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Date: Mon, 18 Feb 2013 11:59:19 -0700
From: David Ahern <dsahern@gmail.com>
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To: Stephane Eranian <eranian@google.com>
CC: Pawel Moll <pawel.moll@arm.com>, Steven Rostedt <rostedt@goodmis.org>,
        John Stultz <john.stultz@linaro.org>,
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        "ak@linux.intel.com" <ak@linux.intel.com>,
        Pekka Enberg <penberg@gmail.com>,
        Robert Richter <robert.richter@amd.com>, tglx <tglx@linutronix.de>
Subject: Re: [RFC] perf: need to expose sched_clock to correlate user samples
 with kernel samples
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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