Re: [PATCH v6] serial/8250: Use fifo in 8250 console driver

[Andy Shevchenko]

On Fri, Apr 01, 2022 at 04:46:42PM -0300, Wander Lairson Costa wrote:
> Note: I am using a small test app + driver located at [0] for the
> problem description. serco is a driver whose write function dispatches
> to the serial controller. sertest is a user-mode app that writes n bytes
> to the serial console using the serco driver.
> 
> While investigating a bug in the RHEL kernel, I noticed that the serial
> console throughput is way below the configured speed of 115200 bps in
> a HP Proliant DL380 Gen9. I was expecting something above 10KB/s, but
> I got 2.5KB/s.
> 
> $ time ./sertest -n 2500 /tmp/serco
> 
> real    0m0.997s
> user    0m0.000s
> sys     0m0.997s
> 
> With the help of the function tracer, I then noticed the serial
> controller was taking around 410us seconds to dispatch one single byte:
> 
> $ trace-cmd record -p function_graph -g serial8250_console_write \
>    ./sertest -n 1 /tmp/serco
> 
> $ trace-cmd report
> 
>             |  serial8250_console_write() {
>  0.384 us   |    _raw_spin_lock_irqsave();
>  1.836 us   |    io_serial_in();
>  1.667 us   |    io_serial_out();
>             |    uart_console_write() {
>             |      serial8250_console_putchar() {
>             |        wait_for_xmitr() {
>  1.870 us   |          io_serial_in();
>  2.238 us   |        }
>  1.737 us   |        io_serial_out();
>  4.318 us   |      }
>  4.675 us   |    }
>             |    wait_for_xmitr() {
>  1.635 us   |      io_serial_in();
>             |      __const_udelay() {
>  1.125 us   |        delay_tsc();
>  1.429 us   |      }
> ...
> ...
> ...
>  1.683 us   |      io_serial_in();
>             |      __const_udelay() {
>  1.248 us   |        delay_tsc();
>  1.486 us   |      }
>  1.671 us   |      io_serial_in();
>  411.342 us |    }
> 
> In another machine, I measured a throughput of 11.5KB/s, with the serial
> controller taking between 80-90us to send each byte. That matches the
> expected throughput for a configuration of 115200 bps.
> 
> This patch changes the serial8250_console_write to use the 16550 fifo
> if available. In my benchmarks I got around 25% improvement in the slow
> machine, and no performance penalty in the fast machine.

...

> +	use_fifo = (up->capabilities & UART_CAP_FIFO) &&
> +		/*
> +		 * BCM283x requires to check the fifo
> +		 * after each byte.
> +		 */
> +		!(up->capabilities & UART_CAP_MINI) &&

Perhaps you need to also comment why we are using tx_loadsz and not fifosize.

> +		up->tx_loadsz > 1 &&
> +		(up->fcr & UART_FCR_ENABLE_FIFO) &&
> +		port->state &&
> +		test_bit(TTY_PORT_INITIALIZED, &port->state->port.iflags) &&
> +		/*
> +		 * After we put a data in the fifo, the controller will send
> +		 * it regardless of the CTS state. Therefore, only use fifo
> +		 * if we don't use control flow.
> +		 */
> +		!(up->port.flags & UPF_CONS_FLOW);

-- 
With Best Regards,
Andy Shevchenko