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