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

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

[Michal Hocko]

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.

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).

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
-- 
2.20.1

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

[Qian Cai]

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

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

[Michal Hocko]

On Tue 03-09-19 11:02:46, Qian Cai wrote:
> 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.

Please report those. Most cases I have seen so far just turned out to
work as expected and memory hogs just used oom_score_adj or similar.

> 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.

Nothing really prevents enabling the sysctl when doing OOM oriented
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.

The main question here is whether that information is useful by
_default_ because it is certainly not free. It takes both time to crawl
all processes and cpu cycles to get that information to the console
because printk is not free either. So if it more of "nice to have" than
necessary for oom analysis then it should be disabled by default IMHO.

-- 
Michal Hocko
SUSE Labs

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

[Qian Cai]

On Tue, 2019-09-03 at 17:13 +0200, Michal Hocko wrote:
> On Tue 03-09-19 11:02:46, Qian Cai wrote:
> > 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.
> 
> Please report those. Most cases I have seen so far just turned out to
> work as expected and memory hogs just used oom_score_adj or similar.
> 
> > 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.
> 
> Nothing really prevents enabling the sysctl when doing OOM oriented
> 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.
> 
> The main question here is whether that information is useful by
> _default_ because it is certainly not free. It takes both time to crawl
> all processes and cpu cycles to get that information to the console
> because printk is not free either. So if it more of "nice to have" than
> necessary for oom analysis then it should be disabled by default IMHO.

It also feels like more a band-aid micro-optimization with the side-effect that
affecting debuggability, as there could be loads of console output anyway during
a kernel OOM event including failed allocation warnings. I suppose if you want
to change the default behavior, the bar is high with more data and
justification.

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

[Michal Hocko]

On Tue 03-09-19 11:32:58, Qian Cai wrote:
> On Tue, 2019-09-03 at 17:13 +0200, Michal Hocko wrote:
> > On Tue 03-09-19 11:02:46, Qian Cai wrote:
> > > 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.
> > 
> > Please report those. Most cases I have seen so far just turned out to
> > work as expected and memory hogs just used oom_score_adj or similar.
> > 
> > > 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.
> > 
> > Nothing really prevents enabling the sysctl when doing OOM oriented
> > 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.
> > 
> > The main question here is whether that information is useful by
> > _default_ because it is certainly not free. It takes both time to crawl
> > all processes and cpu cycles to get that information to the console
> > because printk is not free either. So if it more of "nice to have" than
> > necessary for oom analysis then it should be disabled by default IMHO.
> 
> It also feels like more a band-aid micro-optimization with the side-effect that
> affecting debuggability, as there could be loads of console output anyway during
> a kernel OOM event including failed allocation warnings. I suppose if you want
> to change the default behavior, the bar is high with more data and
> justification.

Any specific idea what that justification should be?

Because this is not something that you could measure very easily. It is
very subjective and far from black and white. And I am fully aware of
that. Hence RFC. That is why we should apply some common sense
and cost/benefit evaluation.

The cost of an additional output should be quite clear. Now we can
argue about the benefit. I argue that for an absolute majority of oom
report I have seen throughout past many years the task list was the
least useful information of the report. Sure I could go there and double
check that the victim was selected as designed. In minority cases I
could use that the task list to confirm that expected-to-be-victim had
OOM_SCORE_ADJ_MIN for some reason and that was why something esle has
been selected. Those configuration issues tend to be reproducible and
are easier to debug with sysctl enabled.

All that being said, arguments tend to weigh more towards IMO.
-- 
Michal Hocko
SUSE Labs

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

[Tetsuo Handa]

On 2019/09/03 23:45, Michal Hocko wrote:
> It's primary purpose is
> to help analyse oom victim selection decision.

I disagree, for I use the process list for understanding what / how many
processes are consuming what kind of memory (without crashing the system)
for anomaly detection purpose. Although we can't dump memory consumed by
e.g. file descriptors, disabling dump_tasks() loose that clue, and is
problematic for me.

