linux-kernel - Re: INFO: rcu_sched detected stalls on CPUs/tasks with `kswapd` and `mem_cgroup_shrink

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Date:   Wed, 30 Nov 2016 12:09:44 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Donald Buczek <buczek@...gen.mpg.de>
Cc:     Paul Menzel <pmenzel@...gen.mpg.de>, dvteam@...gen.mpg.de,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: INFO: rcu_sched detected stalls on CPUs/tasks with `kswapd` and
 `mem_cgroup_shrink_node`

[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?

> 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