Re: [PATCH 1/1] cputime: Make the reported utime+stime correspond to the actual runtime.

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jul 07, 2015 at 02:51:36AM +0200, Frederic Weisbecker wrote:
> On Thu, Jul 02, 2015 at 03:07:01PM +0200, Peter Zijlstra wrote:
> > +	/*
> > +	 * Make sure stime doesn't go backwards; this preserves monotonicity
> > +	 * for utime because rtime is monotonic.
> > +	 *
> > +	 *  utime_i+1 = rtime_i+1 - stime_i
> 
> I'm not sure what is meant by _i+1.

Since we have a discrete set of elements, we can enumerate them and _i
is the i-th element in the (ordered) set. _i+1 is the i+1-th element,
and so on.

> I guess stime_i means prev->stime. stime_i+1 the new update of prev->stime
> But then what is rtime_i and rtime_i+1 since we have no scaled rtime?

still the previous and the next value.

 rtime_i+1 >= rtime_i

just means that every next rtime value must be equal or greater than the
last, IOW. rtime must be monotonic.

> > +	 *            = rtime_i+1 - (rtime_i - stime_i)
> > +	 *            = (rtime_i+1 - rtime_i) + stime_i
> > +	 *            >= stime_i
> > +	 */
> > +	if (stime < prev->stime)
> > +		stime = prev->stime;
> > +	utime = rtime - stime;
> > +
> > +	/*
> > +	 * Make sure utime doesn't go backwards; this still preserves
> > +	 * monotonicity for stime, analogous argument to above.
> > +	 */
> > +	if (utime < prev->utime) {
> > +		utime = prev->utime;
> > +		stime = rtime - utime;
> 
> I see, so we are guaranteed that this final stime won't get below
> prev->stime because older prev->stime + prev->utime <= newest rtime. I
> guess that's more or less what's in the comments above :-)

Indeed.

> > +	}
> >  
> > +update:
> > +	prev->stime = stime;
> > +	prev->utime = utime;
> >  out:
> >  	*ut = prev->utime;
> >  	*st = prev->stime;
> > +	raw_spin_unlock(&prev->lock);
> >  }
> >  
> >  void task_cputime_adjusted(struct task_struct *p, cputime_t *ut, cputime_t *st)
> 
> Ok I scratched my head a lot on this patch and the issues behind and it looks
> good to me. I worried about introducing a spinlock but we had two cmpxchg before
> that. The overhead is close.

Its slightly worse, I had to change the raw_spin_lock, to
raw_spin_lock_irqsave() because Ingo managed to trigger a lockdep splat
with sighand lock taking this lock, and sighand lock is IRQ-safe.