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Message-ID: <aN_PsplsDMpiSHTQ@pathway.suse.cz>
Date: Fri, 3 Oct 2025 15:29:22 +0200
From: Petr Mladek <pmladek@...e.com>
To: Andrew Murray <amurray@...goodpenguin.co.uk>
Cc: Steven Rostedt <rostedt@...dmis.org>,
John Ogness <john.ogness@...utronix.de>,
Sergey Senozhatsky <senozhatsky@...omium.org>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] printk: Introduce console_flush_one_record
On Sat 2025-09-27 23:05:35, 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.
>
> Reviewed-by: Petr Mladek <pmladek@...e.com>
> Signed-off-by: Andrew Murray <amurray@...goodpenguin.co.uk>
> ---
> kernel/printk/printk.c | 159 +++++++++++++++++++++++++++++++------------------
> 1 file changed, 100 insertions(+), 59 deletions(-)
>
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 0efbcdda9aaba9d8d877df5e4f1db002d3a596bc..060d4919de320fe21fd7aca73ba497e27c4ff334 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -3161,6 +3161,100 @@ static inline void printk_kthreads_check_locked(void) { }
>
> #endif /* CONFIG_PRINTK */
>
> +
> +/*
> + * Print out one record for each console.
> + *
> + * @do_cond_resched is set by the caller. It can be true only in schedulable
> + * context.
> + *
> + * @next_seq is set to the sequence number after the last available record.
> + * The value is valid only when this function returns true.
This actually is not true. This function tries to flush one record on all usable
consoles. And @next_seq is set to the highest already emitted sequence
number. But some consoles might be (far) behind.
A more precise description would be something like:
* @next_seq is set to the sequence number after the last available record.
* The value is valid only when there is at least one usable console
* (@any_usable == true) and all usable consoles were flushed
* (function returns false && @handover == false &&
* other_cpu_in_panic() == false)
Huh, it is complicated. This is why I suggested to change the semantic
of @any_usable in the 2nd patch. It would be cleared when the emit
was interrupted (handover or other_cpu_in_panic()).
Best Regards,
Petr
> + * @handover will be set to true if a printk waiter has taken over the
> + * 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.
> + *
> + * 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
> + * of the consoles. It may also indicate that there were no usable consoles,
> + * the context has been lost or there is a panic suitation. Regardless the
> + * reason, the caller should assume it is not useful to immediately try again.
> + *
> + * Requires the console_lock.
> + */
> +static bool console_flush_one_record(bool do_cond_resched, u64 *next_seq, bool *handover,
> + bool *any_usable)
> +{
> + struct console_flush_type ft;
> + struct console *con;
> + bool any_progress;
> + int cookie;
> +
> + 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;
> +
> + /*
> + * console_flush_one_record() 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 (do_cond_resched)
> + cond_resched();
> + }
> + console_srcu_read_unlock(cookie);
> +
> + return any_progress;
> +
> +abandon:
> + console_srcu_read_unlock(cookie);
> + return false;
> +}
> +
> /*
> * Print out all remaining records to all consoles.
> *
> @@ -3186,77 +3280,24 @@ 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_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;
> -
> -abandon:
> - console_srcu_read_unlock(cookie);
> - return false;
> }
>
> static void __console_flush_and_unlock(void)
>
> --
> 2.34.1
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