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From: Wander Costa <wcosta@redhat.com>
To: David Laight <David.Laight@aculab.com>
Cc: "Wander Lairson Costa" <wander@redhat.com>,
	"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
	"Jiri Slaby" <jirislaby@kernel.org>,
	"Johan Hovold" <johan@kernel.org>,
	"Maciej W. Rozycki" <macro@orcam.me.uk>,
	"Serge Semin" <fancer.lancer@gmail.com>,
	"Lukas Wunner" <lukas@wunner.de>, "Pali Rohár" <pali@kernel.org>,
	"open list:SERIAL DRIVERS" <linux-serial@vger.kernel.org>,
	"open list" <linux-kernel@vger.kernel.org>,
	"rostedt@goodmis.org" <rostedt@goodmis.org>,
	"senozhatsky@chromium.org" <senozhatsky@chromium.org>,
	"andre.goddard@gmail.com" <andre.goddard@gmail.com>,
	"sudipm.mukherjee@gmail.com" <sudipm.mukherjee@gmail.com>,
	"andy.shevchenko@gmail.com" <andy.shevchenko@gmail.com>,
	"jonathanh@nvidia.com" <jonathanh@nvidia.com>,
	"phil@raspberrypi.com" <phil@raspberrypi.com>
Subject: Re: [PATCH v4 1/5] serial/8250: Use fifo in 8250 console driver
Date: Thu, 17 Mar 2022 09:27:10 -0300	[thread overview]
Message-ID: <CAAq0SUnGtOgJ5PWZXZ8QHoOxMJEPi1kuuuNeEhGq2iu73oF9AA@mail.gmail.com> (raw)
In-Reply-To: <bb5f03ec092f462c9656f224895bb224@AcuMS.aculab.com>

On Wed, Mar 16, 2022 at 12:27 PM David Laight <David.Laight@aculab.com> wrote:
>
> From: Wander Lairson Costa
> > Sent: 16 March 2022 14:37
> >
> > 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:
>
> Did you verify the baud rate?
>

Yes.



> Or is there some horrid serial redirection going on.
> It is even possible there is a bios smm interrupt
> chugging through on another cpu core.
>

I would be surprised if that were the case because I see the problem
even in low system activity. Someone in a previous patch-revision
theorized this might be due to a bad serial controller emulator
implemented in hardware (or something in this line of thought).


  reply	other threads:[~2022-03-17 12:27 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-03-16 14:36 [PATCH v4 0/5] tty/8250: Use fifo in 8250 console driver Wander Lairson Costa
2022-03-16 14:36 ` [PATCH v4 1/5] serial/8250: " Wander Lairson Costa
2022-03-16 15:27   ` David Laight
2022-03-17 12:27     ` Wander Costa [this message]
2022-03-16 14:36 ` [PATCH v4 2/5] serial/8250: Use the cache value of the FCR register Wander Lairson Costa
2022-03-17  8:29   ` Ilpo Järvinen
2022-03-17 12:03     ` Wander Costa
2022-03-16 14:36 ` [PATCH v4 3/5] serial/8250: Use tx_loadsz as the transmitter fifo size Wander Lairson Costa
2022-03-16 14:36 ` [PATCH v4 4/5] serial/8250: exclude BCM283x from console_fifo_write Wander Lairson Costa
2022-03-16 14:36 ` [PATCH v4 5/5] serial/8250: Only use fifo after the port is initialized in console_write Wander Lairson Costa
2022-03-17  7:06   ` Jiri Slaby
2022-03-17 12:22     ` Wander Costa
2022-03-17  8:43   ` Ilpo Järvinen
2022-03-17 12:23     ` Wander Costa
2022-03-16 16:13 ` [PATCH v4 0/5] tty/8250: Use fifo in 8250 console driver Andy Shevchenko
2022-03-16 16:14   ` Andy Shevchenko
2022-03-16 16:37   ` Wander Costa
2022-03-17 15:47 ` Steven Rostedt

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