linux-kernel - Re: [PATCH v6 1/4] sched/clock: interface to allow timestamps early in boot

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Message-ID: <20170927180950.GD439@worktop>
Date:   Wed, 27 Sep 2017 20:09:50 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dou Liyang <douly.fnst@...fujitsu.com>
Cc:     Pasha Tatashin <pasha.tatashin@...cle.com>, linux@...linux.org.uk,
        schwidefsky@...ibm.com, heiko.carstens@...ibm.com,
        john.stultz@...aro.org, sboyd@...eaurora.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, mingo@...hat.com, tglx@...utronix.de,
        hpa@...or.com
Subject: Re: [PATCH v6 1/4] sched/clock: interface to allow timestamps early
 in boot

On Wed, Sep 27, 2017 at 08:05:48PM +0200, Peter Zijlstra wrote:
> On Wed, Sep 27, 2017 at 09:52:36PM +0800, Dou Liyang wrote:
> > We do not want to do that. Because, we use "notsc" to support Dynamic
> > Reconfiguration[1].
> > 
> > AFAIK, this feature enables hot-add system board which contains CPUs
> > and memories. But the CPUs in different board may have different TSCs
> > which are not consistent with the TSC from the existing CPUs. If we hot-add
> > a board directly, the machine may happen the inconsistency of
> > TSC.
> > 
> > We make our effort to specify the same TSC value as existing one through
> > hardware and firmware, but it is hard. So we recommend to specify
> > "notsc" option in command line for users who want to use Dynamic
> > Reconfiguration.
> 
> Oh gawd, that's horrific. And in my book a good reason to kill that
> option.

That is, even with unsynchronized TSC we're better off using RDTSC. The
whole mess in kernel/sched/clock.c is all about getting semi sensible
results out of unsynchronized TSC.

There really is no reason to artificially kill TSC usage.