Re: + printk-do-cond_resched-between-lines-while-outputting-to-consoles.patch added to -mm tree

Re: + printk-do-cond_resched-between-lines-while-outputting-to-consoles.patch added to -mm tree

[Sergey Senozhatsky]

Hello,

On (12/02/15 15:57), akpm@linux-foundation.org wrote:
[..]
> @console_may_schedule tracks whether console_sem was acquired through lock
> or trylock.  If the former, we're inside a sleepable context and
> console_conditional_schedule() performs cond_resched().  This allows
> console drivers which use console_lock for synchronization to yield while
> performing time-consuming operations such as scrolling.
> 
> However, the actual console outputting is performed while holding irq-safe
> logbuf_lock, so console_unlock() clears @console_may_schedule before
> starting outputting lines.  Also, only a few drivers call
> console_conditional_schedule() to begin with.  This means that when a lot
> of lines need to be output by console_unlock(), for example on a console
> registration, the task doing console_unlock() may not yield for a long
> time on a non-preemptible kernel.
> 
> If this happens with a slow console devices, for example a serial console,
> the outputting task may occupy the cpu for a very long time.  Long enough
> to trigger softlockup and/or RCU stall warnings, which in turn pile more
> messages, sometimes enough to trigger the next cycle of warnings
> incapacitating the system.
> 
> Fix it by making console_unlock() insert cond_resched() between lines if
> @console_may_schedule.

CPU2 still can cause lots of troubles. consider

CPU0		CPU1			CPU2
printk		
...		printk_deferred		
printk					wake_up_klogd
						wake_up_klogd_work_func
							console_trylock
								console_unlock

printk_deferred() may be issued by scheduler, for example.

	-ss

> Signed-off-by: Tejun Heo <tj@kernel.org>
> Reported-by: Calvin Owens <calvinowens@fb.com>
> Acked-by: Jan Kara <jack@suse.com>
> Cc: Dave Jones <davej@codemonkey.org.uk>
> Cc: Kyle McMartin <kyle@kernel.org>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
> 
>  kernel/printk/printk.c |   16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff -puN kernel/printk/printk.c~printk-do-cond_resched-between-lines-while-outputting-to-consoles kernel/printk/printk.c
> --- a/kernel/printk/printk.c~printk-do-cond_resched-between-lines-while-outputting-to-consoles
> +++ a/kernel/printk/printk.c
> @@ -2234,13 +2234,24 @@ void console_unlock(void)
>  	static u64 seen_seq;
>  	unsigned long flags;
>  	bool wake_klogd = false;
> -	bool retry;
> +	bool do_cond_resched, retry;
>  
>  	if (console_suspended) {
>  		up_console_sem();
>  		return;
>  	}
>  
> +	/*
> +	 * Console drivers are called under logbuf_lock, so
> +	 * @console_may_schedule should be cleared before; however, we may
> +	 * end up dumping a lot of lines, for example, if called from
> +	 * console registration path, and should invoke cond_resched()
> +	 * between lines if allowable.  Not doing so can cause a very long
> +	 * scheduling stall on a slow console leading to RCU stall and
> +	 * softlockup warnings which exacerbate the issue with more
> +	 * messages practically incapacitating the system.
> +	 */
> +	do_cond_resched = console_may_schedule;
>  	console_may_schedule = 0;
>  
>  	/* flush buffered message fragment immediately to console */
> @@ -2312,6 +2323,9 @@ skip:
>  		call_console_drivers(level, ext_text, ext_len, text, len);
>  		start_critical_timings();
>  		local_irq_restore(flags);
> +
> +		if (do_cond_resched)
> +			cond_resched();
>  	}
>  	console_locked = 0;
>  
> _
> 
> Patches currently in -mm which might be from tj@kernel.org are
> 
> printk-do-cond_resched-between-lines-while-outputting-to-consoles.patch
> 
> --
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> 

Re: + printk-do-cond_resched-between-lines-while-outputting-to-consoles.patch added to -mm tree

[Sergey Senozhatsky]

