Messages in this thread | | | Date | Tue, 28 May 2013 12:07:42 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH tip/core/rcu 6/7] rcu: Drive quiescent-state-forcing delay from HZ |
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* Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> On Wed, May 15, 2013 at 11:20:55AM +0200, Ingo Molnar wrote: > > > > * Paul E. McKenney <paulmck@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 <bp@alien8.de> > > > Reported-by: Bj?rn Mork <bjorn@mork.no> > > > Reported-by: Joerg Roedel <joro@8bytes.org> > > > Signed-off-by: Paul E. McKenney <paulmck@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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