Messages in this thread | | | Date | Mon, 5 Mar 2018 22:18:48 -0500 | From | Steven Rostedt <> | Subject | Re: Would you help to tell why async printk solution was not taken to upstream kernel ? |
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On Tue, 6 Mar 2018 11:43:58 +0900 Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> wrote:
> 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. >
If you have a complex setup happening like above, you most likely have printks happening on multiple CPUs which means the work load will be spread out across those CPUs.
-- Steve
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