Re: [Xen-devel] Re: [PATCH] x86: unconditionally mark TSC unstable under Xen

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

On 07/15/2010 08:57 AM, Stefano Stabellini wrote:
> On Wed, 14 Jul 2010, Jeremy Fitzhardinge wrote:
>   
>> Subject: [PATCH] xen: disable xen_sched_clock by default
>>
>> xen_sched_clock only counts unstolen time.  In principle this should
>> be useful to the Linux scheduler so that it knows how much time a process
>> actually consumed.  But in practice this doesn't work very well as the
>> scheduler expects the sched_clock time to be synchronized between
>> cpus.  It also uses sched_clock to measure the time a task spends
>> sleeping, in which case "unstolen time" isn't meaningful.
>>
>> So just use plain xen_clocksource_read to return wallclock nanoseconds
>> for sched_clock.
>>
>>     
> I think that in this context is worth mentioning that
> xen_clocksource_read ends up calling pvclock_get_nsec_offset that calls
> native_read_tsc.
>   

That's different.  That's not a general rdtsc, but a specific use of the
tsc within the Xen clock ABI.  We know and expect that the raw tsc value
could be dubious, but we also have the Xen-provided corrections to apply
to it.

    J

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