Re: [PATCH v6] serial/8250: Use fifo in 8250 console driver

[Wander Costa]

On Mon, Apr 4, 2022 at 6:32 AM Andy Shevchenko
<andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
>
> On Fri, Apr 01, 2022 at 04:46:42PM -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:
> >
> > $ 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.
>
> ...
>
> > +     use_fifo = (up->capabilities & UART_CAP_FIFO) &&
> > +             /*
> > +              * BCM283x requires to check the fifo
> > +              * after each byte.
> > +              */
> > +             !(up->capabilities & UART_CAP_MINI) &&
>
> Perhaps you need to also comment why we are using tx_loadsz and not fifosize.
>

Maybe it is better to document their difference in the struct
declaration and not in a random usage.


[snip]