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

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Date:	Thu, 06 Feb 2014 15:24:31 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Jiri Olsa <jolsa@...hat.com>
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

Hi Jiri,

On Wed, 5 Feb 2014 15:33:29 +0100, Jiri Olsa wrote:
> 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.

Right.  As I read the code, it works "if (!attr->sample_period)" case only.

>
> 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.. ;)

Agreed.  But at least we should support whatever user wants IMHO..

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

:)

Thanks,
Namhyung
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