From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Date: Mon, 24 Jul 2017 16:20:33 -0700
> It looks like the system isn't letting the rcu_sched grace-period kthread
> run:
>
> [402138.240512] rcu_sched kthread starved for 2757 jiffies! g53669 c53668 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1
>
> This kthread tried to wait for a few jiffies (the exact number depends
> on HZ and the number of CPUs), but 2,757 jiffies have elapsed and it is
> still waiting. This kthread is responsible for detecting idle CPUs and
> reporting quiescent states on their behalf, so if this kthread doesn't
> get a chance to run, then the stall warnings you are seeing are expected
> behavior.
>
> I am seeing someething like sort of like this in my rcutorture runs,
> but only when I boot with nr_cpus quite a bit bigger than maxcpus, as in
> something like nr_cpus=43 and maxcpus=8. This causes 8 CPUs to be brought
> online at the usual time, and the other 35 come online some time later.
> One difference from your situation is that I see the grace-period
> kthread in ->state=0x401 (TASK_WAKING) instead of your ->state=0x1.
> If I send extra wakeups to the grace-period kthread (which shouldn't be
> needed), it does make progress, but then other kthreads fall into that
> same half-woken state.
>
> So now that I ahve shared the full extent of my ignorance on this topic,
> any ideas? ;-)
Shoing my ignorance as well, after reading this, for some reason this
commit below sticks out to me. Maybe I should do a bisect and see if
it lands on this commit.
That would take a while as it's hard to forcibly set this thing off.
====================
commit f92c734f02cbf10e40569facff82059ae9b61920
Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Date: Mon Apr 10 15:40:35 2017 -0700
rcu: Prevent rcu_barrier() from starting needless grace periods
Currently rcu_barrier() uses call_rcu() to enqueue new callbacks
on each CPU with a non-empty callback list. This works, but means
that rcu_barrier() forces grace periods that are not otherwise needed.
The key point is that rcu_barrier() never needs to wait for a grace
period, but instead only for all pre-existing callbacks to be invoked.
This means that rcu_barrier()'s new callbacks should be placed in
the callback-list segment containing the last pre-existing callback.
This commit makes this change using the new rcu_segcblist_entrain()
function.
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
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