Re: [nacked] mm-oom-avoid-printk-iteration-under-rcu.patch removed from -mm tree

[Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>]

On 2020/04/23 20:07, Michal Hocko wrote:
> The existing loglevels we use are not really carved in stone and we can
> prioritize some more than others. dump_tasks is KERN_INFO already and
> this is quite a low priority so you shouldn't really miss much when
> omitting it. But I wouldn't mind making it KERN_DEBUG.

Do you mean

-       pr_info("[%7d] %5d %5d %8lu %8lu %8ld %8lu         %5hd %s\n",
+       pr_debug("[%7d] %5d %5d %8lu %8lu %8ld %8lu         %5hd %s\n",

in order to suppress printing to consoles? Wow! That will also suppress
saving to log files because syslog daemon is likely configured not to save
KERN_DEBUG level messages.

>> - tune the dump_tasks specifically （vm.oom_dump_tasks）
>> All the consumers are effected.
>> The logfile is fast enough, so we expect that these dump_tasks could
>> be printed into the logfile.
>> The console is so slow that we don't want to print into it.
>> A possilbe way to fix it is improve vm.oom_dump_tasks.
>>     vm.oom_dump_tasks : 1 - dump into all consumers
>>                                          2 - don't dump into console
>>                                          0 - don't dump into any of
> 
> How would that be implemented. I do not know of a way to tell printk
> which consoles to use for the output.  Anyway, isn't this something
> that can be configured on the printk level. In other words send only
> important information to slow consoles?

Last year I proposed
https://lkml.kernel.org/r/1550896930-12324-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp
and Sergey Senozhatsky commented

  "This is a bit of a strange issue, to be honest. If OOM prints too
   many messages then we might want to do some work on the OOM side."

. I was thinking that printing to consoles is a requirement for oom_dump_tasks .

If we can agree with not printing dump_tasks() output to consoles, a trivial
patch shown below will solve the problem. Those who cannot run syslog daemon
in userspace might disagree, but this will be the simplest answer.

 include/linux/kern_levels.h |    3 +++
 include/linux/printk.h      |    1 +
 kernel/printk/printk.c      |    7 ++++++-
 mm/oom_kill.c               |    7 ++++---
 4 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/include/linux/kern_levels.h b/include/linux/kern_levels.h
index bf2389c26ae3..cd69a9cb3c2a 100644
--- a/include/linux/kern_levels.h
+++ b/include/linux/kern_levels.h
@@ -23,6 +23,9 @@
  */
 #define KERN_CONT	KERN_SOH "c"
 
+/* Annotation for "don't print to consoles". */
+#define KERN_NO_CONSOLES KERN_SOH "S"
+
 /* integer equivalents of KERN_<LEVEL> */
 #define LOGLEVEL_SCHED		-2	/* Deferred messages from sched code
 					 * are set to this special level */
diff --git a/include/linux/printk.h b/include/linux/printk.h
index e061635e0409..da338b81c2e1 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -19,6 +19,7 @@ static inline int printk_get_level(const char *buffer)
 		switch (buffer[1]) {
 		case '0' ... '7':
 		case 'c':	/* KERN_CONT */
+		case 'S':       /* KERN_NO_CONSOLES */
 			return buffer[1];
 		}
 	}
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 9a9b6156270b..ed51641af087 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -361,6 +361,7 @@ static int console_msg_format = MSG_FORMAT_DEFAULT;
  */
 
 enum log_flags {
+	LOG_NO_CONSOLES = 1,    /* don't print to consoles */
 	LOG_NEWLINE	= 2,	/* text ended with a newline */
 	LOG_CONT	= 8,	/* text is a fragment of a continuation line */
 };
@@ -1959,6 +1960,9 @@ int vprintk_store(int facility, int level,
 				break;
 			case 'c':	/* KERN_CONT */
 				lflags |= LOG_CONT;
+				break;
+			case 'S':       /* KERN_NO_CONSOLES */
+				lflags |= LOG_NO_CONSOLES;
 			}
 
 			text_len -= 2;
@@ -2453,7 +2457,8 @@ void console_unlock(void)
 			break;
 
 		msg = log_from_idx(console_idx);
-		if (suppress_message_printing(msg->level)) {
+		if ((msg->flags & LOG_NO_CONSOLES) ||
+		    suppress_message_printing(msg->level)) {
 			/*
 			 * Skip record we have buffered and already printed
 			 * directly to the console when we received it, and
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index dfc357614e56..0b487c13a2c9 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -400,7 +400,7 @@ static int dump_task(struct task_struct *p, void *arg)
 		return 0;
 	}
 
-	pr_info("[%7d] %5d %5d %8lu %8lu %8ld %8lu         %5hd %s\n",
+	pr_info(KERN_NO_CONSOLES "[%7d] %5d %5d %8lu %8lu %8ld %8lu         %5hd %s\n",
 		task->pid, from_kuid(&init_user_ns, task_uid(task)),
 		task->tgid, task->mm->total_vm, get_mm_rss(task->mm),
 		mm_pgtables_bytes(task->mm),
@@ -423,8 +423,9 @@ static int dump_task(struct task_struct *p, void *arg)
  */
 static void dump_tasks(struct oom_control *oc)
 {
-	pr_info("Tasks state (memory values in pages):\n");
-	pr_info("[  pid  ]   uid  tgid total_vm      rss pgtables_bytes swapents oom_score_adj name\n");
+	pr_info_once("Tasks state is sent to syslog.\n");
+	pr_info(KERN_NO_CONSOLES "Tasks state (memory values in pages):\n");
+	pr_info(KERN_NO_CONSOLES "[  pid  ]   uid  tgid total_vm      rss pgtables_bytes swapents oom_score_adj name\n");
 
 	if (is_memcg_oom(oc))
 		mem_cgroup_scan_tasks(oc->memcg, dump_task, oc);