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

On Mon, Dec 20, 2021 at 02:02:11PM -0300, Wander Costa wrote:

On Mon, Dec 20, 2021 at 12:45 PM Greg Kroah-Hartman
[off-list ref] wrote:

quoted

On Thu, Nov 04, 2021 at 02:17:31PM -0300, wander@redhat.com wrote:

quoted

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
+                              */

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?

The v1 patch [1] didn't have this extra bit. Andy suggested [2] to add
it so we only enabled this new code on tested controllers as a
precaution.
If it doesn't make sense to you, feel free to consider the v1 patch [1].

[1] https://lore.kernel.org/all/20211029201402.428284-1-wander@redhat.com/ (local)
[2] https://lore.kernel.org/all/CAHp75Vf6DjNcPWpE4Dh3SuzUMJbFQjq1UNCkrCa60uw35SpqKg@mail.gmail.com/ (local)

I do like [1] better, but can you fix up the checkpatch issues and
resend it:

CHECK: Alignment should match open parenthesis
#116: FILE: drivers/tty/serial/8250/8250_port.c:3342:
+static void serial8250_console_fifo_write(struct uart_8250_port *up,
+	        const char *s, unsigned int count)

CHECK: Logical continuations should be on the previous line
#156: FILE: drivers/tty/serial/8250/8250_port.c:3412:
+	use_fifo = (up->capabilities & UART_CAP_FIFO)
+	        && port->fifosize > 1

CHECK: Logical continuations should be on the previous line
#157: FILE: drivers/tty/serial/8250/8250_port.c:3413:
+	        && port->fifosize > 1
+	        && (serial_port_in(port, UART_FCR) & UART_FCR_ENABLE_FIFO)

CHECK: Logical continuations should be on the previous line
#163: FILE: drivers/tty/serial/8250/8250_port.c:3419:
+	         */
+	        && !(up->port.flags & UPF_CONS_FLOW);



thanks,

greg k-h