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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

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
> <gregkh@linuxfoundation.org> wrote:
> >
> > On Thu, Nov 04, 2021 at 02:17:31PM -0300, wander@redhat.com wrote:
> > > From: Wander Lairson Costa <wander@redhat.com>
> > >
> > > 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@redhat.com>
> > > ---
> > >  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/
> [2] https://lore.kernel.org/all/CAHp75Vf6DjNcPWpE4Dh3SuzUMJbFQjq1UNCkrCa60uw35SpqKg@mail.gmail.com/

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