixing
> > it to perf_event_attr::sample_period.
> 
> All 3 patches look good.  But I found something strange.  When we
> setup/config evsel attrs following code is used:
> 
>  util/evsel.c::perf_evsel__config()
> 
> 	/*
> 	 * We default some events to a 1 default interval. But keep
> 	 * it a weak assumption overridable by the user.
> 	 */
> 	if (!attr->sample_period || (opts->user_freq != UINT_MAX &&
> 				     opts->user_interval != ULLONG_MAX)) {
> 		if (opts->freq) {
> 			perf_evsel__set_sample_bit(evsel, PERIOD);
> 			attr->freq		= 1;
> 			attr->sample_freq	= opts->freq;
> 		} else {
> 			attr->sample_period = opts->default_interval;
> 		}
> 	}

yes, I think thats right.. we should use || instead of &&

It will allow to change period for event types with predefined
attr->sample_period like tracepoints.

However, I tried with tracepoints and even with this fix
and following command line:

  # perf record -e syscalls:sys_enter_read -c 2 ls

you'll still get samples with period 1. The reason is in
kernel code:

static void perf_swevent_event(struct perf_event *event, u64 nr,
                               struct perf_sample_data *data,
                               struct pt_regs *regs)
{
...
        if ((event->attr.sample_type & PERF_SAMPLE_PERIOD) && !event->attr.freq) {
                data->period = nr;
                return perf_swevent_overflow(event, 1, data, regs);

bacause above condition is true for tracepoints.

It looks like a bug, but I'm not sure how handy it'd be
set period other than 1 for tracepoints thought.. ;)

Maybe it's not that big issue in comparison of screwing
up other software events processing.

I haven't checked other software events.

jirka
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