Re: 3.0-git15 Atomic scheduling in pidmap_init

Paul E. McKenney
2011-08-24T23:12:59Z

On Thu, Aug 25, 2011 at 12:45:00AM +0200, Frederic Weisbecker wrote:
> On Thu, Aug 18, 2011 at 02:55:40PM -0700, Paul E. McKenney wrote:
> > On Thu, Aug 18, 2011 at 02:23:34PM -0700, Paul E. McKenney wrote:
> > > On Thu, Aug 18, 2011 at 02:00:34PM -0700, Andrew Morton wrote:
> > > > On Thu, 18 Aug 2011 11:35:23 -0700
> > > > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
> > > >
> > > > > On Wed, Aug 17, 2011 at 07:17:50PM -0400, Josh Boyer wrote:
> > > > > > On Thu, Aug 18, 2011 at 01:06:44AM +0200, Frederic Weisbecker wrote:
> > > > > > > On Wed, Aug 17, 2011 at 07:02:19PM -0400, Josh Boyer wrote:
> > > > > > > > On Wed, Aug 17, 2011 at 03:49:16PM -0700, Paul E. McKenney wrote:
> > > > > > > > > On Wed, Aug 17, 2011 at 06:37:35PM -0400, Josh Boyer wrote:
> > > > > > > > > > On Mon, Aug 15, 2011 at 08:20:52AM -0700, Paul E. McKenney wrote:
> > > > > > > > > > > On Mon, Aug 15, 2011 at 10:04:17AM -0400, Josh Boyer wrote:
> > > > > > > > > > > > > Please see the attached.
> > > > > > > > > > > >
> > > > > > > > > > > > Fixed it up quickly to apply on top of -rc2 and it seems to solve the
> > > > > > > > > > > > problem nicely. Thanks for the patch.
> > > > > > > > > > >
> > > > > > > > > > > Good to hear! I guess I should keep it, then. ;-)
> > > > > > > > > >
> > > > > > > > > > Hey Paul, were you going to send this to Linus for -rc3? I haven't seen
> > > > > > > > > > it come across LKML yet.
> > > > > > > > >
> > > > > > > > > I might... But does it qualify as a regression? That part of the
> > > > > > > > > code hasn't changed for some time now.
> > > > > > > >
> > > > > > > > It's a fix for a problem that is newly surfaced in 3.1. A regression,
> > > > > > > > likely not since it's been there forever, but new debugging options
> > > > > > > > uncovered it. I'm pretty sure the -rc stage takes fixes even if they
> > > > > > > > aren't regressions.
> > > > > > >
> > > > > > > Nope, after -rc1 only regressions fixes are taken (most of the time).
> > > > > >
> > > > > > Sigh.
> > > > > >
> > > > > > Look, either way I'm carrying this patch in Fedora because it fixes
> > > > > > a bug that is actually being reported by users (and by abrtd as well).
> > > > > > If you both want to wait until 3.2 to actually submit it to Linus,
> > > > > > then OK.
> > > > > >
> > > > > > Honestly, I'm just glad we actually run with the debug options enabled
> > > > > > (which seems to be a rare thing) so bugs like this are actually found.
> > > > > > Thanks for the fix.
> > > > >
> > > > > I am sorry, but I didn't make the rules! And I must carry the fix
> > > > > longer as well, if that makes you feel any better.
> > > >
> > > > bah, we're not that anal. The patch fixes a bug and prevents a nasty
> > > > warning spew. Please, send it to Linus.
> > >
> > > Given your Acked-by and Josh's Tested-by I might consider it. ;-)
> > >
> > > Speaking of which, Josh, does this patch help Nicolas and Michal?
> > >
> > > > We appear to be referring to the patch "rcu: Avoid having just-onlined
> > > > CPU resched itself when RCU is idle"? If so, the changelog doesn't
> > > > even mention that the patch fixes a scheduling-while-atomic warning and
> > > > the changelog fails to refer to the redhat bug report. These omissions
> > > > should be repaired, please.
> > >
> > > OK... But I cannot bring myself to believe that my fix does more than
> > > hide some other bug. Which is OK, I will just say that in the changelog.
> >
> > And here is this patch ported to v3.1-rc2, FYI.
> >
> > Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> > rcu: Avoid having just-onlined CPU resched itself when RCU is idle
> >
> > CPUs set rdp->qs_pending when coming online to resolve races with
> > grace-period start. However, this means that if RCU is idle, the
> > just-onlined CPU might needlessly send itself resched IPIs. Adjust the
> > online-CPU initialization to avoid this, and also to correctly cause
> > the CPU to respond to the current grace period if needed.
> >
> > This patch is believed to fix or otherwise suppress problems in
> > https://bugzilla.redhat.com/show_bug.cgi?id=726877, however, the
> > relationship is not apparent to this patch's author.
> >
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> >
> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > index ba06207..6986d34 100644
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -1865,8 +1865,6 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible)
> >
> > /* Set up local state, ensuring consistent view of global state. */
> > raw_spin_lock_irqsave(&rnp->lock, flags);
> > - rdp->passed_quiesc = 0; /* We could be racing with new GP, */
> > - rdp->qs_pending = 1; /* so set up to respond to current GP. */
> > rdp->beenonline = 1; /* We have now been online. */
> > rdp->preemptible = preemptible;
> > rdp->qlen_last_fqs_check = 0;
> > @@ -1891,8 +1889,15 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible)
> > rnp->qsmaskinit |= mask;
> > mask = rnp->grpmask;
> > if (rnp == rdp->mynode) {
> > - rdp->gpnum = rnp->completed; /* if GP in progress... */
> > + /*
> > + * If there is a grace period in progress, we will
> > + * set up to wait for it next time we run the
> > + * RCU core code.
> > + */
> > + rdp->gpnum = rnp->completed;
> > rdp->completed = rnp->completed;
> > + rdp->passed_quiesc = 0;
> > + rdp->qs_pending = 1;
>
> In the previous version you had rdp->qs_pending = 0 here.
> If it's set to 0 I can understand that it fixes the problem.
> Otherwise, set to 1 I don't know how it fixes the thing.

Indeed, this was a bogus patch. The version I posted on the -rcu git
tree a few days ago has the correct "rdp->qs_pending = 0". I thought
that I had chased down all the bogus copies, but obviously not. :-(

> Should it perhaps set it to 1 only if we have rnp->gpnum > rnp->completed ?

I would rather keep it simple. If rnp->gpnum > rnp->completed, then
the newly onlined CPU will notice and adjust appropriately soon enough.

Thanx, Paul

> > rdp->passed_quiesc_completed = rnp->completed - 1;
> > }
> > raw_spin_unlock(&rnp->lock); /* irqs already disabled. */