Re: rcu self-detected stall messages on OMAP3, 4 boards

[Paul E. McKenney]

On Mon, Sep 24, 2012 at 03:11:34PM +0530, Shilimkar, Santosh wrote:
> On Sun, Sep 23, 2012 at 3:29 AM, Paul E. McKenney
> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> > On Sat, Sep 22, 2012 at 01:10:43PM -0700, Paul E. McKenney wrote:
> >> On Sat, Sep 22, 2012 at 06:42:08PM +0000, Paul Walmsley wrote:
> >> > On Fri, 21 Sep 2012, Paul E. McKenney wrote:
> 
> [...]
> 
> >
> > And here is a patch.  I am still having trouble reproducing the problem,
> > but figured that I should avoid serializing things.
> >
> >                                                         Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> >  b/kernel/rcutree.c |    4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > rcu: Fix day-one dyntick-idle stall-warning bug
> >
> > Each grace period is supposed to have at least one callback waiting
> > for that grace period to complete.  However, if CONFIG_NO_HZ=n, an
> > extra callback-free grace period is no big problem -- it will chew up
> > a tiny bit of CPU time, but it will complete normally.  In contrast,
> > CONFIG_NO_HZ=y kernels have the potential for all the CPUs to go to
> > sleep indefinitely, in turn indefinitely delaying completion of the
> > callback-free grace period.  Given that nothing is waiting on this grace
> > period, this is also not a problem.
> >
> > Unless RCU CPU stall warnings are also enabled, as they are in recent
> > kernels.  In this case, if a CPU wakes up after at least one minute
> > of inactivity, an RCU CPU stall warning will result.  The reason that
> > no one noticed until quite recently is that most systems have enough
> > OS noise that they will never remain absolutely idle for a full minute.
> > But there are some embedded systems with cut-down userspace configurations
> > that get into this mode quite easily.
> >
> > All this begs the question of exactly how a callback-free grace period
> > gets started in the first place.  This can happen due to the fact that
> > CPUs do not necessarily agree on which grace period is in progress.
> > If a CPU still believes that the grace period that just completed is
> > still ongoing, it will believe that it has callbacks that need to wait
> > for another grace period, never mind the fact that the grace period
> > that they were waiting for just completed.  This CPU can therefore
> > erroneously decide to start a new grace period.
> >
> > Once this CPU notices that the earlier grace period completed, it will
> > invoke its callbacks.  It then won't have any callbacks left.  If no
> > other CPU has any callbacks, we now have a callback-free grace period.
> >
> > This commit therefore makes CPUs check more carefully before starting a
> > new grace period.  This new check relies on an array of tail pointers
> > into each CPU's list of callbacks.  If the CPU is up to date on which
> > grace periods have completed, it checks to see if any callbacks follow
> > the RCU_DONE_TAIL segment, otherwise it checks to see if any callbacks
> > follow the RCU_WAIT_TAIL segment.  The reason that this works is that
> > the RCU_WAIT_TAIL segment will be promoted to the RCU_DONE_TAIL segment
> > as soon as the CPU figures out that the old grace period has ended.
> >
> > This change is to cpu_needs_another_gp(), which is called in a number
> > of places.  The only one that really matters is in rcu_start_gp(), where
> > the root rcu_node structure's ->lock is held, which prevents any
> > other CPU from starting or completing a grace period, so that the
> > comparison that determines whether the CPU is missing the completion
> > of a grace period is stable.
> >
> > Signed-off-by: Paul E. McKenney <paul.mckenney@xxxxxxxxxx>
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> >
> As already confirmed by Paul W and others, I too no longer see the rcu dumps
> any more with above patch. Thanks a lot for the fix.

Glad it finally works!

							Thanx, Paul

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