Re: [PATCH v2] mm, oom: make the calculation of oom badness more accurate

[Michal Hocko <mhocko@kernel.org>]

On Thu 09-07-20 11:53:07, Yafang Shao wrote:
> Recently we found an issue on our production environment that when memcg
> oom is triggered the oom killer doesn't chose the process with largest
> resident memory but chose the first scanned process. Note that all
> processes in this memcg have the same oom_score_adj, so the oom killer
> should chose the process with largest resident memory.
> 
> Bellow is part of the oom info, which is enough to analyze this issue.
> [7516987.983223] memory: usage 16777216kB, limit 16777216kB, failcnt 52843037
> [7516987.983224] memory+swap: usage 16777216kB, limit 9007199254740988kB, failcnt 0
> [7516987.983225] kmem: usage 301464kB, limit 9007199254740988kB, failcnt 0
> [...]
> [7516987.983293] [ pid ]   uid  tgid total_vm      rss pgtables_bytes swapents oom_score_adj name
> [7516987.983510] [ 5740]     0  5740      257        1    32768        0          -998 pause
> [7516987.983574] [58804]     0 58804     4594      771    81920        0          -998 entry_point.bas
> [7516987.983577] [58908]     0 58908     7089      689    98304        0          -998 cron
> [7516987.983580] [58910]     0 58910    16235     5576   163840        0          -998 supervisord
> [7516987.983590] [59620]     0 59620    18074     1395   188416        0          -998 sshd
> [7516987.983594] [59622]     0 59622    18680     6679   188416        0          -998 python
> [7516987.983598] [59624]     0 59624  1859266     5161   548864        0          -998 odin-agent
> [7516987.983600] [59625]     0 59625   707223     9248   983040        0          -998 filebeat
> [7516987.983604] [59627]     0 59627   416433    64239   774144        0          -998 odin-log-agent
> [7516987.983607] [59631]     0 59631   180671    15012   385024        0          -998 python3
> [7516987.983612] [61396]     0 61396   791287     3189   352256        0          -998 client
> [7516987.983615] [61641]     0 61641  1844642    29089   946176        0          -998 client
> [7516987.983765] [ 9236]     0  9236     2642      467    53248        0          -998 php_scanner
> [7516987.983911] [42898]     0 42898    15543      838   167936        0          -998 su
> [7516987.983915] [42900]  1000 42900     3673      867    77824        0          -998 exec_script_vr2
> [7516987.983918] [42925]  1000 42925    36475    19033   335872        0          -998 python
> [7516987.983921] [57146]  1000 57146     3673      848    73728        0          -998 exec_script_J2p
> [7516987.983925] [57195]  1000 57195   186359    22958   491520        0          -998 python2
> [7516987.983928] [58376]  1000 58376   275764    14402   290816        0          -998 rosmaster
> [7516987.983931] [58395]  1000 58395   155166     4449   245760        0          -998 rosout
> [7516987.983935] [58406]  1000 58406 18285584  3967322 37101568        0          -998 data_sim
> [7516987.984221] oom-kill:constraint=CONSTRAINT_MEMCG,nodemask=(null),cpuset=3aa16c9482ae3a6f6b78bda68a55d32c87c99b985e0f11331cddf05af6c4d753,mems_allowed=0-1,oom_memcg=/kubepods/podf1c273d3-9b36-11ea-b3df-246e9693c184,task_memcg=/kubepods/podf1c273d3-9b36-11ea-b3df-246e9693c184/1f246a3eeea8f70bf91141eeaf1805346a666e225f823906485ea0b6c37dfc3d,task=pause,pid=5740,uid=0
> [7516987.984254] Memory cgroup out of memory: Killed process 5740 (pause) total-vm:1028kB, anon-rss:4kB, file-rss:0kB, shmem-rss:0kB
> [7516988.092344] oom_reaper: reaped process 5740 (pause), now anon-rss:0kB, file-rss:0kB, shmem-rss:0kB
> 
> We can find that the first scanned process 5740 (pause) was killed, but its
> rss is only one page. That is because, when we calculate the oom badness in
> oom_badness(), we always ignore the negtive point and convert all of these
> negtive points to 1. Now as oom_score_adj of all the processes in this
> targeted memcg have the same value -998, the points of these processes are
> all negtive value. As a result, the first scanned process will be killed.
> 
> The oom_socre_adj (-998) in this memcg is set by kubelet, because it is a
> a Guaranteed pod, which has higher priority to prevent from being killed by
> system oom.
> 
> To fix this issue, we should make the calculation of oom point more
> accurate. We can achieve it by convert the chosen_point from 'unsigned
> long' to 'long'.
> 
> Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> ---
>  drivers/tty/sysrq.c |  1 +
>  fs/proc/base.c      |  7 ++++++-
>  include/linux/oom.h |  4 ++--
>  mm/memcontrol.c     |  1 +
>  mm/oom_kill.c       | 19 ++++++++-----------
>  mm/page_alloc.c     |  1 +
>  6 files changed, 19 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
> index 7c95afa9..e83fd46 100644
> --- a/drivers/tty/sysrq.c
> +++ b/drivers/tty/sysrq.c
> @@ -382,6 +382,7 @@ static void moom_callback(struct work_struct *ignored)
>  		.memcg = NULL,
>  		.gfp_mask = gfp_mask,
>  		.order = -1,
> +		.chosen_points = LONG_MIN,

It would be better to do the initialization only once when we start
evaluating tasks (select_bad_process).

>  	};
>  
>  	mutex_lock(&oom_lock);
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index d86c0af..bf16406 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -551,8 +551,13 @@ static int proc_oom_score(struct seq_file *m, struct pid_namespace *ns,
>  {
>  	unsigned long totalpages = totalram_pages() + total_swap_pages;
>  	unsigned long points = 0;
> +	long badness;
>  
> -	points = oom_badness(task, totalpages) * 1000 / totalpages;
> +	badness = oom_badness(task, totalpages);
> +	if (badness != LONG_MIN) {
> +		/* Let's keep the range of points as [0, 2000]. */
> +		points = (1000 + badness * 1000 / (long)totalpages) * 2 / 3;
> +	}
>  	seq_printf(m, "%lu\n", points);

This doesn't really work for OOM_SCORE_ADJ_MIN cases because they
will simply print LONG_MIN rather than 0.

So you want
	/*
	 * Special case OOM_SCORE_ADJ_MIN for all others scale the
	 * badness value into [0, 2000] range which we have been
	 * exporting for a long time so userspace might depend on it
	 */
	if (badness == LONG_MIN)
		badness = 0;
	else
		points = (1000 + badness * 1000 / (long)totalpages) * 2 / 3

FTR. In my other email I was proposing to scale usage to the [-1000, 1000]
range by
	points = adj + usage * 1000/ totalpages

this would make the math slightly easier to follow but then I've
realized that this would be much less precise so what you have is
better. Btw. we used to do that in the past until a7f638f999ff4
which has changed that for this very reason.

> @@ -107,7 +107,7 @@ static inline vm_fault_t check_stable_address_space(struct mm_struct *mm)
>  
>  bool __oom_reap_task_mm(struct mm_struct *mm);
>  
> -extern unsigned long oom_badness(struct task_struct *p,
> +long oom_badness(struct task_struct *p,
>  		unsigned long totalpages);

This is not really necessary.
  
With that being addressed, you can add
Acked-by: Michal Hocko <mhocko@suse.com>

Thanks!
-- 
Michal Hocko
SUSE Labs