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

[Michal Hocko]

On Wed 04-09-19 05:52:43, Tetsuo Handa wrote:
> On 2019/09/03 23:45, Michal Hocko wrote:
> > It's primary purpose is
> > to help analyse oom victim selection decision.
> 
> I disagree, for I use the process list for understanding what / how many
> processes are consuming what kind of memory (without crashing the system)
> for anomaly detection purpose. Although we can't dump memory consumed by
> e.g. file descriptors, disabling dump_tasks() loose that clue, and is
> problematic for me.

Does anything really prevent you from enabling this by sysctl though? Or
do you claim that this is a general usage pattern and therefore the
default change is not acceptable or do you want a changelog to be
updated?

-- 
Michal Hocko
SUSE Labs

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

[David Rientjes]

On Wed, 4 Sep 2019, Michal Hocko wrote:

> > > It's primary purpose is
> > > to help analyse oom victim selection decision.
> > 
> > I disagree, for I use the process list for understanding what / how many
> > processes are consuming what kind of memory (without crashing the system)
> > for anomaly detection purpose. Although we can't dump memory consumed by
> > e.g. file descriptors, disabling dump_tasks() loose that clue, and is
> > problematic for me.
> 
> Does anything really prevent you from enabling this by sysctl though? Or
> do you claim that this is a general usage pattern and therefore the
> default change is not acceptable or do you want a changelog to be
> updated?
> 

I think the motivation is that users don't want to need to reproduce an 
oom kill to figure out why: they want to be able to figure out which 
process had higher than normal memory usage.  If oom is the normal case 
then they ceratinly have the ability to disable it by disabling the 
sysctl, but that seems like something better to opt-out of rather than 
need to opt-in to and reproduce.

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

[Tetsuo Handa]

On 2019/09/05 5:04, David Rientjes wrote:
> On Wed, 4 Sep 2019, Michal Hocko wrote:
> 
>>>> It's primary purpose is
>>>> to help analyse oom victim selection decision.
>>>
>>> I disagree, for I use the process list for understanding what / how many
>>> processes are consuming what kind of memory (without crashing the system)
>>> for anomaly detection purpose. Although we can't dump memory consumed by
>>> e.g. file descriptors, disabling dump_tasks() loose that clue, and is
>>> problematic for me.
>>
>> Does anything really prevent you from enabling this by sysctl though? Or
>> do you claim that this is a general usage pattern and therefore the
>> default change is not acceptable or do you want a changelog to be
>> updated?
>>
> 
> I think the motivation is that users don't want to need to reproduce an 
> oom kill to figure out why: they want to be able to figure out which 
> process had higher than normal memory usage.

Right. Users can't reproduce an OOM kill to figure out why. Those who do
failure analysis for users (e.g. technical staff at support center) need to
figure out system's state when an OOM kill happened. And installing dynamic
hooks like SystemTap / eBPF is hardly acceptable for users.

What I don't like is that Michal refuses to solve OOM stalling problem,
does not try to understand how difficult to avoid problems caused by
thoughtless printk(), and instead recommending to disable oom_dump_tasks.

There is nothing that prevents users from enabling oom_dump_tasks by sysctl.
But that requires a solution for OOM stalling problem. Since I know how
difficult to avoid problems caused by printk() flooding, I insist that
we need "mm,oom: Defer dump_tasks() output." patch.

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

[Michal Hocko]

On Thu 05-09-19 22:39:47, Tetsuo Handa wrote:
[...]
> There is nothing that prevents users from enabling oom_dump_tasks by sysctl.
> But that requires a solution for OOM stalling problem.

You can hardly remove stalling if you are not reducing the amount of
output or get it into a different context. Whether the later is
reasonable is another question but you are essentially losing "at the
OOM event state".

> Since I know how
> difficult to avoid problems caused by printk() flooding, I insist that
> we need "mm,oom: Defer dump_tasks() output." patch.

insisting is not a way to cooperate.
-- 
Michal Hocko
SUSE Labs

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

[Qian Cai]

On Tue, 2019-09-03 at 17:13 +0200, Michal Hocko wrote:
> On Tue 03-09-19 11:02:46, Qian Cai wrote:
> > 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.
> 
> Please report those. Most cases I have seen so far just turned out to
> work as expected and memory hogs just used oom_score_adj or similar.

