linux-kernel - RE: [PATCH v4 1/5] serial/8250: Use fifo in 8250 console driver

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Date:   Wed, 16 Mar 2022 15:27:26 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Wander Lairson Costa' <wander@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Johan Hovold <johan@...nel.org>,
        "Maciej W. Rozycki" <macro@...am.me.uk>,
        Serge Semin <fancer.lancer@...il.com>,
        Lukas Wunner <lukas@...ner.de>,
        Pali Rohár <pali@...nel.org>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
CC:     "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "senozhatsky@...omium.org" <senozhatsky@...omium.org>,
        "andre.goddard@...il.com" <andre.goddard@...il.com>,
        "sudipm.mukherjee@...il.com" <sudipm.mukherjee@...il.com>,
        "andy.shevchenko@...il.com" <andy.shevchenko@...il.com>,
        "jonathanh@...dia.com" <jonathanh@...dia.com>,
        "phil@...pberrypi.com" <phil@...pberrypi.com>
Subject: RE: [PATCH v4 1/5] serial/8250: Use fifo in 8250 console driver

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?

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.

	David

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