On (12/03/15 10:11), Sergey Senozhatsky wrote:
> On (12/02/15 15:57), akpm@linux-foundation.org wrote:
> [..]
> > @console_may_schedule tracks whether console_sem was acquired through lock
> > or trylock.  If the former, we're inside a sleepable context and
> > console_conditional_schedule() performs cond_resched().  This allows
> > console drivers which use console_lock for synchronization to yield while
> > performing time-consuming operations such as scrolling.
> > 
> > However, the actual console outputting is performed while holding irq-safe
> > logbuf_lock, so console_unlock() clears @console_may_schedule before
> > starting outputting lines.  Also, only a few drivers call
> > console_conditional_schedule() to begin with.  This means that when a lot
> > of lines need to be output by console_unlock(), for example on a console
> > registration, the task doing console_unlock() may not yield for a long
> > time on a non-preemptible kernel.
> > 
> > If this happens with a slow console devices, for example a serial console,
> > the outputting task may occupy the cpu for a very long time.  Long enough
> > to trigger softlockup and/or RCU stall warnings, which in turn pile more
> > messages, sometimes enough to trigger the next cycle of warnings
> > incapacitating the system.
> > 
> > Fix it by making console_unlock() insert cond_resched() between lines if
> > @console_may_schedule.
> 
> CPU2 still can cause lots of troubles. consider
> 
> CPU0		CPU1			CPU2
> printk		
> ...		printk_deferred		
> printk					wake_up_klogd
> 						wake_up_klogd_work_func
> 							console_trylock
> 								console_unlock
> 
> printk_deferred() may be issued by scheduler, for example.

IOW, may be we can start limiting the number of bytes printed in console_unlock()
from irq contexts. Which is quite ugly, yes. We basically don't know how much time
we spend in call_console_drivers(); some of the consoles can do 'internal' spin_lock
loops in ->write() handlers, etc. So something like this (below) probably will not
really help, but still it's not always OK to do `while (1)' loop in console_unlock()
for irqs.

	-ss

(not even compile tested)

---

 kernel/printk/printk.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 9da39e7..221a230 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2235,6 +2235,7 @@ void console_unlock(void)
 	unsigned long flags;
 	bool wake_klogd = false;
 	bool do_cond_resched, retry;
+	int printed, irq_count = irq_count();
 
 	if (console_suspended) {
 		up_console_sem();
@@ -2257,6 +2258,7 @@ void console_unlock(void)
 	/* flush buffered message fragment immediately to console */
 	console_cont_flush(text, sizeof(text));
 again:
+	printed = 0;
 	for (;;) {
 		struct printk_log *msg;
 		size_t ext_len = 0;
@@ -2326,6 +2328,8 @@ skip:
 
 		if (do_cond_resched)
 			cond_resched();
+		if (irq_count && printed > LOG_LINE_MAX)
+			break;
 	}
 	console_locked = 0;
 
@@ -2344,7 +2348,7 @@ skip:
 	 * flush, no worries.
 	 */
 	raw_spin_lock(&logbuf_lock);
-	retry = console_seq != log_next_seq;
+	retry = (console_seq != log_next_seq) && !!irq_count;
 	raw_spin_unlock_irqrestore(&logbuf_lock, flags);
 
 	if (retry && console_trylock())

Re: + printk-do-cond_resched-between-lines-while-outputting-to-consoles.patch added to -mm tree

[Jan Kara]

