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 8d91f8b ("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: linux-fbdev@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Fixes: 5f53ca3 ("printk: Implement legacy printer kthread for PREEMPT_RT")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Petr Mladek <pmladek@suse.com> # from printk() POV
Signed-off-by: Helge Deller <deller@gmx.de>

Author: Sebastian Andrzej Siewior

drivers/tty/vt/vt.c

@@ -3230,7 +3230,6 @@ static int do_con_write(struct tty_struct *tty, const u8 *buf, int count)
 			goto rescan_last_byte;
 	}
 	con_flush(vc, &draw);
-	console_conditional_schedule();
 	notify_update(vc);
 
 	return n;

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 {

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 *);

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;