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Date: Wed, 22 Dec 2021 08:22:07 -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 Tue, Dec 21, 2021 at 5:12 AM Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
>
> 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@...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/
>
> I do like [1] better, but can you fix up the checkpatch issues and
> resend it:
>
Sure. No problem.
> 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
>
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