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

[Pasha Tatashin]

Hi Dou,

This makes sense. The current sched_clock_early() approach does not 
break it because with notsc TSC is used early in boot, and later 
stopped. But, notsc must stay.

Peter,

So, we could either expend sched_clock() with another static branch for 
early clock, or use what I proposed. IMO, the later is better, but 
either way works for me.

Thank you,
Pasha

On 09/27/2017 09:52 AM, Dou Liyang wrote:
> Hi Pasha, Peter
>
> At 09/27/2017 09:16 PM, Pasha Tatashin wrote:
> > Hi Peter,
> >
> > I am totally happy with removing notsc. This certainly simplifies the
> > sched_clock code. Are there any issues with removing existing kernel
> > parameters that I should be aware of?
>
> 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.
>
> [1] 
> http://www.fujitsu.com/global/products/computing/servers/mission-critical/primequest/technology/availability/dynamic-reconfiguration.html