Messages in this thread | | | Date | Wed, 30 Nov 2016 03:54:42 -0800 | From | "Paul E. McKenney" <> | Subject | Re: INFO: rcu_sched detected stalls on CPUs/tasks with `kswapd` and `mem_cgroup_shrink_node` |
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On Wed, Nov 30, 2016 at 03:53:20AM -0800, Paul E. McKenney wrote: > On Wed, Nov 30, 2016 at 12:09:44PM +0100, Michal Hocko wrote: > > [CCing Paul] > > > > On Wed 30-11-16 11:28:34, Donald Buczek wrote: > > [...] > > > shrink_active_list gets and releases the spinlock and calls cond_resched(). > > > This should give other tasks a chance to run. Just as an experiment, I'm > > > trying > > > > > > --- a/mm/vmscan.c > > > +++ b/mm/vmscan.c > > > @@ -1921,7 +1921,7 @@ static void shrink_active_list(unsigned long > > > nr_to_scan, > > > spin_unlock_irq(&pgdat->lru_lock); > > > > > > while (!list_empty(&l_hold)) { > > > - cond_resched(); > > > + cond_resched_rcu_qs(); > > > page = lru_to_page(&l_hold); > > > list_del(&page->lru); > > > > > > and didn't hit a rcu_sched warning for >21 hours uptime now. We'll see. > > > > This is really interesting! Is it possible that the RCU stall detector > > is somehow confused? > > No, it is not confused. Again, cond_resched() is not a quiescent > state unless it does a context switch. Therefore, if the task running > in that loop was the only runnable task on its CPU, cond_resched() > would -never- provide RCU with a quiescent state. > > In contrast, cond_resched_rcu_qs() unconditionally provides RCU > with a quiescent state (hence the _rcu_qs in its name), regardless > of whether or not a context switch happens. > > It is therefore expected behavior that this change might prevent > RCU CPU stall warnings.
I should add... This assumes that CONFIG_PREEMPT=n. So what is CONFIG_PREEMPT?
Thanx, Paul
> > > Is preemption disabled for another reason? > > > > I do not think so. I will have to double check the code but this is a > > standard sleepable context. Just wondering what is the PREEMPT > > configuration here? > > -- > > Michal Hocko > > SUSE Labs > >
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