Re: [PATCH v3 1/1] tty: serial: Use fifo in 8250 console driver

[Wander Costa]

On Tue, Jan 25, 2022 at 10:07 AM Sebastian Andrzej Siewior
<bigeasy@xxxxxxxxxxxxx> wrote:
>
> On 2021-12-22 08:28:30 [-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:
>
> was this the HW access or did this include the wait-for-fifo empty?
>
> > $ 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 |    }
>
> So this includes waiting for empty slot. It is wait_for_xmitr() only.
>
> > 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.
>
> Either the HW is slow on starting to work, or…
>
> What I noticed now in -rc1 is this output during boot:
>
> |[    6.370196] ACPI: button: Power Button [PWRF]
> |[    6.443501] Serial: 8250/16550 driver, 4 ports, IRQ sharing enabled
> |[0I 15
> |      [0I 15
> |            [No
> |               [ld
> |                  [a2
> |                     [a20tm
> |                           [a2nct
> |                                 [s
> |[s
> |[s
> |[s
> …
> |[sk65,
> |      [rt
> |         [Pe
> |            [a
> |               [    6.873611] ata1: SATA link down (SStatus 0 SControl 300)
> |[    6.879680] ata3: SATA link down (SStatus 0 SControl 300)
>
> The kernel buffer reports here:
>
> |[    6.370196] ACPI: button: Power Button [PWRF]
> |[    6.443501] Serial: 8250/16550 driver, 4 ports, IRQ sharing enabled
> |[    6.450643] 00:03: ttyS0 at I/O 0x3f8 (irq = 4, base_baud = 115200) is a 16550A
> |[    6.451625] 00:04: ttyS1 at I/O 0x2f8 (irq = 3, base_baud = 115200) is a 16550A
> |[    6.453808] Non-volatile memory driver v1.3
> |[    6.475688] loop: module loaded
> |[    6.476401] ahci 0000:00:1f.2: version 3.0
> |[    6.487238] ahci 0000:00:1f.2: AHCI 0001.0300 32 slots 6 ports 6 Gbps 0x3f impl SATA mode
>
> I did remove the last few lines but it appears that since the
> initialisation of the port some of the lines got lost.
>
> Do you see the same?

There is another thread[1] reporting some issues with this patch.
There, this diff seems to fix the problems, could you please try and
report if it works for you too?

diff --git a/drivers/tty/serial/8250/8250_port.c
b/drivers/tty/serial/8250/8250_port.c
index 2abb3de11a48..d3a93e5d55f7 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -3343,7 +3343,7 @@ static void serial8250_console_fifo_write(struct
uart_8250_port *up,
 {
        int i;
        const char *end = s + count;
-       unsigned int fifosize = up->port.fifosize;
+       unsigned int fifosize = up->tx_loadsz;
        bool cr_sent = false;

        while (s != end) {
@@ -3409,8 +3409,8 @@ void serial8250_console_write(struct
uart_8250_port *up, const char *s,
        }

        use_fifo = (up->capabilities & UART_CAP_FIFO) &&
-               port->fifosize > 1 &&
-               (serial_port_in(port, UART_FCR) & UART_FCR_ENABLE_FIFO) &&
+               up->tx_loadsz > 1 &&
+               (up->fcr & UART_FCR_ENABLE_FIFO) &&
                /*
                 * After we put a data in the fifo, the controller will send
                 * it regardless of the CTS state. Therefore, only use fifo

[1] https://lore.kernel.org/all/fa42a60c-954a-acc0-3962-f00427153f78@xxxxxxxxxx/


>
> > Signed-off-by: Wander Lairson Costa <wander@xxxxxxxxxx>
>
> Sebastian
>