linux-kernel - [PATCH v3 0/3] printk: Release console_lock between printing records in legacy thread

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Message-Id: <20251020-printk_legacy_thread_console_lock-v3-0-00f1f0ac055a@thegoodpenguin.co.uk>
Date: Mon, 20 Oct 2025 16:38:04 +0100
From: Andrew Murray <amurray@...goodpenguin.co.uk>
To: Petr Mladek <pmladek@...e.com>, Steven Rostedt <rostedt@...dmis.org>, 
 John Ogness <john.ogness@...utronix.de>, 
 Sergey Senozhatsky <senozhatsky@...omium.org>
Cc: linux-kernel@...r.kernel.org, 
 Andrew Murray <amurray@...goodpenguin.co.uk>
Subject: [PATCH v3 0/3] printk: Release console_lock between printing
 records in legacy thread

The legacy printer kthread uses console_lock and
__console_flush_and_unlock to flush records to the console which
holds the console_lock being held for the entire flush. This
results in large waiting times for console_lock waiters
especially where there is a large volume of records or where the
console is slow (e.g. serial). During boot, this contention causes
delays in the filp_open call in console_on_rootfs.
   
Let's instead release and reacquire console_lock in between
printing individual records.

Signed-off-by: Andrew Murray <amurray@...goodpenguin.co.uk>
---
Changes in v3:
- Rebased on v6.18-rc2
- Reordering of variables and minor tweak to comments in console_flush_one_record
  in first patch
- Improve semantics of console_flush_one_record in the second patch through different
  use of return value and by replacing any_usable with try_again.
- Update third patch to use newer version of console_flush_one_record
- Link to v2: https://lore.kernel.org/r/20250927-printk_legacy_thread_console_lock-v2-0-cff9f063071a@thegoodpenguin.co.uk

Changes in v2:
- Move any_usable=false to console_flush_all in the 'introduce
  console_flush_one_record' patch to match original implementation.
- Add Petr's console_flush_one_record() code cleanup patch
- Open code flushing implementation in legacy_kthread_func instead
  of introducing new console_flush functions.
- Link to v1: https://lore.kernel.org/r/20250915-printk_legacy_thread_console_lock-v1-0-f34d42a9bcb3@thegoodpenguin.co.uk

---
Andrew Murray (2):
      printk: Introduce console_flush_one_record
      printk: Use console_flush_one_record for legacy printer kthread

Petr Mladek (1):
      printk: console_flush_one_record() code cleanup

 kernel/printk/printk.c | 186 +++++++++++++++++++++++++++++++------------------
 1 file changed, 119 insertions(+), 67 deletions(-)
---
base-commit: 211ddde0823f1442e4ad052a2f30f050145ccada
change-id: 20250914-printk_legacy_thread_console_lock-1c27f59bf990

Best regards,
-- 
Andrew Murray <amurray@...goodpenguin.co.uk>