Re: [RFC][PATCH v4 1/2] printk: Make printk() completely async

[Byungchul Park <byungchul.park@lge.com>]

On Tue, Mar 15, 2016 at 11:07:38PM +0900, Sergey Senozhatsky wrote:
> 
> something like this (*minimally tested so far*).
> 
> -- move wake_up() and friends under the logbuf section; so we can detect
                                          ^^^^^^^^^^^^^^
					  section protected by logbuf_lock?

>    printk() recursion from wake_up()

Excuse me, but I fear that it can cause an unnecessary deadlock.
Furthermore it cannot be handled if it is caused by logbuf_lock. IMHO,
nothing including synced printk can help us in this case. Is there
something I missed? I'am sorry if I did, but could you let me know if so?

I mean that it would be better to keep the wake_up and friend out of the
critical section by logbuf_lock.

Thanks,
Byungchul

> 
> -- in recursion_bug branch switch to sync printk. we don't know why did we
>    recurse, may be because of wake_up()->spin_lock(). doing
>    kthread_stop()->wake_up_process() can be unsafe, I guess, just set
>    `printk_sync' to true.
> 
> 
> Signed-off-by: Jan Kara <jack@suse.cz>
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> ---
>  Documentation/kernel-parameters.txt |  10 ++
>  kernel/printk/printk.c              | 202 +++++++++++++++++++++++++-----------
>  2 files changed, 154 insertions(+), 58 deletions(-)
> 
> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> index 1e58ae9..454999e 100644
> --- a/Documentation/kernel-parameters.txt
> +++ b/Documentation/kernel-parameters.txt
> @@ -3114,6 +3114,16 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>  	printk.time=	Show timing data prefixed to each printk message line
>  			Format: <bool>  (1/Y/y=enable, 0/N/n=disable)
>  
> +	printk.synchronous=
> +			By default kernel messages are printed to console
> +			asynchronously (except during early boot or when oops
> +			is happening). That avoids kernel stalling behind slow
> +			serial console and thus avoids softlockups, interrupt
> +			timeouts, or userspace timing out during heavy printing.
> +			However for debugging problems, printing messages to
> +			console immediately may be desirable. This option
> +			enables such behavior.
> +
>  	processor.max_cstate=	[HW,ACPI]
>  			Limit processor to maximum C-state
>  			max_cstate=9 overrides any DMI blacklist limit.
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index d5fd844..38baed1 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -46,6 +46,7 @@
>  #include <linux/utsname.h>
>  #include <linux/ctype.h>
>  #include <linux/uio.h>
> +#include <linux/kthread.h>
>  
>  #include <asm/uaccess.h>
>  #include <asm-generic/sections.h>
> @@ -284,6 +285,105 @@ static char __log_buf[__LOG_BUF_LEN] __aligned(LOG_ALIGN);
>  static char *log_buf = __log_buf;
>  static u32 log_buf_len = __LOG_BUF_LEN;
>  
> +/*
> + * When true, printing to console will happen synchronously unless someone else
> + * is already printing messages.
> + *
> + * The default value on UP systems is 'true'.
> + */
> +static bool __read_mostly printk_sync = !IS_ENABLED(CONFIG_SMP);
> +module_param_named(synchronous, printk_sync, bool, S_IRUGO | S_IWUSR);
> +MODULE_PARM_DESC(synchronous, "make printing to console synchronous");
> +
> +/* Printing kthread for async vprintk_emit() */
> +static struct task_struct *printk_thread;
> +/* Wait for printing wakeups from async vprintk_emit() */
> +static DECLARE_WAIT_QUEUE_HEAD(printing_wait);
> +
> +static int printing_func(void *data)
> +{
> +	while (1) {
> +		DECLARE_WAITQUEUE(wait, current);
> +		set_current_state(TASK_INTERRUPTIBLE);
> +		add_wait_queue(&printing_wait, &wait);
> +
> +		schedule();
> +		remove_wait_queue(&printing_wait, &wait);
> +
> +		console_lock();
> +		console_unlock();
> +	}
> +
> +	return 0;
> +}
> +
> +static int __init init_printk_thread(void)
> +{
> +	if (printk_sync)
> +		return 0;
> +
> +	printk_thread = kthread_run(printing_func, NULL, "printk");
> +	BUG_ON(IS_ERR(printk_thread));
> +	return 0;
> +}
> +late_initcall(init_printk_thread);
> +
> +/*
> + * Delayed printk version, for scheduler-internal messages:
> + */
> +#define PRINTK_PENDING_WAKEUP	(1<<0)
> +#define PRINTK_PENDING_OUTPUT	(1<<1)
> +
> +static DEFINE_PER_CPU(int, printk_pending);
> +
> +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 (!printk_sync && printk_thread) {
> +			wake_up(&printing_wait);
> +		} else {
> +			/*
> +			 * If trylock fails, someone else is doing
> +			 * the printing
> +			 */
> +			if (console_trylock())
> +				console_unlock();
> +		}
> +	}
> +
> +	if (pending & PRINTK_PENDING_WAKEUP)
> +		wake_up_interruptible(&log_wait);
> +}
> +
> +static DEFINE_PER_CPU(struct irq_work, wake_up_klogd_work) = {
> +	.func = wake_up_klogd_work_func,
> +	.flags = IRQ_WORK_LAZY,
> +};
> +
> +void wake_up_klogd(void)
> +{
> +	preempt_disable();
> +	if (waitqueue_active(&log_wait)) {
