On Thu, Nov 04, 2021 at 02:17:31PM -0300, wander@xxxxxxxxxx wrote:
> From: Wander Lairson Costa <wander@xxxxxxxxxx>
>
> 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 <wander@xxxxxxxxxx>
> ---
> 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
> + */
Why do you need a new bit? Why can't you just do this change for all
devices that have a fifo? Why would you _not_ want to do this for all
devices that have a fifo?
thanks,
greg k-h
