Re: [PATCH RFC 1/2] printk: Introduce console_flush_one_record

[Petr Mladek <pmladek@suse.com>]

On Mon 2025-09-15 13:43:05, Andrew Murray wrote:
> console_flush_all prints all remaining records to all usable consoles
> whilst its caller holds console_lock. 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).
> 
> Let's extract the parts of this function which print a single record
> into a new function named console_flush_one_record. This can later
> be used for functions that will release and reacquire console_lock
> between records.
> 
> This commit should not change existing functionality.
> 
> Signed-off-by: Andrew Murray <amurray@thegoodpenguin.co.uk>

The code shufling looks correct to me. Feel free to use:

Reviewed-by: Petr Mladek <pmladek@suse.com>

See a nit below.

> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -3186,77 +3281,24 @@ static inline void printk_kthreads_check_locked(void) { }
>   */
>  static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handover)
>  {
> -	struct console_flush_type ft;
> -	bool any_usable = false;
> -	struct console *con;
> +	bool any_usable;
>  	bool any_progress;
> -	int cookie;
>  
>  	*next_seq = 0;
>  	*handover = false;
>  
>  	do {
> -		any_progress = false;
> -
> -		printk_get_console_flush_type(&ft);
> -
> -		cookie = console_srcu_read_lock();
> -		for_each_console_srcu(con) {
> -			short flags = console_srcu_read_flags(con);
> -			u64 printk_seq;
> -			bool progress;
> +		any_progress = console_flush_one_record(do_cond_resched, next_seq, handover,
> +							&any_usable);
>  
> -			/*
> -			 * console_flush_all() is only responsible for nbcon
> -			 * consoles when the nbcon consoles cannot print via
> -			 * their atomic or threaded flushing.
> -			 */
> -			if ((flags & CON_NBCON) && (ft.nbcon_atomic || ft.nbcon_offload))
> -				continue;
> -
> -			if (!console_is_usable(con, flags, !do_cond_resched))
> -				continue;
> -			any_usable = true;
> -
> -			if (flags & CON_NBCON) {
> -				progress = nbcon_legacy_emit_next_record(con, handover, cookie,
> -									 !do_cond_resched);
> -				printk_seq = nbcon_seq_read(con);
> -			} else {
> -				progress = console_emit_next_record(con, handover, cookie);
> -				printk_seq = con->seq;
> -			}
> -
> -			/*
> -			 * If a handover has occurred, the SRCU read lock
> -			 * is already released.
> -			 */
> -			if (*handover)
> -				return false;
> -
> -			/* Track the next of the highest seq flushed. */
> -			if (printk_seq > *next_seq)
> -				*next_seq = printk_seq;
> -
> -			if (!progress)
> -				continue;
> -			any_progress = true;
> -
> -			/* Allow panic_cpu to take over the consoles safely. */
> -			if (other_cpu_in_panic())
> -				goto abandon;
> +		if (*handover)
> +			return false;
>  
> -			if (do_cond_resched)
> -				cond_resched();
> -		}
> -		console_srcu_read_unlock(cookie);
> +		if (other_cpu_in_panic())
> +			return false;
>  	} while (any_progress);
>  
>  	return any_usable;

The semantic of the function was always complicated. It results
into duplicated checks in the callers.

I think that we could do slightly better in this particular case.
But I would do the refactoring in a separate patch to make this
one easier for review.

I played with the code and came up with:

From 7a3c930a12d7c0157f404a4a834d1f92f3070799 Mon Sep 17 00:00:00 2001
From: Petr Mladek <pmladek@suse.com>
Date: Thu, 18 Sep 2025 16:57:58 +0200
Subject: [RFC] printk: console_flush_one_record() code cleanup

console_flush_one_record() and console_flush_all() duplicate several
checks. They both want to tell the caller that consoles are not
longer usable in this context because it has lost the lock or
the lock has to be reserved for the panic CPU.

Pass this information by clearing the @any_usable parameter value
which has the same effect.

Signed-off-by: Petr Mladek <pmladek@suse.com>
---
 kernel/printk/printk.c | 20 +++++++++-----------
 1 file changed, 9 insertions(+), 11 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 53daab5cdee5..2cceedcc3d34 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3175,7 +3175,8 @@ static inline void printk_kthreads_check_locked(void) { }
  * console_lock, in which case the caller is no longer holding the
  * console_lock. Otherwise it is set to false.
  *
- * @any_usable will be set to true if there are any usable consoles.
+ * @any_usable will be set to true if there are any usable consoles
+ * in this context.
  *
  * Returns true when there was at least one usable console and a record was
  * flushed. A returned false indicates there were no records to flush for any
@@ -3230,7 +3231,7 @@ static bool console_flush_one_record(bool do_cond_resched, u64 *next_seq, bool *
 		 * is already released.
 		 */
 		if (*handover)
-			return false;
+			goto unusable;
 
 		/* Track the next of the highest seq flushed. */
 		if (printk_seq > *next_seq)
@@ -3242,7 +3243,7 @@ static bool console_flush_one_record(bool do_cond_resched, u64 *next_seq, bool *
 
 		/* Allow panic_cpu to take over the consoles safely. */
 		if (other_cpu_in_panic())
-			goto abandon;
+			goto unusable_srcu;
 
 		if (do_cond_resched)
 			cond_resched();
@@ -3251,8 +3252,10 @@ static bool console_flush_one_record(bool do_cond_resched, u64 *next_seq, bool *
 
 	return any_progress;
 
-abandon:
+unusable_srcu:
 	console_srcu_read_unlock(cookie);
+unusable:
+	*any_usable = false;
 	return false;
 }
 
@@ -3288,14 +3291,9 @@ static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handove
 	*handover = false;
 
 	do {
-		any_progress = console_flush_one_record(do_cond_resched, next_seq, handover,
-							&any_usable);
+		any_progress = console_flush_one_record(do_cond_resched, next_seq,
+							handover, &any_usable);
 
-		if (*handover)
-			return false;
-
-		if (other_cpu_in_panic())
-			return false;
 	} while (any_progress);
 
 	return any_usable;
-- 
2.51.0

Best Regards,
Petr