linux-kernel - Re: [RFC] perf: need to expose sched_clock to correlate user samples with kernel samples

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Message-ID: <51227A07.5020508@gmail.com>
Date:	Mon, 18 Feb 2013 11:59:19 -0700
From:	David Ahern <dsahern@...il.com>
To:	Stephane Eranian <eranian@...gle.com>
CC:	Pawel Moll <pawel.moll@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	John Stultz <john.stultz@...aro.org>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"mingo@...e.hu" <mingo@...e.hu>, Paul Mackerras <paulus@...ba.org>,
	Anton Blanchard <anton@...ba.org>,
	Will Deacon <Will.Deacon@....com>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	Pekka Enberg <penberg@...il.com>,
	Robert Richter <robert.richter@....com>,
	tglx <tglx@...utronix.de>
Subject: Re: [RFC] perf: need to expose sched_clock to correlate user samples
 with kernel samples

On 2/18/13 8:16 AM, Stephane Eranian wrote:
> Hi,
>
>
> I think the advantage of the ioctl() is that is reuses existing infrastructure.
> The downside is that to get the timestamp you need at a minimum:
>
> uint64_t get_perf_timestamp(void)
> {
>     struct perf_event_attr attr;
>     uint64_t ts = 0;
>     int fd;
>
>     memset(&attr, 0, sizeof(attr));
>
>     /* pick a dummy SW event (no PMU HW resource allocated), keep it disabled */
>     attr.type = PERF_TYPE_SOFTWARE;
>     attr.config =  PERF_COUNT_SW_CPU_CLOCK; /* dummy event */
>     attr.disabled = 1;
>
>     /* attach to self in per-thread mode */
>     fd = perf_event_open(&attr, 0, -1, -1, 0);
>     if (fd == -1)
>         return 0;
>
>    ioctl(fd, PERF_EVENT_IOC_GET_TIME, &ts);
>    close(fd);
>
>    return ts;
> }
>

That's the approach I took with an update to my perf_clock to 
time-of-day series. Specific patch:
 
https://github.com/dsahern/linux/commit/7e6f40fca5f8cdbee1cd46d42b11aee71d0ffd34

and series:
https://github.com/dsahern/linux/commits/perf-time-of-day-3.7

David
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