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

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Message-ID: <20200801174904.GA2740416@kernel.org>
Date:   Sat, 1 Aug 2020 14:49:04 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     David Ahern <dsahern@...il.com>
Cc:     peterz@...radead.org, Andi Kleen <ak@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>, Jiri Olsa <jolsa@...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

Em Fri, Jul 31, 2020 at 06:46:21PM -0600, David Ahern escreveu:
> On 7/31/20 12:05 PM, peterz@...radead.org wrote:
> > 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.

> Every userspace component logs in walltime. You can say that is
> misguided, but that is the way it is. The missing piece is the ability
> to correlate kernel events to userspace logs.

> > What happens if the servers have (per accident or otherwise) different
> > DST settings, or someone does a clock_setttime() for giggles.
 
> Yes, someone *could* change the time. Someone *could* start ntpd or
> other time server in the middle of a session. While technically such
> things can happen, that is not real life in most environments (e.g.,
> Data center servers). ntpd (or other) is started at boot, and it is just
> the little misc adjustments that happen over time.
 
> We could add tracepoints and detect the changes and invalidate the
> reference time. We could add tracepoints to track the adjustments and
> update the reference time. In my experience over 9+ years using this
> tool (out of tree patches) that has never been the problem.
 
> > All you really want is a clock that runs at the same rate but is not
> > subject to random jumps and user foibles.

> All I want is to compare user logs to a kernel event via timestamps.

Can we have both possibilities and leave the decision on which one to
use in the hands of those who have a gun to shoot wherever they want,
maybe in the foot?

- Arnaldo