* Thomas Gleixner <t...@linutronix.de> wrote:
> On Thu, 4 Jun 2015, John Stultz wrote:
> > On Wed, Jun 3, 2015 at 5:56 PM, Jeremiah Mahler <jmmah...@gmail.com> wrote:
> > So I suspect the problem is the change to clock_was_set_seq in
> > timekeeping_update is done prior to mirroring the time state to the
> > shadow-timekeeper. Thus the next time we do update_wall_time() the
> > updated sequence is overwritten by whats in the shadow copy. The
> > attached patch moving the modification up seems to avoid the issue for
> > me.
>
> Duh, yes.
>
> > Thomas: Looking at the problematic change, I'm not a big fan of it. Caching
> > timekeeping state here in the hrtimer code has been a source of bugs in the
> > past, and I'm not sure I see how avoiding copying 24bytes is that big of a
> > win. Especially since it adds more state to the timekeeper and hrtimer base
> > that we have to read and mange.
>
> It's not about copying 24 bytes. It's about touching 3 cache lines for
> nothing.
> In situations where we run high frequency periodic timers on clock monotonic
> and
> nothing is going on in the other clock domains, which is a pretty common
> situation, this is measurable in terms of cache utilization. [...]
It's not just about 'touching': it's about _dirtying_ cachelines from a
globally
executed function (timekeeping), which is then accessed by per-CPU
functionality
(hrtimers).
That makes it far more expensive, it has similar scalability limiting effects
as a
global lock - while if we do it smart it can perform as essentially lockless
code
in most cases.
Thanks,
Ingo
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