[PATCH v3] printk: allow direct console printing to be enabled always

[Jason A. Donenfeld]

In 5.19, there are some changes in printk message ordering /
interleaving which leads to confusion. The most obvious (and benign)
example appears on system boot, in which the "Run /init as init process"
message gets intermixed with the messages that init actually writes() to
stdout. For example, here's a snippet from build.wireguard.com:

    [    0.469732] Freeing unused kernel image (initmem) memory: 4576K
    [    0.469738] Write protecting the kernel read-only data: 10240k
    [    0.473823] Freeing unused kernel image (text/rodata gap) memory: 2044K
    [    0.475228] Freeing unused kernel image (rodata/data gap) memory: 1136K
    [    0.475236] Run /init as init process

        WireGuard Test Suite on Linux 5.19.0-rc2+ x86_64

    [+] Mounting filesystems...
    [+] Module self-tests:
     *  allowedips self-tests: pass
     *  nonce counter self-tests: pass
     *  ratelimiter self-tests: pass
    [+] Enabling logging...
    [+] Launching tests...
    [    0.475237]   with arguments:
    [    0.475238]     /init
    [    0.475238]   with environment:
    [    0.475239]     HOME=/
    [    0.475240]     TERM=linux
    [+] ip netns add wg-test-46-0
    [+] ip netns add wg-test-46-1

Before the "with arguments:" and such would print prior to the
"wireguard test suite on linux 5.19" banner. Now it shows after.

I see the same thing with "Freeing unused kernel image (text/rodata gap)
memory" printing interwoven into the console of my initramfs on my
laptop. And so forth.

But the bigger issue for me is that it makes it very confusing to
interpret CI results later on. Prior, I would nice a nice correlation
of:

[+] some userspace command
[    1.2345 ] some kernel log output
[+] some userspace command
[    1.2346 ] some kernel log output
[+] some userspace command
[    1.2347 ] some kernel log output

Now, the kernel log outputs are all over the place and out of order with
the sequence of commands. This makes debugging issues somewhat tricky,
because post hoc ergo propter hoc winds up being a good intuition to
follow when tracking down bugs, and now the post hoc part is muddled.

This is caused by threaded printk. In order to restore this in debugging
sessions and in CI, this commit adds the ability to always use direct
printk, either set by default at compile time, or overridden with a
runtime command line switch.

Cc: John Ogness <john.ogness@xxxxxxxxxxxxx>
Cc: Petr Mladek <pmladek@xxxxxxxx>
Cc: Marco Elver <elver@xxxxxxxxxx>
Fixes: 09c5ba0aa2fc ("printk: add kthread console printers")
Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
---
 Documentation/admin-guide/kernel-parameters.txt |  7 +++++++
 kernel/printk/printk.c                          | 11 +++++++++++
 lib/Kconfig.debug                               | 12 ++++++++++++
 3 files changed, 30 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 2522b11e593f..04cec66802d1 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4424,6 +4424,13 @@
 	printk.time=	Show timing data prefixed to each printk message line
 			Format: <bool>  (1/Y/y=enable, 0/N/n=disable)
 
+	printk.direct=	Rather than using kthreads for printk output, always
+			attempt to write to the console immediately. This has
+			performance implications, but will result in a more
+			faithful ordering and interleaving with other
+			processes writing to the console.
+			Format: <bool>  (1/Y/y=enable, 0/N/n=disable)
+
 	processor.max_cstate=	[HW,ACPI]
 			Limit processor to maximum C-state
 			max_cstate=9 overrides any DMI blacklist limit.
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index b095fb5f5f61..b7f8f2904f2c 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -178,6 +178,14 @@ static int __init control_devkmsg(char *str)
 }
 __setup("printk.devkmsg=", control_devkmsg);
 
+static bool printk_direct_only __initdata = IS_ENABLED(CONFIG_PRINTK_DIRECT);
+
+static int __init control_printk_direct_only(char *str)
+{
+	return kstrtobool(str, &printk_direct_only);
+}
+__setup("printk.direct=", control_printk_direct_only);
+
 char devkmsg_log_str[DEVKMSG_STR_MAX_SIZE] = "ratelimit";
 #if defined(CONFIG_PRINTK) && defined(CONFIG_SYSCTL)
 int devkmsg_sysctl_set_loglvl(struct ctl_table *table, int write,
@@ -3605,6 +3613,9 @@ static int __init printk_activate_kthreads(void)
 {
 	struct console *con;
 
+	if (printk_direct_only)
+		return 0;
+
 	console_lock();
 	printk_kthreads_available = true;
 	for_each_console(con)
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 2e24db4bff19..1acfb4971ec7 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -35,6 +35,18 @@ config PRINTK_CALLER
 	  no option to enable/disable at the kernel command line parameter or
 	  sysfs interface.
 
+config PRINTK_DIRECT
+	bool "Attempt to flush printk output immediately"
+	depends on PRINTK
+	help
+	  Rather than using kthreads for printk output, always attempt to write
+	  to the console immediately. This has performance implications, but
+	  will result in a more faithful ordering and interleaving with other
+	  processes writing to the console.
+
+	  Say N here unless you really need this. This may also be controlled
+	  at boot time with printk.direct=0/1.
+
 config STACKTRACE_BUILD_ID
 	bool "Show build ID information in stacktraces"
 	depends on PRINTK
-- 
2.35.1