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

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