linux-kernel - Re: [PATCH v4 1/3] perf: Use monotonic clock as a source for timestamps

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Date:	Fri, 16 Jan 2015 14:41:55 +0200
From:	Adrian Hunter <adrian.hunter@...el.com>
To:	Pawel Moll <pawel.moll@....com>,
	Richard Cochran <richardcochran@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	John Stultz <john.stultz@...aro.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Christopher Covington <cov@...eaurora.org>,
	Namhyung Kim <namhyung@...nel.org>,
	David Ahern <dsahern@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tomeu Vizoso <tomeu@...euvizoso.net>
CC:	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [PATCH v4 1/3] perf: Use monotonic clock as a source for timestamps

On 06/11/14 18:51, Pawel Moll wrote:
> Until now, perf framework never defined the meaning of the timestamps
> captured as PERF_SAMPLE_TIME sample type. The values were obtaining
> from local (sched) clock, which is unavailable in userspace. This made
> it impossible to correlate perf data with any other events. Other
> tracing solutions have the source configurable (ftrace) or just share
> a common time domain between kernel and userspace (LTTng).
> 
> Follow the trend by using monotonic clock, which is readily available
> as POSIX CLOCK_MONOTONIC.
> 
> Also add a sysctl "perf_sample_time_clk_id" attribute which can be used
> by the user to obtain the clk_id to be used with POSIX clock API (eg.
> clock_gettime()) to obtain a time value comparable with perf samples.
> 
> Old behaviour can be restored by using "perf_use_local_clock" kernel
> parameter.
> 

Don't forget this breaks the relationship to TSC. So you will need something
like below. Also patch 3 needs to be done first and extended to cover TSC so
there is no gap when we cannot get TSC.

diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index 143e5f5..b6e833d 100644
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -1922,21 +1922,6 @@ void arch_perf_update_userpage(struct
perf_event_mmap_page *userpg, u64 now)
        userpg->cap_user_time_zero = 0;
        userpg->cap_user_rdpmc = x86_pmu.attr_rdpmc;
        userpg->pmc_width = x86_pmu.cntval_bits;
-
-       if (!sched_clock_stable())
-               return;
-
-       data = cyc2ns_read_begin();
-
-       userpg->cap_user_time = 1;
-       userpg->time_mult = data->cyc2ns_mul;
-       userpg->time_shift = data->cyc2ns_shift;
-       userpg->time_offset = data->cyc2ns_offset - now;
-
-       userpg->cap_user_time_zero = 1;
-       userpg->time_zero = data->cyc2ns_offset;
-
-       cyc2ns_read_end(data);
 }

 /*


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