tty: serial: Use fifo in 8250 console driver
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. Signed-off-by:Wander Lairson Costa <wander@redhat.com> Link: https://lore.kernel.org/r/20211222112831.1968392-2-wander@redhat.com Signed-off-by:
Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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