From 8e9bf8b9e8c0a3e1ef16dd48260a113f65ed01d2 Mon Sep 17 00:00:00 2001 From: Sebastian Andrzej Siewior Date: Mon, 26 Jan 2026 19:08:36 +0100 Subject: [PATCH] printk, vt, fbcon: Remove console_conditional_schedule() 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 Cc: Helge Deller Cc: linux-fbdev@vger.kernel.org Cc: dri-devel@lists.freedesktop.org Fixes: 5f53ca3ff83b4 ("printk: Implement legacy printer kthread for PREEMPT_RT") Signed-off-by: Sebastian Andrzej Siewior Acked-by: Greg Kroah-Hartman Acked-by: Petr Mladek # from printk() POV Signed-off-by: Helge Deller --- 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 index 84de274d24cab..3bdbd4c52c2fb 100644 --- a/drivers/tty/vt/vt.c +++ b/drivers/tty/vt/vt.c @@ -3230,7 +3230,6 @@ rescan_last_byte: goto rescan_last_byte; } con_flush(vc, &draw); - console_conditional_schedule(); notify_update(vc); return n; diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c index 36e380797a9e4..98387c42e4e59 100644 --- a/drivers/video/fbdev/core/fbcon.c +++ b/drivers/video/fbdev/core/fbcon.c @@ -1608,12 +1608,10 @@ static void fbcon_redraw_move(struct vc_data *vc, struct fbcon_display *p, start = s; } } - console_conditional_schedule(); s++; } while (s < le); if (s > start) fbcon_putcs(vc, start, s - start, dy, x); - console_conditional_schedule(); dy++; } } @@ -1649,14 +1647,12 @@ static void fbcon_redraw_blit(struct vc_data *vc, struct fb_info *info, } scr_writew(c, d); - console_conditional_schedule(); s++; d++; } while (s < le); if (s > start) par->bitops->bmove(vc, info, line + ycount, x, line, x, 1, s - start); - console_conditional_schedule(); if (ycount > 0) line++; else { @@ -1704,13 +1700,11 @@ static void fbcon_redraw(struct vc_data *vc, int line, int count, int offset) } } scr_writew(c, d); - console_conditional_schedule(); s++; d++; } while (s < le); if (s > start) fbcon_putcs(vc, start, s - start, line, x); - console_conditional_schedule(); if (offset > 0) line++; else { diff --git a/include/linux/console.h b/include/linux/console.h index 1346f0b4cd8b3..5520e4477ad7e 100644 --- a/include/linux/console.h +++ b/include/linux/console.h @@ -697,7 +697,6 @@ extern int unregister_console(struct console *); extern void console_lock(void); extern int console_trylock(void); extern void console_unlock(void); -extern void console_conditional_schedule(void); extern void console_unblank(void); extern void console_flush_on_panic(enum con_flush_mode mode); extern struct tty_driver *console_device(int *); diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 4ddeb55e1207e..a181394604d12 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -3416,22 +3416,6 @@ void console_unlock(void) } EXPORT_SYMBOL(console_unlock); -/** - * console_conditional_schedule - yield the CPU if required - * - * If the console code is currently allowed to sleep, and - * if this CPU should yield the CPU to another task, do - * so here. - * - * Must be called within console_lock();. - */ -void __sched console_conditional_schedule(void) -{ - if (console_may_schedule) - cond_resched(); -} -EXPORT_SYMBOL(console_conditional_schedule); - void console_unblank(void) { bool found_unblank = false; -- 2.47.3