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

Jason A. Donenfeld
2022-06-17T13:39:04Z

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@linutronix.de>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Marco Elver <elver@google.com>
Fixes: 09c5ba0aa2fc ("printk: add kthread console printers")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
Documentation/admin-guide/kernel-parameters.txt | 8 ++++++++
init/Kconfig | 12 ++++++++++++
kernel/printk/printk.c | 12 ++++++++++++
3 files changed, 32 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 8090130b544b..a960c47a2002 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4389,6 +4389,14 @@
printk.time= Show timing data prefixed to each printk message line
Format: <bool> (1/Y/y=enable, 0/N/n=disable)

+ printk.always_direct=
+ Rather than using kthreads for printk output, always
+ 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/init/Kconfig b/init/Kconfig
index c7900e8975f1..7676897f2321 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -798,6 +798,18 @@ config PRINTK_INDEX

There is no additional runtime cost to printk with this enabled.

+config PRINTK_ALWAYS_DIRECT
+ bool "Flush printk output immediately"
+ depends on PRINTK
+ help
+ Rather than using kthreads for printk output, always 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.always_direct=0/1.
+
#
# Architectures with an unreliable sched_clock() should select this:
#
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index ea3dd55709e7..d9f419a88429 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 always_direct_printk = IS_ENABLED(CONFIG_PRINTK_ALWAYS_DIRECT);
+
+static int __init control_always_direct_printk(char *str)
+{
+ return kstrtobool(str, &always_direct_printk);
+}
+__setup("printk.always_direct=", control_always_direct_printk);
+
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,
@@ -471,6 +479,10 @@ void printk_prefer_direct_exit(void)
*/
static inline bool allow_direct_printing(void)
{
+ /* If the user has explicitly enabled this to be on always. */
+ if (always_direct_printk)
+ return true;
+
/*
* Checking kthread availability is a possible race because the
* kthread printers can become permanently disabled during runtime.
--
2.35.1