Here is the one where oom01 should be one to be killed.

[92598.855697][ T2588] Swap cache stats: add 105240923, delete 105250445, find
42196/101577
[92598.893970][ T2588] Free swap  = 16383612kB
[92598.913482][ T2588] Total swap = 16465916kB
[92598.932938][ T2588] 7275091 pages RAM
[92598.950212][ T2588] 0 pages HighMem/MovableOnly
[92598.971539][ T2588] 1315554 pages reserved
[92598.990698][ T2588] 16384 pages cma reserved
[92599.010760][ T2588] Tasks state (memory values in pages):
[92599.036265][ T2588] [  pid  ]   uid  tgid total_vm      rss pgtables_bytes
swapents oom_score_adj name
[92599.080129][ T2588]
[   1662]     0  1662    29511     1034   290816      244             0 systemd-
journal
[92599.126163][ T2588]
[   2586]   998  2586   508086        0   368640     1838             0 polkitd
[92599.168706][ T2588]
[   2587]     0  2587    52786        0   421888      500             0 sssd
[92599.210082][ T2588]
[   2588]     0  2588    31223        0   139264      195             0
irqbalance
[92599.255606][ T2588]
[   2589]    81  2589    18381        0   167936      217          -900 dbus-
daemon
[92599.303678][ T2588]
[   2590]     0  2590    97260      193   372736      573             0
NetworkManager
[92599.348957][ T2588]
[   2594]     0  2594    95350        1   229376      758             0 rngd
[92599.390216][ T2588]
[   2598]   995  2598     7364        0    94208      103             0 chronyd
[92599.432447][ T2588]
[   2629]     0  2629   106234      399   442368     3836             0 tuned
[92599.473950][ T2588]
[   2638]     0  2638    23604        0   212992      240         -1000 sshd
[92599.515158][ T2588]
[   2642]     0  2642    10392        0   102400      138             0
rhsmcertd
[92599.560435][ T2588]
[   2691]     0  2691    21877        0   208896      277             0 systemd-
logind
[92599.605035][ T2588]
[   2700]     0  2700     3916        0    69632       45             0 agetty
[92599.646750][ T2588]
[   2705]     0  2705    23370        0   225280      393             0 systemd
[92599.688063][ T2588]
[   2730]     0  2730    37063        0   294912      667             0 (sd-pam)
[92599.729028][ T2588]
[   2922]     0  2922     9020        0    98304      232             0 crond
[92599.769130][ T2588]
[   3036]     0  3036    37797        1   307200      305             0 sshd
[92599.813768][ T2588]
[   3057]     0  3057    37797        0   303104      335             0 sshd
[92599.853450][ T2588]
[   3065]     0  3065     6343        1    86016      163             0 bash
[92599.892899][ T2588] [  38249]     0
38249    58330      293   221184      246             0 rsyslogd
[92599.934457][ T2588] [  11329]     0
11329    55131       73   454656      396             0 sssd_nss
[92599.976240][ T2588] [  11331]     0
11331    54424        1   434176      610             0 sssd_be
[92600.017106][ T2588] [  25247]     0
25247    25746        1   212992      300         -1000 systemd-udevd
[92600.060539][ T2588] [  25391]     0
25391     2184        0    65536       32             0 oom01
[92600.100648][ T2588] [  25392]     0
25392     2184        0    65536       39             0 oom01
[92600.143516][ T2588] oom-
kill:constraint=CONSTRAINT_NONE,nodemask=(null),cpuset=/,mems_allowed=0-
1,global_oom,task_memcg=/system.slice/tuned.service,task=tuned,pid=2629,uid=0
[92600.213724][ T2588] Out of memory: Killed process 2629 (tuned) total-
vm:424936kB, anon-rss:328kB, file-rss:1268kB, shmem-rss:0kB, UID:0
pgtables:442368kB oom_score_adj:0
[92600.297832][  T305] oom_reaper: reaped process 2629 (tuned), now anon-
rss:0kB, file-rss:0kB, shmem-rss:0kB


> 
> > 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.

