Re: [PATCH v2] tty: serial: Use fifo in 8250 console driver

From: Wander Lairson Costa <redacted>

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.

Recently I got a report of a soft lockup while loading a bunch a
scsi_debug devices (> 500).

While investigating it, I noticed that the serial console throughput
(called by the printk code) is way below the configured speed of 115200
bps in a HP Proliant DL380 Gen9 server. I was expecting something above
10KB/s, but I got 2.5KB/s. I then built a simple driver [0] to isolate
the console from the printk code. Here it is:

$ 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 artificial benchmark I could get a throughput
increase up to 100% in some cases, but in the real case described at the
beginning the gain was of about 25%.

[0] https://github.com/walac/serial-console-test

Signed-off-by: Wander Lairson Costa <redacted>
---
 drivers/tty/serial/8250/8250.h      |  3 ++
 drivers/tty/serial/8250/8250_port.c | 63 +++++++++++++++++++++++++----
 2 files changed, 59 insertions(+), 7 deletions(-)

diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
index 6473361525d1..c711bf118cc1 100644
--- a/drivers/tty/serial/8250/8250.h
+++ b/drivers/tty/serial/8250/8250.h

@@ -83,6 +83,9 @@ struct serial8250_config {
 #define UART_CAP_MINI	BIT(17)	/* Mini UART on BCM283X family lacks:
 					 * STOP PARITY EPAR SPAR WLEN5 WLEN6
 					 */
+#define UART_CAP_CWFIFO BIT(18) /* Use the UART Fifo in
+				 * serial8250_console_write
+				 */