Re: INFO: rcu_sched detected stalls on CPUs/tasks with `kswapd` and `mem_cgroup_shrink_node`

[Michal Hocko]

On Wed 30-11-16 03:53:20, 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.

Sorry for being dense here. But why cannot we hide the QS handling into
cond_resched()? I mean doesn't every current usage of cond_resched
suffer from the same problem wrt RCU stalls?

> 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

-- 
Michal Hocko
SUSE Labs