Re: [PATCH v2 3/3] printk: Use console_flush_one_record for legacy printer kthread

[Petr Mladek <pmladek@suse.com>]

Here is the same patch with the alternative semantic of
console_flush_one_record() as proposed at
https://lore.kernel.org/all/aN_3id2CF7ivC42R@pathway.suse.cz/

The return value from console_flush_all() can be ignored.
It is enough to check whether @try_again is true.

I hope that this semantic is more straightforward.

Here to go:

From 51667c506c13f662d222c7907e6a055ae77639a0 Mon Sep 17 00:00:00 2001
From: Andrew Murray <amurray@thegoodpenguin.co.uk>
Date: Sat, 27 Sep 2025 23:05:37 +0100
Subject: [PATCH 3/3] printk: Use console_flush_one_record for legacy printer
 kthread

The legacy printer kthread uses console_lock and
__console_flush_and_unlock to flush records to the console. This
approach results in the console_lock being held for the entire
duration of a flush. This can result in large waiting times for
those waiting for console_lock especially where there is a large
volume of records or where the console is slow (e.g. serial). This
contention is observed during boot, as the call to filp_open in
console_on_rootfs will delay progression to userspace until any
in-flight flush is completed.

Let's instead use console_flush_one_record and release/reacquire
the console_lock between records.

On a PocketBeagle 2, with the following boot args:
"console=ttyS2,9600 initcall_debug=1 loglevel=10"

Without this patch:

[    5.613166] +console_on_rootfs/filp_open
[    5.643473] mmc1: SDHCI controller on fa00000.mmc [fa00000.mmc] using ADMA 64-bit
[    5.643823] probe of fa00000.mmc returned 0 after 258244 usecs
[    5.710520] mmc1: new UHS-I speed SDR104 SDHC card at address 5048
[    5.721976] mmcblk1: mmc1:5048 SD32G 29.7 GiB
[    5.747258]  mmcblk1: p1 p2
[    5.753324] probe of mmc1:5048 returned 0 after 40002 usecs
[   15.595240] ti_sci_pm_domains 44043000.system-controller:power-controller: sync_state() pending due to 30040000.pruss
[   15.595282] ti_sci_pm_domains 44043000.system-controller:power-controller: sync_state() pending due to e010000.watchdog
[   15.595297] ti_sci_pm_domains 44043000.system-controller:power-controller: sync_state() pending due to e000000.watchdog
[   15.595437] ti_sci_pm_domains 44043000.system-controller:power-controller: sync_state() pending due to 30300000.crc
[  146.275961] -console_on_rootfs/filp_open ...

and with:

[    5.477122] +console_on_rootfs/filp_open
[    5.595814] mmc1: SDHCI controller on fa00000.mmc [fa00000.mmc] using ADMA 64-bit
[    5.596181] probe of fa00000.mmc returned 0 after 312757 usecs
[    5.662813] mmc1: new UHS-I speed SDR104 SDHC card at address 5048
[    5.674367] mmcblk1: mmc1:5048 SD32G 29.7 GiB
[    5.699320]  mmcblk1: p1 p2
[    5.705494] probe of mmc1:5048 returned 0 after 39987 usecs
[    6.418682] -console_on_rootfs/filp_open ...
...
[   15.593509] ti_sci_pm_domains 44043000.system-controller:power-controller: sync_state() pending due to 30040000.pruss
[   15.593551] ti_sci_pm_domains 44043000.system-controller:power-controller: sync_state() pending due to e010000.watchdog
[   15.593566] ti_sci_pm_domains 44043000.system-controller:power-controller: sync_state() pending due to e000000.watchdog
[   15.593704] ti_sci_pm_domains 44043000.system-controller:power-controller: sync_state() pending due to 30300000.crc

Where I've added a printk surrounding the call in console_on_rootfs to filp_open.

Suggested-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Andrew Murray <amurray@thegoodpenguin.co.uk>
---
 kernel/printk/printk.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 6c846d2d37d9..885506fa0178 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3644,17 +3644,27 @@ static bool legacy_kthread_should_wakeup(void)
 
 static int legacy_kthread_func(void *unused)
 {
-	for (;;) {
-		wait_event_interruptible(legacy_wait, legacy_kthread_should_wakeup());
+	bool try_again;
+
+wait_for_event:
+	wait_event_interruptible(legacy_wait,
+				 legacy_kthread_should_wakeup());
+
+	do {
+		bool handover = false;
+		u64 next_seq;
 
 		if (kthread_should_stop())
-			break;
+			return 0;
 
 		console_lock();
-		__console_flush_and_unlock();
-	}
+		console_flush_one_record(true, &next_seq, &handover, &try_again);
+		if (!handover)
+			__console_unlock();
 
-	return 0;
+	} while (try_again);
+
+	goto wait_for_event;
 }
 
 static bool legacy_kthread_create(void)
-- 
2.51.0