Re: [PATCH 3/7] kernel: Avoid softlockups in stop_machine() during heavy printing

[Jan Kara]

  Hmph, sorry for the x/7 numbering. Patch 7 was the debug patch which I
didn't send...

								Honza

On Mon 26-10-15 05:52:46, Jan Kara wrote:
> From: Jan Kara <jack@xxxxxxx>
> 
> 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.
> 
> Signed-off-by: Jan Kara <jack@xxxxxxx>
> ---
>  include/linux/console.h | 11 +++++++++++
>  kernel/printk/printk.c  | 25 +++++++++++++++++++++++++
>  kernel/stop_machine.c   |  9 +++++++++
>  3 files changed, 45 insertions(+)
> 
> diff --git a/include/linux/console.h b/include/linux/console.h
> index bd194343c346..96da462cdfeb 100644
> --- a/include/linux/console.h
> +++ b/include/linux/console.h
> @@ -150,6 +150,17 @@ extern int console_trylock(void);
>  extern void console_unlock(void);
>  extern void console_conditional_schedule(void);
>  extern void console_unblank(void);
> +#ifdef CONFIG_SMP
> +extern void printk_log_buf_drain(void);
> +#else
> +/*
> + * In non-SMP kernels there won't be much to drain so save some code for tiny
> + * kernels.
> + */
> +static inline void printk_log_buf_drain(void)
> +{
> +}
> +#endif
>  extern struct tty_driver *console_device(int *);
>  extern void console_stop(struct console *);
>  extern void console_start(struct console *);
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index b9bb4a7a6dff..8dc6c146d022 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2488,6 +2488,31 @@ struct tty_driver *console_device(int *index)
>  	return driver;
>  }
>  
> +/* For non-SMP kernels this function isn't used and would be pointless anyway */
> +#ifdef CONFIG_SMP
> +/*
> + * 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 printk_log_buf_drain(void)
> +{
> +	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);
> +		if (!retry || console_suspended)
> +			break;
> +		/* Cycle console_sem to wait for outstanding printing */
> +		console_lock();
> +		console_unlock();
> +	}
> +}
> +#endif
> +
>  /*
>   * Prevent further output on the passed console device so that (for example)
>   * serial drivers can disable console output before suspending a port, and can
> diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
> index 12484e5d5c88..e9496b4a3825 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 @@ static int __stop_machine(cpu_stop_fn_t fn, void *data, const struct cpumask *cp
>  		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.
> +	 */
> +	printk_log_buf_drain();
> +
>  	/* 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);
> -- 
> 2.1.4
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
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