Re: [PATCH v3] memcg: fix soft lockup in the OOM process

[Chen Ridong <chenridong@huaweicloud.com>]

On 2025/1/4 0:18, Michal Koutný wrote:
> Hello.
> 
> On Tue, Dec 24, 2024 at 02:52:38AM +0000, Chen Ridong <chenridong@huaweicloud.com> wrote:
>> A soft lockup issue was found in the product with about 56,000 tasks were
>> in the OOM cgroup, it was traversing them when the soft lockup was
>> triggered.
> 
> Why is this softlockup a problem? 
> It's lot of tasks afterall and possibly a slow console (given looking
> for a victim among the comparable number didn't trigger it).
> 

It's not a slow console, but rather 'console pressure'. When a lot of
tasks apply to the console, it can make 'pr_info' slow. In my case,
these tasks will apply to the console. I reproduced this issue using a
test ko that creates many tasks, all of which just call 'pr_info'.

Best regards,
Ridong

>> To fix this issue, call 'cond_resched' in the 'mem_cgroup_scan_tasks'
>> function per 1000 iterations. For global OOM, call
>> 'touch_softlockup_watchdog' per 1000 iterations to avoid this issue.
> 
> This only hides the issue. It could be similarly fixed by simply
> decreasing loglevel= ;-)
> 
> cond_resched() in the memcg case may be OK but the arbitrary touch for
> global situation may hide possibly useful troubleshooting information.
> (Yeah, cond_resched() won't fit inside RCU section as in other global
> task iterations.)
> 
> 0.02€,
> Michal