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 12:43:44 +0100
From:   Donald Buczek <buczek@...gen.mpg.de>
To:     Michal Hocko <mhocko@...nel.org>
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`

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.

>> 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?

buczek@...l:~$ zcat /proc/config.gz |grep PREE
CONFIG_PREEMPT_NOTIFIERS=y
# CONFIG_PREEMPT_NONE is not set
CONFIG_PREEMPT_VOLUNTARY=y
# CONFIG_PREEMPT is not set

Thanks
   Donald

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