On Fri, 2 Jun 2017, Haris Okanovic wrote:
> On 05/26/2017 03:50 PM, Thomas Gleixner wrote:
> > > > static void expire_timers(struct timer_base *base)
> > > > {
> > > > struct hlist_head *head;
> > > > + int expCount = base->expired_count;
> >
> > No camel case for heavens sake!
> >
> > And this requires:
> >
> > cnt = READ_ONCE(base->expired_count);
> >
> > > > - while (base->expired_count--) {
> > > > - head = base->expired_lists + base->expired_count;
> > > > + while (expCount--) {
> > > > + head = base->expired_lists + expCount;
> > > > __expire_timers(base, head);
> > > > }
> >
> > Plus a comment.
>
> Fixed, thanks.
>
> Are your recommending READ_ONCE() purely for documentation purposes?
Yes.
> > The other thing I noticed was this weird condition which does not do the
> > look ahead when base->clk is back for some time.
>
> The soft interrupt fires unconditionally if base->clk hasn't advanced in some
> time to limit how long cpu spends in hard interrupt context.
That makes no sense.
> > Why don't you use the
> > existing optimization which uses the bitmap for fast forward?
> >
>
> Are you referring to forward_timer_base()/base->next_expiry? I think it's only
> updated in the nohz case. Can you share function name/line number(s) if you're
> thinking of something else.
I think just using collect_expired_timers() should be enough. In the !NOHZ
case the base shouldn't be that far back, right?
> > The other issue I have is that this can race at all. If you raised the
> > softirq in the look ahead then you should not go into that function until
> > the softirq has actually completed. There is no point in wasting time in
> > the hrtimer interrupt if the softirq is running anyway.
> >
>
> Makes sense. Skipping the large `if` block in run_local_timers() when
> `local_softirq_pending() & TIMER_SOFTIRQ`.
No. You need your own state tracking. The TIMER_SOFTIRQ bit is cleared when
the softirq is invoked, but that does not mean that it finished running.
run_local_timers()
{
lock(base->lock);
if (!base->softirq_activated)
if (base_has_timers_to_expire()) {
base->softirq_activated = true;
raise_softirq(TIMER_SOFTIRQ);
}
}
unlock(base->lock);
}
timer_softirq()
{
lock(base->lock);
expire_timers();
base->softirq_activated = false;
unlock(base->lock);
}
That way you avoid any operation in the tick interrupt as long as the soft
interrupt processing has not completed.
Thanks,
tglx
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