Messages in this thread | | | Date | Wed, 30 Nov 2016 03:53:20 -0800 | From | "Paul E. McKenney" <> | Subject | Re: INFO: rcu_sched detected stalls on CPUs/tasks with `kswapd` and `mem_cgroup_shrink_node` |
| |
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.
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 >
| |