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

[Nick Piggin <nickpiggin@yahoo.com.au>]

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