linux-kernel - Re: [PATCH 0/6] perf tools: Add wallclock time conversion support

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Message-ID: <20200731172006.GJ2638@hirez.programming.kicks-ass.net>
Date:   Fri, 31 Jul 2020 19:20:06 +0200
From:   peterz@...radead.org
To:     David Ahern <dsahern@...il.com>
Cc:     Jiri Olsa <jolsa@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Michael Petlan <mpetlan@...hat.com>,
        Ian Rogers <irogers@...gle.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Geneviève Bastien <gbastien@...satic.net>,
        Wang Nan <wangnan0@...wei.com>,
        Jeremie Galarneau <jgalar@...icios.com>
Subject: Re: [PATCH 0/6] perf tools: Add wallclock time conversion support

On Thu, Jul 30, 2020 at 07:21:54PM -0600, David Ahern wrote:
> On 7/30/20 4:14 PM, peterz@...radead.org wrote:
> > On Thu, Jul 30, 2020 at 11:39:44PM +0200, Jiri Olsa wrote:
> > 
> >> The patchset is adding the ability to display TOD/wallclock timestamp
> >> in 'perf script' output and in 'perf data convert --to-ctf' subcommand,
> >> so the converted CTF data contain TOD/wallclock timestamps.
> > 
> > But why? Wallclock is a horrible piece of crap. Why would you want to do
> > this?
> > 
> 
> Same reason I brought this up 9+ years ago: userspace lives on
> time-of-day, and troubleshooting is based on correlating timestamps from
> multiple sources. To correlate a perf event to syslog or an application
> log, we need time-of-day.

You need a sync'ed CLOCK_MONOTONIC for that, CLOCK_REALTIME 'aka'
wallclock is a trainwreck, you don't ever want that.