Re: [v3, 3/3] MIPS: VDSO: Add implementations of gettimeofday() and clock_gettime()

[David Daney]

On 10/27/2015 02:15 PM, Leonid Yegoshin wrote:
> On 10/27/2015 02:02 PM, David Daney wrote:
> > On 10/27/2015 01:46 PM, Leonid Yegoshin wrote:
> > [...]
> > > And finally. clock scaling - what we would do if there are two CPUs with
> > > different clock ratios in system? It seems like common kernel timing
> > > subsystem can handle that.
> >
> > The code that executes in userspace must have access to a consistent
> > clock source.  If you are running on a SMP system that doesn't have
> > synchronized CP0.Count registers, then your gettimeofday() cannot use
> > CP0.Count (RDHWR $2).
>
> Right, I agree.
>
> > As far as I know, CP0.Count is the only available counter visible to
> > userspace, so you would have to disable the accelerated versions of
> > gettimeofday() where you cannot assert that the counters are always
> > synchronized.
>
> Any system with GIC may have access to the same GIC global counter in a
> special separate page available for mapping by user in RO mode and it
> seems Alex did that.
>
> Besides that this GIC global counter is used as a major system
> clocksource in systems with GIC.

Yes, I had forgotten about the GIC thing.

In any event, there is a set of systems where we could run into problems 
with unsynchronized clocks.  There needs to be an easy way to 
enable/disable the gettimeofday() acceleration at run time based on the 
properties of the counter (GIC, CP0.Count, or whatever) chosen at boot time.

For example, On OCTEON single-chip systems we synchronize the the 
counters and they don't drift.  So, we can use CPO.Count.  However, on 
two-chip NUMA configurations there may be clock drift between the two 
chips, so CPO.Count cannot be used as a clocksource.  We have a single 
kernel image that runs on both types of systems, so we have to be able 
to enable/disable the gettimeofday() acceleration.

David Daney


> - Leonid

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