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.
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