Re: [RFC PATCH] mm, oom: disable dump_tasks by default

[Qian Cai <cai@lca.pw>]

On Tue, 2019-09-03 at 16:45 +0200, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.com>
> 
> dump_tasks has been introduced by quite some time ago fef1bdd68c81
> ("oom: add sysctl to enable task memory dump"). It's primary purpose is
> to help analyse oom victim selection decision. This has been certainly
> useful at times when the heuristic to chose a victim was much more
> volatile. Since a63d83f427fb ("oom: badness heuristic rewrite")
> situation became much more stable (mostly because the only selection
> criterion is the memory usage) and reports about a wrong process to
> be shot down have become effectively non-existent.

Well, I still see OOM sometimes kills wrong processes like ssh, systemd
processes while LTP OOM tests with staight-forward allocation patterns. I just
have not had a chance to debug them fully. The situation could be worse with
more complex allocations like random stress or fuzzy testing.

> 
> dump_tasks can generate a lot of output to the kernel log. It is not
> uncommon that even relative small system has hundreds of tasks running.
> Generating a lot of output to the kernel log both makes the oom report
> less convenient to process and also induces a higher load on the printk
> subsystem which can lead to other problems (e.g. longer stalls to flush
> all the data to consoles).

It is only generate output for the victim process where I tested on those large
NUMA machines and the output is fairly manageable.

> 
> Therefore change the default of oom_dump_tasks to not print the task
> list by default. The sysctl remains in place for anybody who might need
> to get this additional information. The oom report still provides an
> information about the allocation context and the state of the MM
> subsystem which should be sufficient to analyse most of the oom
> situations.
> 
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
>  mm/oom_kill.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index eda2e2a0bdc6..d0353705c6e6 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -52,7 +52,7 @@
>  
>  int sysctl_panic_on_oom;
>  int sysctl_oom_kill_allocating_task;
> -int sysctl_oom_dump_tasks = 1;
> +int sysctl_oom_dump_tasks;
>  
>  /*
>   * Serializes oom killer invocations (out_of_memory()) from all contexts to