linux-kernel - Re: [PATCH 3/4] printk: Defer printing to irq work when we printed too much

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Date:	Wed, 21 Aug 2013 15:06:26 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Jan Kara <jack@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, mhocko@...e.cz, hare@...e.de
Subject: Re: [PATCH 3/4] printk: Defer printing to irq work when we printed
 too much

On Wed, 21 Aug 2013 10:08:31 +0200
Jan Kara <jack@...e.cz> wrote:

> A CPU can be caught in console_unlock() for a long time (tens of seconds
> are reported by our customers) when other CPUs are using printk heavily
> and serial console makes printing slow. Despite serial console drivers
> are calling touch_nmi_watchdog() this triggers softlockup warnings
> because interrupts are disabled for the whole time console_unlock() runs
> (e.g. vprintk() calls console_unlock() with interrupts disabled). Thus
> IPIs cannot be processed and other CPUs get stuck spinning in calls like
> smp_call_function_many(). Also RCU eventually starts reporting lockups.
> 
> In my artifical testing I can also easily trigger a situation when disk
> disappears from the system apparently because commands to / from it
> could not be delivered for long enough. This is why just silencing
> watchdogs isn't a reliable solution to the problem and we simply have to
> avoid spending too long in console_unlock() with interrupts disabled.
> 
> The solution this patch works toward is to postpone printing to a later
> moment / different CPU when we already printed over 1000 characters in
> current console_unlock() invocation. This is a crude heuristic but
> measuring time we spent printing doesn't seem to be really viable - we
> cannot rely on high resolution time being available and with interrupts
> disabled jiffies are not updated. The value 1000 was chosen so that
> things are still bearable with 9600 baud serial console and OTOH it
> shouldn't cause offloading of printing in common cases.
> 

BTW, you can also test by running:

 # cd /sys/kernel/debug/tracing
 # echo function > current_tracer
 # sleep 1
 # echo z > /proc/sysrq-trigger


> Signed-off-by: Jan Kara <jack@...e.cz>

Reviewed-by: Steven Rostedt <rostedt@...dmis.org>

-- Steve
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