* Paul E. McKenney <paul...@linux.vnet.ibm.com> wrote:
> On Wed, May 15, 2013 at 11:20:55AM +0200, Ingo Molnar wrote:
> >
> > * Paul E. McKenney <paul...@linux.vnet.ibm.com> wrote:
> >
> > > rcu: Fix comparison sense in rcu_needs_cpu()
> > >
> > > Commit c0f4dfd4f (rcu: Make RCU_FAST_NO_HZ take advantage of numbered
> > > callbacks) introduced a bug that can result in excessively long grace
> > > periods. This bug reverse the senes of the "if" statement checking
> > > for lazy callbacks, so that RCU takes a lazy approach when there are
> > > in fact non-lazy callbacks. This can result in excessive boot, suspend,
> > > and resume times.
> > >
> > > This commit therefore fixes the sense of this "if" statement.
> > >
> > > Reported-by: Borislav Petkov <b...@alien8.de>
> > > Reported-by: Bj?rn Mork <bj...@mork.no>
> > > Reported-by: Joerg Roedel <j...@8bytes.org>
> > > Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
> > >
> > > diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> > > index 170814d..6d939a6 100644
> > > --- a/kernel/rcutree_plugin.h
> > > +++ b/kernel/rcutree_plugin.h
> > > @@ -1667,7 +1667,7 @@ int rcu_needs_cpu(int cpu, unsigned long *dj)
> > > rdtp->last_accelerate = jiffies;
> > >
> > > /* Request timer delay depending on laziness, and round. */
> > > - if (rdtp->all_lazy) {
> > > + if (!rdtp->all_lazy) {
> > > *dj = round_up(rcu_idle_gp_delay + jiffies,
> > > rcu_idle_gp_delay) - jiffies;
> >
> > Neat - could this explain sporadic long (but not infinite) boot times with
> > NOHZ_FULL?
> >
> > We changed HZ to be at least 1 Hz pretty recently, which might have worked
> > around this bug.
>
> Quite possibly...
>
> Of course, I don't see the boot slowdowns in my testing. :-/
They were pretty sporadic and only popped up (and down) during randconfig
testing. Simple unrelated changes to the .config made them go away -
heisenbugs.
Thanks,
Ingo
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