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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.
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