Re: [PATCH] mm, oom_adj: avoid meaningless loop to find processes sharing mm

[Michal Hocko <mhocko@kernel.org>]

[Cc Oleg]

On Fri 05-10-18 15:32:08, Yong-Taek Lee wrote:
> It is introduced by commit 44a70adec910 ("mm, oom_adj: make sure
> processes sharing mm have same view of oom_score_adj"). Most of
> user process's mm_users is bigger than 1 but only one thread group.
> In this case, for_each_process loop meaninglessly try to find processes
> which sharing same mm even though there is only one thread group.
> 
> My idea is that target task's nr thread is smaller than mm_users if there
> are more thread groups sharing the same mm. So we can skip loop

I remember trying to optimize this but ended up with nothing that would
work reliable. E.g. what prevents a thread terminating right after we
read mm reference count and result in early break and other process
not being updated properly?

> if mm_user and nr_thread are same.
> 
> test result
> while true; do count=0; time while [ $count -lt 10000 ]; do echo -1000 > /proc/
> 1457/oom_score_adj; count=$((count+1)); done; done;

Is this overhead noticeable in a real work usecases though? Or are you
updating oom_score_adj that often really?

> before patch
> 0m00.59s real     0m00.09s user     0m00.51s system
> 0m00.59s real     0m00.14s user     0m00.45s system
> 0m00.58s real     0m00.11s user     0m00.47s system
> 0m00.58s real     0m00.10s user     0m00.48s system
> 0m00.59s real     0m00.11s user     0m00.48s system
> 
> after patch
> 0m00.15s real     0m00.07s user     0m00.08s system
> 0m00.14s real     0m00.10s user     0m00.04s system
> 0m00.14s real     0m00.10s user     0m00.05s system
> 0m00.14s real     0m00.08s user     0m00.07s system
> 0m00.14s real     0m00.08s user     0m00.07s system
> 
> Signed-off-by: Lee YongTaek <ytk.lee@samsung.com>
> ---
>  fs/proc/base.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index f9f72aee6d45..54b2fb5e9c51 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -1056,6 +1056,7 @@ static int __set_oom_adj(struct file *file, int oom_adj,
> bool legacy)
>         struct mm_struct *mm = NULL;
>         struct task_struct *task;
>         int err = 0;
> +       int mm_users = 0;
> 
>         task = get_proc_task(file_inode(file));
>         if (!task)
> @@ -1092,7 +1093,8 @@ static int __set_oom_adj(struct file *file, int oom_adj,
> bool legacy)
>                 struct task_struct *p = find_lock_task_mm(task);
> 
>                 if (p) {
> -                       if (atomic_read(&p->mm->mm_users) > 1) {
> +                       mm_users = atomic_read(&p->mm->mm_users);
> +                       if ((mm_users > 1) && (mm_users != get_nr_threads(p)))
> {
>                                 mm = p->mm;
>                                 atomic_inc(&mm->mm_count);
>                         }
> --
> 
> *

-- 
Michal Hocko
SUSE Labs