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Subject: Re: INFO: rcu_sched detected stalls on CPUs/tasks with `kswapd` and
 `mem_cgroup_shrink_node`
To: Michal Hocko <mhocko@kernel.org>
References: <20161121134130.GB18112@dhcp22.suse.cz>
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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>,
        Peter Zijlstra <peterz@infradead.org>
From: Donald Buczek <buczek@molgen.mpg.de>
Message-ID: <6c8599e5-7f4c-c60b-8954-08168af6343b@molgen.mpg.de>
Date: Fri, 2 Dec 2016 10:14:45 +0100
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On 11/30/16 12:43, Donald Buczek wrote:
> On 11/30/16 12:09, 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?
>
> Wait... 21 hours is not yet a test result.

For the records: We didn't have any stall warnings after 2 days and 20 
hours now and so I'm quite confident, that my above patch fixed the 
problem for v4.8.0. On previous boots the rcu warnings started after 
37,0.2,1,2,0.8 hours uptime.

Now I've applied this patch to stable latest (v4.8.11) on another backup 
machine which suffered even more rcu stalls.

Donald

> [...]

-- 
Donald Buczek
buczek@molgen.mpg.de
Tel: +49 30 8413 1433