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SubjectRe: [RFC][PATCHv2 2/8] printk: introduce printing kernel thread
On Wed 2017-03-29 18:25:05, Sergey Senozhatsky wrote:
> This patch introduces a dedicated printing kernel thread - printk_kthread.
> The main purpose of this kthread is to offload printing to a non-atomic
> and always scheduleable context, which eliminates 4) and makes 1)-3) less
> critical. printk() now just appends log messages to the kernel log buffer
> and wake_up()s printk_kthread instead of locking console_sem and calling
> into potentially unsafe console_unlock().
>
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 2d07678e9ff9..ab6b3b2a68c6 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -445,6 +447,42 @@ static char __log_buf[__LOG_BUF_LEN] __aligned(LOG_ALIGN);
> static char *log_buf = __log_buf;
> static u32 log_buf_len = __LOG_BUF_LEN;
>
> +static struct task_struct *printk_kthread __read_mostly;
> +/*
> + * We can't call into the scheduler (wake_up() printk kthread) during
> + * suspend/kexec/etc. This temporarily switches printk to old behaviour.
> + */
> +static atomic_t printk_emergency __read_mostly;
> +/*
> + * Disable printk_kthread permanently. Unlike `oops_in_progress'
> + * it doesn't go back to 0.
> + */

The comment is not valid once we allow to modify the variable using
the sysfs knob.

> @@ -1765,17 +1803,40 @@ asmlinkage int vprintk_emit(int facility, int level,
>
> printed_len += log_output(facility, level, lflags, dict, dictlen, text, text_len);
>
> + /*
> + * Emergency level indicates that the system is unstable and, thus,
> + * we better stop relying on wake_up(printk_kthread) and try to do
> + * a direct printing.
> + */
> + if (level == LOGLEVEL_EMERG)
> + printk_kthread_disabled = true;
> +
> + set_bit(PRINTK_PENDING_OUTPUT, &printk_pending);
> logbuf_unlock_irqrestore(flags);
>
> /* If called from the scheduler, we can not call up(). */
> if (!in_sched) {
> /*
> - * Try to acquire and then immediately release the console
> - * semaphore. The release will print out buffers and wake up
> - * /dev/kmsg and syslog() users.
> + * Under heavy printing load/slow serial console/etc
> + * console_unlock() can stall CPUs, which can result in
> + * soft/hard-lockups, lost interrupts, RCU stalls, etc.
> + * Therefore we attempt to print the messages to console
> + * from a dedicated printk_kthread, which always runs in
> + * schedulable context.
> */
> - if (console_trylock())
> - console_unlock();
> + if (printk_kthread_enabled()) {
> + printk_safe_enter_irqsave(flags);
> + wake_up_process(printk_kthread);
> + printk_safe_exit_irqrestore(flags);

I am really happy that we have the printk_safe stuff available!

> + } else {
> + /*
> + * Try to acquire and then immediately release the
> + * console semaphore. The release will print out
> + * buffers and wake up /dev/kmsg and syslog() users.
> + */
> + if (console_trylock())
> + console_unlock();
> + }
> }
>
> return printed_len;
> @@ -1882,6 +1943,9 @@ static size_t msg_print_text(const struct printk_log *msg,
> bool syslog, char *buf, size_t size) { return 0; }
> static bool suppress_message_printing(int level) { return false; }
>
> +void printk_emergency_begin(void) {}
> +void printk_emergency_end(void) {}
> +
> #endif /* CONFIG_PRINTK */
>
> #ifdef CONFIG_EARLY_PRINTK
> @@ -2164,6 +2228,13 @@ void console_unlock(void)
> bool do_cond_resched, retry;
>
> if (console_suspended) {
> + /*
> + * Avoid an infinite loop in printk_kthread function
> + * when console_unlock() cannot flush messages because
> + * we suspended consoles. Someone else will print the
> + * messages from resume_console().
> + */
> + clear_bit(PRINTK_PENDING_OUTPUT, &printk_pending);

Great catch!

> up_console_sem();
> return;
> }
> @@ -2182,6 +2253,7 @@ void console_unlock(void)
> console_may_schedule = 0;
>
> again:
> + clear_bit(PRINTK_PENDING_OUTPUT, &printk_pending);

This will not help if new messages appear during
call_console_drivers().

I would move this line after the for(;;) cycle. It will be
cleared when all messages are really handled.

Otherwise, it looks fine to me.

Best Regards,
Petr

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