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>
 <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>
 <29196f89-c35e-f79d-8e4d-2bf73fe930df@molgen.mpg.de>
 <20161130110944.GD18432@dhcp22.suse.cz>
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>
From: Donald Buczek <buczek@molgen.mpg.de>
Message-ID: <099c4569-a010-5414-0934-6af3734d8460@molgen.mpg.de>
Date: Wed, 30 Nov 2016 12:43:44 +0100
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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@null:~$ 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@molgen.mpg.de
Tel: +49 30 8413 1433