On Thu 03-12-15 11:39:33, Sergey Senozhatsky wrote:
> On (12/03/15 10:11), Sergey Senozhatsky wrote:
> > On (12/02/15 15:57), akpm@linux-foundation.org wrote:
> > [..]
> > > @console_may_schedule tracks whether console_sem was acquired through lock
> > > or trylock.  If the former, we're inside a sleepable context and
> > > console_conditional_schedule() performs cond_resched().  This allows
> > > console drivers which use console_lock for synchronization to yield while
> > > performing time-consuming operations such as scrolling.
> > > 
> > > However, the actual console outputting is performed while holding irq-safe
> > > logbuf_lock, so console_unlock() clears @console_may_schedule before
> > > starting outputting lines.  Also, only a few drivers call
> > > console_conditional_schedule() to begin with.  This means that when a lot
> > > of lines need to be output by console_unlock(), for example on a console
> > > registration, the task doing console_unlock() may not yield for a long
> > > time on a non-preemptible kernel.
> > > 
> > > If this happens with a slow console devices, for example a serial console,
> > > the outputting task may occupy the cpu for a very long time.  Long enough
> > > to trigger softlockup and/or RCU stall warnings, which in turn pile more
> > > messages, sometimes enough to trigger the next cycle of warnings
> > > incapacitating the system.
> > > 
> > > Fix it by making console_unlock() insert cond_resched() between lines if
> > > @console_may_schedule.
> > 
> > CPU2 still can cause lots of troubles. consider
> > 
> > CPU0		CPU1			CPU2
> > printk		
> > ...		printk_deferred		
> > printk					wake_up_klogd
> > 						wake_up_klogd_work_func
> > 							console_trylock
> > 								console_unlock
> > 
> > printk_deferred() may be issued by scheduler, for example.
> 
> IOW, may be we can start limiting the number of bytes printed in console_unlock()
> from irq contexts. Which is quite ugly, yes. We basically don't know how much time
> we spend in call_console_drivers(); some of the consoles can do 'internal' spin_lock
> loops in ->write() handlers, etc. So something like this (below) probably will not
> really help, but still it's not always OK to do `while (1)' loop in console_unlock()
> for irqs.

What we really want is pushing the printing into async context (unless
forced by debug option or oops in progress). Because what you do here fixes
only a small fraction of the problem space. I have patches which fix more
of it (https://lkml.org/lkml/2015/10/26/16) but they are still not enough
because on large machines e.g. udev times out because printing messages
about inserted hardware over serial console just takes too long.

								Honza
> 
> ---
> 
>  kernel/printk/printk.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 9da39e7..221a230 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2235,6 +2235,7 @@ void console_unlock(void)
>  	unsigned long flags;
>  	bool wake_klogd = false;
>  	bool do_cond_resched, retry;
> +	int printed, irq_count = irq_count();
>  
>  	if (console_suspended) {
>  		up_console_sem();
> @@ -2257,6 +2258,7 @@ void console_unlock(void)
>  	/* flush buffered message fragment immediately to console */
>  	console_cont_flush(text, sizeof(text));
>  again:
> +	printed = 0;
>  	for (;;) {
>  		struct printk_log *msg;
>  		size_t ext_len = 0;
> @@ -2326,6 +2328,8 @@ skip:
>  
>  		if (do_cond_resched)
>  			cond_resched();
> +		if (irq_count && printed > LOG_LINE_MAX)
> +			break;
>  	}
>  	console_locked = 0;
>  
> @@ -2344,7 +2348,7 @@ skip:
>  	 * flush, no worries.
>  	 */
>  	raw_spin_lock(&logbuf_lock);
> -	retry = console_seq != log_next_seq;
> +	retry = (console_seq != log_next_seq) && !!irq_count;
>  	raw_spin_unlock_irqrestore(&logbuf_lock, flags);
>  
>  	if (retry && console_trylock())
> 
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: + printk-do-cond_resched-between-lines-while-outputting-to-consoles.patch added to -mm tree

[Sergey Senozhatsky]

On (12/03/15 10:57), Jan Kara wrote:
[..]
> > > CPU2 still can cause lots of troubles. consider
> > > 
> > > CPU0		CPU1			CPU2
> > > printk		
> > > ...		printk_deferred		
> > > printk					wake_up_klogd
> > > 						wake_up_klogd_work_func
> > > 							console_trylock
> > > 								console_unlock
> > > 
> > > printk_deferred() may be issued by scheduler, for example.
> > 
> > IOW, may be we can start limiting the number of bytes printed in console_unlock()
> > from irq contexts. Which is quite ugly, yes. We basically don't know how much time
> > we spend in call_console_drivers(); some of the consoles can do 'internal' spin_lock
> > loops in ->write() handlers, etc. So something like this (below) probably will not
> > really help, but still it's not always OK to do `while (1)' loop in console_unlock()
> > for irqs.
> 
> What we really want is pushing the printing into async context (unless
> forced by debug option or oops in progress). Because what you do here fixes
> only a small fraction of the problem space. I have patches which fix more
> of it (https://lkml.org/lkml/2015/10/26/16) but they are still not enough
> because on large machines e.g. udev times out because printing messages
> about inserted hardware over serial console just takes too long.

absolutely agree. thanks for the link!

	-ss