Re: [PATCH] Revert "timers: Don't wake ktimersoftd on every tick"

[Haris Okanovic]

On 05/26/2017 03:50 PM, Thomas Gleixner wrote:
> On Fri, 26 May 2017, Haris Okanovic wrote:
>
> > Oh crap. I think I see the problem. I decrement expired_count before
> > processing the list. Dropping the lock permits another run of
> > tick_find_expired()->find_expired_timers() in the middle of __expire_timers()
> > since it uses expired_count==0 as a condition.
> >
> > This should fix it, but I'll wait for Anna-Maria's test next week before
> > submitting a patch.
> >
> > >  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?
All reads and writes to base->expired_count happen while base->lock is 
held. It just can't reach zero until expired_lists is ready to be rewritten.

> > >         base->expired_count = 0;
>
> Anna-Maria spotted the same issue, but I voted for the revert right now
> because I was worried about the consistency of base->clk under all
> circumstances.
>
> 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.

> 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.

> 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`.

> Thanks,
>
> 	tglx

I also ran Anna-Maria's test for 12h without failure; I.e. no "Stalled" 
messages. It fails withing 10-15m on my qemu VM without the fix (4-core 
Nehalem, 1GB RAM).

You can view a diff at
https://github.com/harisokanovic/linux/tree/dev/hokanovi/timers-race

-- Haris