On Wed, Jun 15, 2016 at 07:02:38PM +0200, Thomas Gleixner wrote:
> On Wed, 15 Jun 2016, Paul E. McKenney wrote:
> > On Mon, Jun 13, 2016 at 09:15:14AM -0700, Paul E. McKenney wrote:
> > > On Mon, Jun 13, 2016 at 08:40:50AM -0000, Thomas Gleixner wrote:
> > > > The current timer wheel has some drawbacks:
> > > >
> > > > 1) Cascading
> > > >
> > > > Cascading can be an unbound operation and is completely pointless in
> > > > most
> > > > cases because the vast majority of the timer wheel timers are
> > > > canceled or
> > > > rearmed before expiration.
> > > >
> > > > 2) No fast lookup of the next expiring timer
> > > >
> > > > In NOHZ scenarios the first timer soft interrupt after a long NOHZ
> > > > period
> > > > must fast forward the base time to current jiffies. As we have no
> > > > way to
> > > > find the next expiring timer fast, the code loops and increments the
> > > > base
> > > > time by one and checks for expired timers in each step. I've
> > > > observed loops
> > > > lasting 1 ms!
> > > >
> > > > There are some other issues caused by the above, but they are minor
> > > > compare to
> > > > those.
> > >
> > > For SMP configurations, this passes light rcutorture testing. For UP
> > > builds, it complains about undefined symbols. Builds succeed with
> > > the following kneejerk patch. Am retesting rcutorture.
> >
> > And with the patch below, testing goes as well with your patch stack as
> > it does without it. So, with that patch (or equivalent):
> >
> > Tested-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
> >
> > There were some complaints about increasing the size of the tiny
> > configuration, FYI.
>
> I know. The extra storage space for the deferrable stuff makes it larger along
> with the extra code for avoiding all the crap which the current wheel suffers
> from :) Do the tiny people need NOHZ?
If it makes the code bigger, I would hope that they don't need it.
> > So, just out of curiosity, does anyone still run -rt on single-CPU systems?
>
> Of course :)
Hey, had to ask! ;-)
Thanx, Paul
