On 1/27/26 15:24, Greg Kroah-Hartman wrote:
On Mon, Jan 26, 2026 at 07:08:36PM +0100, Sebastian Andrzej Siewior wrote:
quoted
do_con_write(), fbcon_redraw.*() invoke console_conditional_schedule()
which is a conditional scheduling point based on printk's internal
variables console_may_schedule. It may only be used if the console lock
is acquired for instance via console_lock() or console_trylock().
Prinkt sets the internal variable to 1 (and allows to schedule)
if the console lock has been acquired via console_lock(). The trylock
does not allow it.
The console_conditional_schedule() invocation in do_con_write() is
invoked shortly before console_unlock().
The console_conditional_schedule() invocation in fbcon_redraw.*()
original from fbcon_scroll() / vt's con_scroll() which originate from a
line feed.
In console_unlock() the variable is set to 0 (forbids to schedule) and
it tries to schedule while making progress printing. This is brand new
compared to when console_conditional_schedule() was added in v2.4.9.11.
In v2.6.38-rc3, console_unlock() (started its existence) iterated over
all consoles and flushed them with disabled interrupts. A scheduling
attempt here was not possible, it relied that a long print scheduled
before console_unlock().
Since commit 8d91f8b15361d ("printk: do cond_resched() between lines
while outputting to consoles"), which appeared in v4.5-rc1,
console_unlock() attempts to schedule if it was allowed to schedule
while during console_lock(). Each record is idealy one line so after
every line feed.
This console_conditional_schedule() is also only relevant on
PREEMPT_NONE and PREEMPT_VOLUNTARY builds. In other configurations
cond_resched() becomes a nop and has no impact.
I'm bringing this all up just proof that it is not required anymore. It
becomes a problem on a PREEMPT_RT build with debug code enabled because
that might_sleep() in cond_resched() remains and triggers a warnings.
This is due to
legacy_kthread_func-> console_flush_one_record -> vt_console_print-> lf
-> con_scroll -> fbcon_scroll
and vt_console_print() acquires a spinlock_t which does not allow a
voluntary schedule. There is no need to fb_scroll() to schedule since
console_flush_one_record() attempts to schedule after each line.
!PREEMPT_RT is not affected because the legacy printing thread is only
enabled on PREEMPT_RT builds.
Therefore I suggest to remove console_conditional_schedule().
Cc: Simona Vetter <simona@ffwll.ch>
Cc: Helge Deller <deller@gmx.de>
Cc: <redacted>
Cc: <redacted>
Fixes: 5f53ca3ff83b4 ("printk: Implement legacy printer kthread for PREEMPT_RT")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
A follow-up to
https://lore.kernel.org/all/20260114145955.d924Z-zu@linutronix.de/ (local)
drivers/tty/vt/vt.c | 1 -
drivers/video/fbdev/core/fbcon.c | 6 ------
include/linux/console.h | 1 -
kernel/printk/printk.c | 16 ----------------
4 files changed, 24 deletions(-)
diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
[....]
No objection from me about removing this if it's not needed anymore!
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
I've added it to the fbdev git tree to get some testing....
Thanks!
Helge