Re: BUG: using smp_processor_id() in preemptible [00000000] code:icedove-bin/5449

[Peter Zijlstra]

On Wed, 2010-05-26 at 16:02 -0700, Andrew Morton wrote:

> How is anyone supposed to use this?  What are the semantics of this
> thing?  What are the units of its return value?  What is the base value
> of its return value?  Does it return different times on different CPUs?
> I assume so, otherwise why does sched_clock_cpu() exist?  <looks at
> the sched_clock_cpu() documentation, collapses in giggles>

The point of the whole sched_clock_cpu() thing is to provide a fairly
high resolution clock with bounded drift between cpus.

It also promises to be monotonic per cpu argument, that is,
sched_clock_cpu(j) will, for a constant j always return a monotonic
increasing timestamp.

It doesn't make much promises about its base (although people tend to
want it to start at 0 on boot, but the users really shouldn't care).

sched_clock() doesn't promise either bounded drift between cpus nor
monotonicity.


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