> +		this_cpu_or(printk_pending, PRINTK_PENDING_WAKEUP);
> +		irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
> +	}
> +	preempt_enable();
> +}
> +
> +int printk_deferred(const char *fmt, ...)
> +{
> +	va_list args;
> +	int r;
> +
> +	va_start(args, fmt);
> +	r = vprintk_emit(0, LOGLEVEL_SCHED, NULL, 0, fmt, args);
> +	va_end(args);
> +
> +	return r;
> +}
> +
>  /* Return log buffer address */
>  char *log_buf_addr_get(void)
>  {
> @@ -1609,6 +1709,8 @@ asmlinkage int vprintk_emit(int facility, int level,
>  			    const char *dict, size_t dictlen,
>  			    const char *fmt, va_list args)
>  {
> +	/* cpu currently holding logbuf_lock in this function */
> +	static unsigned int logbuf_cpu = UINT_MAX;
>  	static bool recursion_bug;
>  	static char textbuf[LOG_LINE_MAX];
>  	char *text = textbuf;
> @@ -1618,12 +1720,17 @@ asmlinkage int vprintk_emit(int facility, int level,
>  	int this_cpu;
>  	int printed_len = 0;
>  	bool in_sched = false;
> -	/* cpu currently holding logbuf_lock in this function */
> -	static unsigned int logbuf_cpu = UINT_MAX;
> +	bool in_panic = console_loglevel == CONSOLE_LOGLEVEL_MOTORMOUTH;
> +	bool sync_print = printk_sync;
>  
>  	if (level == LOGLEVEL_SCHED) {
>  		level = LOGLEVEL_DEFAULT;
> +		/*
> +		 * Deferred sched messages must not be printed
> +		 * synchronously regardless the @printk_sync or @in_panic.
> +		 */
>  		in_sched = true;
> +		sync_print = false;
>  	}
>  
>  	boot_delay_msec(level);
> @@ -1660,6 +1767,7 @@ asmlinkage int vprintk_emit(int facility, int level,
>  		static const char recursion_msg[] =
>  			"BUG: recent printk recursion!";
>  
> +		printk_sync = true;
>  		recursion_bug = false;
>  		/* emit KERN_CRIT message */
>  		printed_len += log_store(0, 2, LOG_PREFIX|LOG_NEWLINE, 0,
> @@ -1749,13 +1857,45 @@ asmlinkage int vprintk_emit(int facility, int level,
>  						 dict, dictlen, text, text_len);
>  	}
>  
> +	/*
> +	 * By default we print message to console asynchronously so that kernel
> +	 * doesn't get stalled due to slow serial console. That can lead to
> +	 * softlockups, lost interrupts, or userspace timing out under heavy
> +	 * printing load.
> +	 *
> +	 * However we resort to synchronous printing of messages during early
> +	 * boot, when synchronous printing was explicitly requested by
> +	 * kernel parameter, or when console_verbose() was called to print
> +	 * everything during panic / oops.
> +	 */
> +	if (!sync_print) {
> +		if (in_sched) {
> +			/*
> +			 * @in_sched messages may come too early, when we don't
> +			 * yet have @printk_thread. We can't print deferred
> +			 * messages directly, because this may deadlock, route
> +			 * them via IRQ context.
> +			 */
> +			__this_cpu_or(printk_pending,
> +					PRINTK_PENDING_OUTPUT);
> +			irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
> +		} else if (printk_thread && !in_panic) {
> +			/*
> +			 * This will wakeup the printing kthread and offload
> +			 * printing to a schedulable context.
> +			 */
> +			wake_up(&printing_wait);
> +		} else {
> +			sync_print = true;
> +		}
> +	}
> +
>  	logbuf_cpu = UINT_MAX;
>  	raw_spin_unlock(&logbuf_lock);
>  	lockdep_on();
>  	local_irq_restore(flags);
>  
> -	/* If called from the scheduler, we can not call up(). */
> -	if (!in_sched) {
> +	if (sync_print) {
>  		lockdep_off();
>  		/*
>  		 * Try to acquire and then immediately release the console
> @@ -2724,60 +2864,6 @@ late_initcall(printk_late_init);
>  
>  #if defined CONFIG_PRINTK
>  /*
> - * Delayed printk version, for scheduler-internal messages:
> - */
> -#define PRINTK_PENDING_WAKEUP	0x01
> -#define PRINTK_PENDING_OUTPUT	0x02
> -
> -static DEFINE_PER_CPU(int, printk_pending);
> -
> -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 trylock fails, someone else is doing the printing */
> -		if (console_trylock())
> -			console_unlock();
> -	}
> -
> -	if (pending & PRINTK_PENDING_WAKEUP)
> -		wake_up_interruptible(&log_wait);
> -}
> -
> -static DEFINE_PER_CPU(struct irq_work, wake_up_klogd_work) = {
> -	.func = wake_up_klogd_work_func,
> -	.flags = IRQ_WORK_LAZY,
> -};
> -
> -void wake_up_klogd(void)
> -{
> -	preempt_disable();
> -	if (waitqueue_active(&log_wait)) {
> -		this_cpu_or(printk_pending, PRINTK_PENDING_WAKEUP);
> -		irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
> -	}
> -	preempt_enable();
> -}
> -
> -int printk_deferred(const char *fmt, ...)
> -{
> -	va_list args;
> -	int r;
> -
> -	preempt_disable();
> -	va_start(args, fmt);
> -	r = vprintk_emit(0, LOGLEVEL_SCHED, NULL, 0, fmt, args);
> -	va_end(args);
> -
> -	__this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
> -	irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
> -	preempt_enable();
> -
> -	return r;
> -}
> -
> -/*
>   * printk rate limiting, lifted from the networking subsystem.
>   *
>   * This enforces a rate limit: not more than 10 kernel messages
> -- 
> 2.8.0.rc0