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printk: Use two irq_works instead per-CPU

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In commit b845b517b5 ("printk: robustify printk") initially got its
delayed wake up which is today implemented via the irq_work. Back then
it was a variable which was assigned to 1 if a pending wake up was
waiting. This variable was checked on each timer tick. Therefore it
makes sense to assign a per-CPU variable with this task to avoid cache
line bouncing when each CPU reads the variable on each timer tick
simultaneously.

Today we have a irq_work which is enqueued onto a list and handled if it
has work items assigned. This list is already per-CPU.

We could replace the per-CPU irq_work and the per-CPU printk_pending with
two irq_work structs: One for PRINTK_PENDING_WAKEUP and the other for
PRINTK_PENDING_OUTPUT.
While unrolling the conditions in __wake_up_klogd() it becomes obvious
that PRINTK_PENDING_WAKEUP is always passed as an argument but never
checked. The return value wq_has_sleeper() of does not really matter
it's sole purpose is to enforce the needed memory barrier.

Redo the irq_work by:
- Use one irq_work (pending_wakeup_work) for klogd wakes. Schedule it
  only if there is a sleeper waiting.
- Use one irq_work (&pending_output_work) for output printing. Schedule
  it only if PRINTK_PENDING_OUTPUT passed.
- Drop unused PRINTK_PENDING_WAKEUP.
- Remove the preempt_disable() section. Its purpose was to ensure that
  printk_pending and wake_up_klogd_work is accessed from the same CPU.

Worst case is defer_console_output() where two irq_work are scheduled but
those get processed one after the other during the HZ tick so it
shouldn't get worse.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: John Ogness <john.ogness@linutronix.de>
Link: https://patch.msgid.link/20260914143412.1sMcp89V@linutronix.de
Signed-off-by: Petr Mladek <pmladek@suse.com>
This commit is contained in:
Sebastian Andrzej Siewior 2026-09-14 16:34:12 +02:00 committed by Petr Mladek
parent 72d907f23e
commit 731383dcf5
1 changed files with 23 additions and 30 deletions
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53
kernel/printk/printk.c
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@ -4593,31 +4593,26 @@ bool pr_flush(int timeout_ms, bool reset_on_progress)
/*
* Delayed printk version, for scheduler-internal messages:
*/
#define PRINTK_PENDING_WAKEUP 0x01
#define PRINTK_PENDING_OUTPUT 0x02
#define PRINTK_PENDING_OUTPUT 0x01
static DEFINE_PER_CPU(int, printk_pending);
static void wake_up_klogd_work_func(struct irq_work *irq_work)
static void pending_wake_fn(struct irq_work *irq_work)
{
int pending = this_cpu_xchg(printk_pending, 0);
if (pending & PRINTK_PENDING_OUTPUT) {
if (force_legacy_kthread()) {
if (printk_legacy_kthread)
wake_up_interruptible(&legacy_wait);
} else {
if (console_trylock())
console_unlock();
}
}
if (pending & PRINTK_PENDING_WAKEUP)
wake_up_interruptible(&log_wait);
wake_up_interruptible(&log_wait);
}
static DEFINE_PER_CPU(struct irq_work, wake_up_klogd_work) =
IRQ_WORK_INIT_LAZY(wake_up_klogd_work_func);
static void pending_output_fn(struct irq_work *irq_work)
{
if (force_legacy_kthread()) {
if (printk_legacy_kthread)
wake_up_interruptible(&legacy_wait);
} else {
if (console_trylock())
console_unlock();
}
}
static struct irq_work pending_wakeup_work = IRQ_WORK_INIT_LAZY(pending_wake_fn);
static struct irq_work pending_output_work = IRQ_WORK_INIT_LAZY(pending_output_fn);
static void __wake_up_klogd(int val)
{
@ -4631,7 +4626,6 @@ static void __wake_up_klogd(int val)
if (WARN_ON_ONCE(console_irqwork_blocked))
return;
preempt_disable();
/*
* Guarantee any new records can be seen by tasks preparing to wait
* before this context checks if the wait queue is empty.
@ -4643,12 +4637,11 @@ static void __wake_up_klogd(int val)
*
* This pairs with devkmsg_read:A and syslog_print:A.
*/
if (wq_has_sleeper(&log_wait) || /* LMM(__wake_up_klogd:A) */
(val & PRINTK_PENDING_OUTPUT)) {
this_cpu_or(printk_pending, val);
irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
}
preempt_enable();
if (wq_has_sleeper(&log_wait)) /* LMM(__wake_up_klogd:A) */
irq_work_queue(&pending_wakeup_work);
if (val & PRINTK_PENDING_OUTPUT)
irq_work_queue(&pending_output_work);
}
/**
@ -4663,7 +4656,7 @@ static void __wake_up_klogd(int val)
*/
void wake_up_klogd(void)
{
__wake_up_klogd(PRINTK_PENDING_WAKEUP);
__wake_up_klogd(0);
}
/**
@ -4684,7 +4677,7 @@ void defer_console_output(void)
* New messages may have been added directly to the ringbuffer
* using vprintk_store(), so wake any waiters as well.
*/
__wake_up_klogd(PRINTK_PENDING_WAKEUP | PRINTK_PENDING_OUTPUT);
__wake_up_klogd(PRINTK_PENDING_OUTPUT);
}
/**