Re: [PATCH printk v3 13/15] printk: add kthread console printers

[Petr Mladek <pmladek@suse.com>]

On Wed 2022-04-20 01:52:35, John Ogness wrote:
> Create a kthread for each console to perform console printing. During
> normal operation (@system_state == SYSTEM_RUNNING), the kthread
> printers are responsible for all printing on their respective
> consoles.
> 
> During non-normal operation, console printing is done as it has been:
> within the context of the printk caller or within irqwork triggered
> by the printk caller, referred to as direct printing.
> 
> Since threaded console printers are responsible for all printing
> during normal operation, this also includes messages generated via
> deferred printk calls. If direct printing is in effect during a
> deferred printk call, the queued irqwork will perform the direct
> printing. To make it clear that this is the only time that the
> irqwork will perform direct printing, rename the flag
> PRINTK_PENDING_OUTPUT to PRINTK_PENDING_DIRECT_OUTPUT.
> 
> Threaded console printers synchronize against each other and against
> console lockers by taking the console lock for each message that is
> printed.
> 
> Note that the kthread printers do not care about direct printing.
> They will always try to print if new records are available. They can
> be blocked by direct printing, but will be woken again once direct
> printing is finished.
> 
> Console unregistration is a bit tricky because the associated
> kthread printer cannot be stopped while the console lock is held.
> A policy is implemented that states: whichever task clears
> con->thread (under the console lock) is responsible for stopping
> the kthread. unregister_console() will clear con->thread while
> the console lock is held and then stop the kthread after releasing
> the console lock.
> 
> For consoles that have implemented the exit() callback, the kthread
> is stopped before exit() is called.
> 
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -390,6 +397,14 @@ void printk_prefer_direct_exit(void)
>  	WARN_ON(atomic_dec_if_positive(&printk_prefer_direct) < 0);
>  }
>  
> +static inline bool allow_direct_printing(void)
> +{
> +	return (!printk_kthreads_available ||
> +		system_state > SYSTEM_RUNNING ||
> +		oops_in_progress ||
> +		atomic_read(&printk_prefer_direct));
> +}
> +
>  DECLARE_WAIT_QUEUE_HEAD(log_wait);
>  /* All 3 protected by @syslog_lock. */
>  /* the next printk record to read by syslog(READ) or /proc/kmsg */
> @@ -2280,10 +2295,10 @@ asmlinkage int vprintk_emit(int facility, int level,
>  	printed_len = vprintk_store(facility, level, dev_info, fmt, args);
>  
>  	/* If called from the scheduler, we can not call up(). */
> -	if (!in_sched) {
> +	if (!in_sched && allow_direct_printing()) {

allow_direct_printing() is racy here. But I think that we could live
with it, see below.


>  		/*
>  		 * The caller may be holding system-critical or
> -		 * timing-sensitive locks. Disable preemption during
> +		 * timing-sensitive locks. Disable preemption during direct
>  		 * printing of all remaining records to all consoles so that
>  		 * this context can return as soon as possible. Hopefully
>  		 * another printk() caller will take over the printing.

[...]

> @@ -3475,10 +3720,14 @@ static void wake_up_klogd_work_func(struct irq_work *irq_work)
>  {
>  	int pending = this_cpu_xchg(printk_pending, 0);
>  
> -	if (pending & PRINTK_PENDING_OUTPUT) {
> +	if (pending & PRINTK_PENDING_DIRECT_OUTPUT) {
> +		printk_prefer_direct_enter();
> +
>  		/* If trylock fails, someone else is doing the printing */
>  		if (console_trylock())
>  			console_unlock();
> +
> +		printk_prefer_direct_exit();
>  	}
>  
>  	if (pending & PRINTK_PENDING_WAKEUP)
> @@ -3503,10 +3752,11 @@ static void __wake_up_klogd(int val)
>  	 * prepare_to_wait_event(), which is called after ___wait_event() adds
>  	 * the waiter but before it has checked the wait condition.
>  	 *
> -	 * This pairs with devkmsg_read:A and syslog_print:A.
> +	 * This pairs with devkmsg_read:A, syslog_print:A, and
> +	 * printk_kthread_func:A.
>  	 */
>  	if (wq_has_sleeper(&log_wait) || /* LMM(__wake_up_klogd:A) */
> -	    (val & PRINTK_PENDING_OUTPUT)) {
> +	    (val & PRINTK_PENDING_DIRECT_OUTPUT)) {
>  		this_cpu_or(printk_pending, val);
>  		irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
>  	}
> @@ -3524,7 +3774,16 @@ 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);
> +	int val = PRINTK_PENDING_WAKEUP;
> +
> +	/*
> +	 * If console deferring was called with preferred direct printing,
> +	 * make the irqwork perform the direct printing.
> +	 */
> +	if (atomic_read(&printk_prefer_direct))
> +		val |= PRINTK_PENDING_DIRECT_OUTPUT;

We actually need:

	/*
	 * Make sure that someone will handle the messages when direct
	 * printing is allowed. It happens when the kthreads are less
	 * reliable or unusable at all.
	 */
	if (allow_direct_printing())
		val |= PRINTK_PENDING_DIRECT_OUTPUT;


It is racy. But the same race is also in vprintk_emit().

False positive is fine. console_flush_all() will bail out when
the direct printing gets disabled in the meantime.

False negative is worse. But we will still queue PRINTK_PENDING_WAKEUP
that will try to wake up the kthreads that should still be around.

And it was always problem even with console_trylock() approach.
Failure means an expectation that someone else is doing the printing.
It might be either a kthread or the current console_lock owner.
But it is never guaranteed because both might be sleeping.

We do our best by calling pr_flush() or console_flush_on_panic()
on various places. Also PRINTK_PENDING_WAKEUP will always try to wake
up the kthreads.


Anyway, we should document this somewhere. At least in the commit
message.

My dream is Documentation/core-api/printk-design.rst but I do not
want to force you to do it ;-)


> +	__wake_up_klogd(val);
>  }
>  
>  void printk_trigger_flush(void)

Otherwise, it looks good.

Best Regards,
Petr