One more thing On (03/06/18 10:52), Sergey Senozhatsky wrote: [..] > > If you know the baud rate, logbuf size * console throughput is actually > > trivial to calculate.
It's trivial when your setup is trivial. In a less trivial case if you set watchdog threshold based on "logbuf size * console throughput" then things are still too bad. So this is what a typical printk over serial console looks like printk() console_unlock() for (;;) { local_irq_save() call_console_drivers() foo_console_write() spin_lock_irqsave(&port->lock, flags); uart_console_write(port, s, count, foo_console_putchar); spin_unlock_irqrestore(&port->lock, flags); local_irq_restore() } Notice that call_console_drivers->foo_console_write spins on port->lock every time it wants to print out a logbuf line. Why does it do this? In short, because of printf(). Yes, printk() may depend on printf(). printf() n_tty_write() uart_write() uart_port_lock(state, flags) // spin_lock_irqsave(&uport->lock, flags) memcpy(circ->buf + circ->head, buf, c); uart_port_unlock(port, flags) // spin_unlock_irqrestore(&port->lock, flags); Now, printf() messages stored in uart circ buffer must be printed to the console. And this is where console's IRQ handler jumps in. A typical IRQ handler does something like this static irqreturn_t foo_console_irq_handler(...) { spin_lock(&port->lock); rx_chars(port, status); tx_chars(port, status); spin_unlock(&port->lock); } Where tx_chars() usually does something like this while (...) { write_char(port, xmit->buf[xmit->tail]); xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1); if (uart_circ_empty(xmit)) break; } Some drivers flush all pending chars, some drivers limit the number of TX chars to some number, e.g. 512. But in any case, printk() -> call_console_drivers() -> foo_console_write() must spin on port->lock as long as foo_console_irq_handler() has chars to TX / RX. Thus, if you have O(logbuf) of kernel messages, and O(circ->buf) of user space messages, then printk() will spend O(logbuf) + O(circ->buf) + O(RX). So the watchdog threshold value based purely on O(logbuf) (printing to _all_ of the consoles) will not always work. -ss