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

[Tetsuo Handa]

On 2019/09/05 23:08, Michal Hocko wrote:
> On Thu 05-09-19 22:39:47, Tetsuo Handa wrote:
> [...]
>> There is nothing that prevents users from enabling oom_dump_tasks by sysctl.
>> But that requires a solution for OOM stalling problem.
> 
> You can hardly remove stalling if you are not reducing the amount of
> output or get it into a different context. Whether the later is
> reasonable is another question but you are essentially losing "at the
> OOM event state".
> 

I am not losing "at the OOM event state". Please find "struct oom_task_info"
(for now) embedded into "struct task_struct" which holds "at the OOM event state".

And my patch moves "printk() from dump_tasks()" from OOM context to WQ context.
Thus, I do remove stalling by defer printing of "struct oom_task_info" until
the OOM killer sends SIGKILL and the OOM reaper starts reclaiming memory.

--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -639,6 +639,21 @@ struct wake_q_node {
 	struct wake_q_node *next;
 };
 
+/* Memory usage and misc info as of invocation of OOM killer. */
+struct oom_task_info {
+	struct list_head list;
+	unsigned int seq;
+	char comm[TASK_COMM_LEN];
+	pid_t pid;
+	uid_t uid;
+	pid_t tgid;
+	unsigned long total_vm;
+	unsigned long mm_rss;
+	unsigned long pgtables_bytes;
+	unsigned long swapents;
+	int score_adj;
+};
+
 struct task_struct {
 #ifdef CONFIG_THREAD_INFO_IN_TASK
 	/*
@@ -1260,7 +1275,7 @@ struct task_struct {
 #ifdef CONFIG_MMU
 	struct task_struct		*oom_reaper_list;
 #endif
-	struct list_head		oom_victim_list;
+	struct oom_task_info		oom_task_info;
 #ifdef CONFIG_VMAP_STACK
 	struct vm_struct		*stack_vm_area;
 #endif

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

[Michal Hocko]

On Fri 06-09-19 19:46:10, Tetsuo Handa wrote:
> On 2019/09/05 23:08, Michal Hocko wrote:
> > On Thu 05-09-19 22:39:47, Tetsuo Handa wrote:
> > [...]
> >> There is nothing that prevents users from enabling oom_dump_tasks by sysctl.
> >> But that requires a solution for OOM stalling problem.
> > 
> > You can hardly remove stalling if you are not reducing the amount of
> > output or get it into a different context. Whether the later is
> > reasonable is another question but you are essentially losing "at the
> > OOM event state".
> > 
> 
> I am not losing "at the OOM event state". Please find "struct oom_task_info"
> (for now) embedded into "struct task_struct" which holds "at the OOM event state".
> 
> And my patch moves "printk() from dump_tasks()" from OOM context to WQ context.

Workers might be blocked for unbound amount of time and so this
information might be printed late.
-- 
Michal Hocko
SUSE Labs

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

[Tetsuo Handa]

On 2019/09/06 20:02, Michal Hocko wrote:
> On Fri 06-09-19 19:46:10, Tetsuo Handa wrote:
>> On 2019/09/05 23:08, Michal Hocko wrote:
>>> On Thu 05-09-19 22:39:47, Tetsuo Handa wrote:
>>> [...]
>>>> There is nothing that prevents users from enabling oom_dump_tasks by sysctl.
>>>> But that requires a solution for OOM stalling problem.
>>>
>>> You can hardly remove stalling if you are not reducing the amount of
>>> output or get it into a different context. Whether the later is
>>> reasonable is another question but you are essentially losing "at the
>>> OOM event state".
>>>
>>
>> I am not losing "at the OOM event state". Please find "struct oom_task_info"
>> (for now) embedded into "struct task_struct" which holds "at the OOM event state".
>>
>> And my patch moves "printk() from dump_tasks()" from OOM context to WQ context.
> 
> Workers might be blocked for unbound amount of time and so this
> information might be printed late.
> 

Yes, but the OOM reaper will quickly reclaim memory. And if WQ is blocked, new WQ
for processing this work will be created (because OOM situation is quickly solved).

Nonetheless if your worry turns out to be a real problem, we can use a dedicated WQ
or offload to e.g. the OOM reaper kernel thread. Anyway, such tuning is beyond the
scope of my patch.