Date: Wed, 30 Nov 2016 12:09:44 +0100
From: Michal Hocko <mhocko@kernel.org>
To: Donald Buczek <buczek@molgen.mpg.de>
Cc: Paul Menzel <pmenzel@molgen.mpg.de>, dvteam@molgen.mpg.de,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Josh Triplett <josh@joshtriplett.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: Re: INFO: rcu_sched detected stalls on CPUs/tasks with `kswapd` and
 `mem_cgroup_shrink_node`
Message-ID: <20161130110944.GD18432@dhcp22.suse.cz>
References: <20161121134130.GB18112@dhcp22.suse.cz>
 <20161121140122.GU3612@linux.vnet.ibm.com>
 <20161121141818.GD18112@dhcp22.suse.cz>
 <20161121142901.GV3612@linux.vnet.ibm.com>
 <68025f6c-6801-ab46-b0fc-a9407353d8ce@molgen.mpg.de>
 <20161124101525.GB20668@dhcp22.suse.cz>
 <583AA50A.9010608@molgen.mpg.de>
 <20161128110449.GK14788@dhcp22.suse.cz>
 <109d5128-f3a4-4b6e-db17-7a1fcb953500@molgen.mpg.de>
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[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