Re: [PATCH 3/4] kernel: Avoid softlockups in stop_machine() during heavy printing

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 19 Aug 2015 17:38:30 +0200 Jan Kara <jack@suse.com> wrote:

> From: Jan Kara <jack@suse.cz>
> 
> When there are lots of messages accumulated in printk buffer, printing
> them (especially over serial console) can take a long time (tens of
> seconds). stop_machine() will effectively make all cpus spin in
> multi_cpu_stop() waiting for the CPU doing printing to print all the
> messages which triggers NMI softlockup watchdog and RCU stall detector
> which add even more to the messages to print. Since machine doesn't do
> anything (except serving interrupts) during this time, also network
> connections are dropped and other disturbances may happen.
> 
> Paper over the problem by waiting for printk buffer to be empty before
> starting to stop CPUs. In theory a burst of new messages can be appended
> to the printk buffer before CPUs enter multi_cpu_stop() so this isn't a 100%
> solution but it works OK in practice and I'm not aware of a reasonably
> simple better solution.

Confused.  Why don't patches 1 and 2 already fix this problem?

>
> ...
>
> @@ -2489,6 +2489,28 @@ struct tty_driver *console_device(int *index)
>  }
>  
>  /*
> + * Wait until all messages accumulated in the printk buffer are printed to
> + * console. Note that as soon as this function returns, new messages may be
> + * added to the printk buffer by other CPUs.
> + */
> +void console_flush(void)

This doesn't seem a very good name.  We already have
console_cont_flush(), cont_flush(), etc.  Can we think of something
more specific?  printk_log_buf_drain() perhaps.

> +{
> +	bool retry;
> +	unsigned long flags;
> +
> +	while (1) {
> +		raw_spin_lock_irqsave(&logbuf_lock, flags);
> +		retry = console_seq != log_next_seq;
> +		raw_spin_unlock_irqrestore(&logbuf_lock, flags);

Does this lock/unlock do anything useful?

> +		if (!retry || console_suspended)
> +			break;
> +		/* Cycle console_sem to wait for outstanding printing */
> +		console_lock();
> +		console_unlock();
> +	}
> +}
> +
> +/*
>   * Prevent further output on the passed console device so that (for example)
>   * serial drivers can disable console output before suspending a port, and can
>   * re-enable output afterwards.
> diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
> index fd643d8c4b42..016d34621d2e 100644
> --- a/kernel/stop_machine.c
> +++ b/kernel/stop_machine.c
> @@ -21,6 +21,7 @@
>  #include <linux/smpboot.h>
>  #include <linux/atomic.h>
>  #include <linux/lglock.h>
> +#include <linux/console.h>
>  
>  /*
>   * Structure to determine completion condition and record errors.  May
> @@ -543,6 +544,14 @@ int __stop_machine(int (*fn)(void *), void *data, const struct cpumask *cpus)
>  		return ret;
>  	}
>  
> +	/*
> +	 * If there are lots of outstanding messages, printing them can take a
> +	 * long time and all cpus would be spinning waiting for the printing to
> +	 * finish thus triggering NMI watchdog, RCU lockups etc. Wait for the
> +	 * printing here to avoid these.
> +	 */
> +	console_flush();

This is pretty pointless if num_possible_cpus==1.  I'd suggest setting
printk_offload_chars=0 in this case, add some early bale-out into
console_flush().  Or something along those lines.

And make console_flush() go away altogether if CONFIG_SMP=n - it's
pointless bloat.

>  	/* Set the initial state and stop all online cpus. */
>  	set_state(&msdata, MULTI_STOP_PREPARE);
>  	return stop_cpus(cpu_online_mask, multi_cpu_stop, &msdata);