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

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Message-ID: <CAAq0SUmVmyALNYUbM5dy3D0=Bp=ukNoNdodc1yxYQjm1SnBgAQ@mail.gmail.com>
Date:   Mon, 20 Dec 2021 14:02:11 -0300
From:   Wander Costa <wcosta@...hat.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Wander Lairson Costa <wander@...hat.com>,
        Jiri Slaby <jirislaby@...nel.org>,
        Andrew Jeffery <andrew@...id.au>,
        "Maciej W. Rozycki" <macro@...am.me.uk>,
        Johan Hovold <johan@...nel.org>,
        Serge Semin <fancer.lancer@...il.com>,
        Pali Rohár <pali@...nel.org>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] tty: serial: Use fifo in 8250 console driver

On Mon, Dec 20, 2021 at 12:45 PM Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
>
> On Thu, Nov 04, 2021 at 02:17:31PM -0300, wander@...hat.com wrote:
> > From: Wander Lairson Costa <wander@...hat.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@...hat.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/

> thanks,
>
> greg k-h
>