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Subject: Re: [RFC PATCH] mm, oom: disable dump_tasks by default
To:     Michal Hocko <mhocko@kernel.org>
Cc:     David Rientjes <rientjes@google.com>, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>
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From:   Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Message-ID: <20ec856d-0f1e-8903-dbe0-bbc8b7a1847a@i-love.sakura.ne.jp>
Date:   Fri, 6 Sep 2019 19:46:10 +0900
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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