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

[Dou Liyang]

Hi Peter,

At 09/28/2017 02:09 AM, Peter Zijlstra wrote:
> 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.

It will be best if we can support TSC sync capability in x86, but seems
is not easy.

Thanks,

	dou.

> There really is no reason to artificially kill TSC usage.