[PATCH printk v3 0/2] nbcon irq_work fixes

[PATCH printk v3 0/2] nbcon irq_work fixes

[John Ogness]

Hi,

This is v3 of a series to address some irq_work issues with nbcon.
Initially motivated as a workaround for hardware-related regressions
reported [0], it turns out that the workaround is actually a nice
improvement for nbcon deferred printing in general.

v2 is here [1].

The changes since v2:

- On nbcon_free(), perform the irq_work flush before stopping the
  kthread printer.

- Rework the commit message of patch 2/2 so that reducing interrupts
  is the primary motivation and the Tegra workaround is a secondary
  "benefit". I really want the Tegra workaround information to stay
  in the commit message because it is historically significant.

John Ogness

[0] https://lore.kernel.org/lkml/f3757a75-0ba1-4558-bf57-f19ab7e59a4c@nvidia.com
[1] https://lore.kernel.org/lkml/20260828140218.232439-1-john.ogness@linutronix.de

John Ogness (2):
  printk/nbcon: Flush nbcon_irq_work in nbcon_free()
  printk/nbcon: Change nbcon_irq_work to IRQ_WORK_LAZY

 kernel/printk/nbcon.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)


base-commit: ffe0486b139e45cd9c9ca2584f04a1910fe4f8a6
-- 
2.47.3

[PATCH printk v3 1/2] printk/nbcon: Flush nbcon_irq_work in nbcon_free()

[John Ogness]

Ensure any pending nbcon_irq_work is flushed before allowing the
console to be recycled.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 kernel/printk/nbcon.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index a5921a84a80ed..68f77afee9106 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -1837,6 +1837,8 @@ void nbcon_free(struct console *con)
 	/* Synchronize the kthread stop. */
 	lockdep_assert_console_list_lock_held();
 
+	irq_work_sync(&con->irq_work);
+
 	if (printk_kthreads_running) {
 		nbcon_kthread_stop(con);
 
-- 
2.47.3

[PATCH printk v3 2/2] printk/nbcon: Change nbcon_irq_work to IRQ_WORK_LAZY

[John Ogness]

Change the nbcon_irq_work to be IRQ_WORK_LAZY, thus not raising
an IRQ upon irq_work queuing. The irq_work is then handled on the
next kernel tick. This additional delay is acceptable because
nbcon_irq_work is only responsible for non-emergency deferred
printing, which is delayed anyway. This has the benefit of not
needing to raise an IRQ for each printk() call.

On a side note, the Tegra20 and Tegra30 platforms can hang if an
irq_work IRQ is raised while entering cpuidle states. This problem
was reproducible by calling printk() while entering cpuidle. So
this change also provides a workaround for these platforms (as long
as they are not running tickless).

Link: https://lore.kernel.org/lkml/f3757a75-0ba1-4558-bf57-f19ab7e59a4c@nvidia.com
Fixes: 76f258bf3f2a ("printk: nbcon: Introduce printer kthreads")
Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 kernel/printk/nbcon.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index 68f77afee9106..d17704fe93ae1 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -1782,7 +1782,7 @@ bool nbcon_alloc(struct console *con)
 	}
 
 	rcuwait_init(&con->rcuwait);
-	init_irq_work(&con->irq_work, nbcon_irq_work);
+	con->irq_work = IRQ_WORK_INIT_LAZY(nbcon_irq_work);
 	atomic_long_set(&ACCESS_PRIVATE(con, nbcon_prev_seq), -1UL);
 	nbcon_state_set(con, &state);
 
-- 
2.47.3

Re: [PATCH printk v3 2/2] printk/nbcon: Change nbcon_irq_work to IRQ_WORK_LAZY

[John Ogness]

On 2026-09-01, John Ogness <john.ogness@linutronix.de> wrote:
> Change the nbcon_irq_work to be IRQ_WORK_LAZY, thus not raising
> an IRQ upon irq_work queuing. The irq_work is then handled on the
> next kernel tick. This additional delay is acceptable because
> nbcon_irq_work is only responsible for non-emergency deferred
> printing, which is delayed anyway. This has the benefit of not
> needing to raise an IRQ for each printk() call.
>
> On a side note, the Tegra20 and Tegra30 platforms can hang if an
> irq_work IRQ is raised while entering cpuidle states. This problem
> was reproducible by calling printk() while entering cpuidle. So
> this change also provides a workaround for these platforms (as long
> as they are not running tickless).
>
> Link: https://lore.kernel.org/lkml/f3757a75-0ba1-4558-bf57-f19ab7e59a4c@nvidia.com
> Fixes: 76f258bf3f2a ("printk: nbcon: Introduce printer kthreads")
> Signed-off-by: John Ogness <john.ogness@linutronix.de>

Sorry, I failed to gather the previous tags:

Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Tested-by: Jon Hunter <jonathanh@nvidia.com>

Re: [PATCH printk v3 1/2] printk/nbcon: Flush nbcon_irq_work in nbcon_free()

[Petr Mladek]

On Tue 2026-09-01 11:37:42, John Ogness wrote:
> Ensure any pending nbcon_irq_work is flushed before allowing the
> console to be recycled.
> 
> Signed-off-by: John Ogness <john.ogness@linutronix.de>

Thanks for moving the call. It looks good to me:

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

Best Regards,
Petr

Re: [PATCH printk v3 0/2] nbcon irq_work fixes

[Petr Mladek]

On Tue 2026-09-01 11:37:41, John Ogness wrote:
> Hi,
> 
> This is v3 of a series to address some irq_work issues with nbcon.
> Initially motivated as a workaround for hardware-related regressions
> reported [0], it turns out that the workaround is actually a nice
> improvement for nbcon deferred printing in general.
> 
> v2 is here [1].
> 
> The changes since v2:
> 
> - On nbcon_free(), perform the irq_work flush before stopping the
>   kthread printer.
> 
> - Rework the commit message of patch 2/2 so that reducing interrupts
>   is the primary motivation and the Tegra workaround is a secondary
>   "benefit". I really want the Tegra workaround information to stay
>   in the commit message because it is historically significant.
> 
> John Ogness
> 
> [0] https://lore.kernel.org/lkml/f3757a75-0ba1-4558-bf57-f19ab7e59a4c@nvidia.com
> [1] https://lore.kernel.org/lkml/20260828140218.232439-1-john.ogness@linutronix.de
> 
> John Ogness (2):
>   printk/nbcon: Flush nbcon_irq_work in nbcon_free()
>   printk/nbcon: Change nbcon_irq_work to IRQ_WORK_LAZY
> 
>  kernel/printk/nbcon.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)

JFYI, the patchset has been comitted into printk/linux.git,
branch for-7.3-irq-work-fixes.

I am going to give it few days in linux-next. Then I will create
a pull request for 7.3 unless any problems got reported.

Best Regards,
Petr