Re: [PATCH v5 0/2] printk: Console owner and waiter logic cleanup

Tejun Heo
2018-01-17T20:06:01Z

Hello, Steven.

On Wed, Jan 17, 2018 at 12:12:51PM -0500, Steven Rostedt wrote:
> From what I gathered, you said an OOM would trigger, and then the
> network console would not be able to allocate memory and it would
> trigger a printk too, and cause an infinite amount of printks.

Yeah, it falls into back-and-forth loop between the OOM code and
netconsole path.

> This could very well be a great place to force offloading. If a printk
> is called from within a printk, at the same context (normal, softirq,
> irq or NMI), then we should trigger the offloading.

I was thinking more of a timeout based approach (ie. if stuck for
longer than X or X messages, offload), but if local feedback loop is
the only thing we're missing after your improvements, detecting that
specific condition definitely works and is likely a better approach in
terms of message delivery guarantee.

> +static void kick_offload_thread(void)
> +{
> + /*
> + * Consoles are triggering printks, offload the printks
> + * to another CPU to hopefully avoid a lockup.
> + */
> +}
...
> @@ -2333,6 +2390,7 @@ void console_unlock(void)
>
> for (;;) {
> struct printk_log *msg;
> + bool offload;
> size_t ext_len = 0;
> size_t len;
>
> @@ -2393,15 +2451,20 @@ void console_unlock(void)
> * waiter waiting to take over.
> */
> console_lock_spinning_enable();
> + offload = recursion_check_start();
>
> stop_critical_timings(); /* don't trace print latency */
> call_console_drivers(ext_text, ext_len, text, len);
> start_critical_timings();
>
> + recursion_check_finish(offload);
> +
> if (console_lock_spinning_disable_and_check()) {
> printk_safe_exit_irqrestore(flags);
> return;
> }
> + if (offload)
> + kick_offload_thread();

Yeah, something like this would definitely work.

Thanks a lot.

--
tejun