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:

quoted

[CCing Paul]

On Wed 30-11-16 11:28:34, Donald Buczek wrote:
[...]

quoted

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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