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From:   David Laight <David.Laight@ACULAB.COM>
To:     '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>,
        =?iso-8859-1?Q?Pali_Roh=E1r?= <pali@kernel.org>,
        "open list:SERIAL DRIVERS" <linux-serial@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>
CC:     "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
Thread-Topic: [PATCH v4 1/5] serial/8250: Use fifo in 8250 console driver
Thread-Index: AQHYOUNg+UxO79+xmUuDTD6Kl8POG6zCIVEg
Date:   Wed, 16 Mar 2022 15:27:26 +0000
Message-ID: <bb5f03ec092f462c9656f224895bb224@AcuMS.aculab.com>
References: <20220316143646.13301-1-wander@redhat.com>
 <20220316143646.13301-2-wander@redhat.com>
In-Reply-To: <20220316143646.13301-2-wander@redhat.com>
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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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