Re: [PATCH tip/core/rcu 6/7] rcu: Drive quiescent-state-forcingdelay from HZ

[Ingo Molnar]

* Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote:

> On Wed, May 15, 2013 at 11:20:55AM +0200, Ingo Molnar wrote:
> > 
> > * Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> 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 <bp@xxxxxxxxx>
> > > Reported-by: Bj?rn Mork <bjorn@xxxxxxx>
> > > Reported-by: Joerg Roedel <joro@xxxxxxxxxx>
> > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > > 
> > > 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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