Re: [PATCH] CPU time clock support in clock_* syscalls

[Nick Piggin]

Roland McGrath wrote:

> > CPU. In which case you don't need to worry about timestamp_last_tick.
>
>
> I don't really understand this comment.  update_cpu_clock is called from
> schedule and from scheduler_tick.  When it was last called by schedule,
> p->timestamp will mark this time.  When it was last called by
> p->scheduler_tick, rq->timestamp_last_tick will mark this time.
> Hence the max of the two is the last time update_cpu_clock was called.

OK I see what its doing - ignore my comments then :P

> > It also seems to conveniently ignore locking when reading those values
> > off another CPU. Not a big deal for dynamic load calculations, but I'm
> > not so sure about your usage...?
>
>
> Here again I don't know what you are talking about.  Nothing is ever read
> "off another CPU".  A thread maintains its own sched_time counter while it
> is running on a CPU.

It seemed like a syscall could read the values from a task currently
running on another CPU. If not, great.

> > Lastly, even when using timestamp_last_tick correctly, I think sched_clock
> > will still drift around slightly, especially if a task switches CPUs a lot
> > (but not restricted to moving CPUs). 
>
>
> Please explain.

As you pointed out, you are only measuring on-cpu time so this shouldn't
be a problem either.

Nick
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