rcu self-detected stall messages on OMAP3, 4 boards

[Shilimkar, Santosh]

On Sun, Sep 23, 2012 at 3:29 AM, Paul E. McKenney
<paulmck at linux.vnet.ibm.com> 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 at linaro.org>
> Signed-off-by: Paul E. McKenney <paulmck at linux.vnet.ibm.com>
>
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.

Regards
Santosh