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

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Date:   Thu, 17 Mar 2022 09:27:10 -0300
From:   Wander Costa <wcosta@...hat.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     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>,
        "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

On Wed, Mar 16, 2022 at 12:27 PM David Laight <David.Laight@...lab.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).