* Frederic Weisbecker <fweis...@gmail.com> wrote:
> 2013/4/10 Ingo Molnar <mi...@kernel.org>:
> >
> > * Frederic Weisbecker <fweis...@gmail.com> wrote:
> >
> >> Of course 128 bits ops are very expensive, so to help you evaluating the
> >> situation, this is going to happen on every call to
> >> task_cputime_adjusted() and
> >> thread_group_adjusted(), namely:
> >
> > It's really only expensive for divisions. Addition and multiplication
> > should be
> > straightforward and relatively low overhead, especially on 64-bit platforms.
>
> Ok, well we still have one division in the scaling path. I'm mostly
> worried about the thread group exit that makes use of it through
> threadgroup_cputime_adjusted(). Not sure if we can avoid that.
I see, scale_stime()'s use of div64_u64_rem(), right?
I swapped out the details already, is there a link or commit ID that explains
where we hit 64-bit multiplication overflow? It's due to accounting in
nanosecs,
spread out across thousands of tasks potentially, right?
But even with nsecs, a 64-bit variable ought to be able to hold hundreds of
years
worth of runtime. How do we overflow?
Thanks,
Ingo
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