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

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Message-ID: <20200731180539.GA2674@hirez.programming.kicks-ass.net>
Date:   Fri, 31 Jul 2020 20:05:39 +0200
From:   peterz@...radead.org
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Jiri Olsa <jolsa@...hat.com>, David Ahern <dsahern@...il.com>,
        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>,
        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 Fri, Jul 31, 2020 at 08:36:12AM -0700, Andi Kleen wrote:
> > yep, we have a customer that needs to compare data from multiple servers
> 
> It's also needed to correlate over different guests on the same machine.
> This is an important use case.

Both these cases you want to sync up CLOCK_MONOTONIC, using walltime is
just utterly misguided.

What happens if the servers have (per accident or otherwise) different
DST settings, or someone does a clock_setttime() for giggles.

All you really want is a clock that runs at the same rate but is not
subject to random jumps and user foibles.