linux-kernel - Re: [PATCH v5 1/1] serial/8250: Use fifo in 8250 console driver

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Date:   Fri, 1 Apr 2022 12:07:05 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Jiri Slaby <jirislaby@...nel.org>
Cc:     Wander Lairson Costa <wander@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Johan Hovold <johan@...nel.org>,
        "Maciej W. Rozycki" <macro@...am.me.uk>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.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>,
        senozhatsky@...omium.org, andre.goddard@...il.com,
        sudipm.mukherjee@...il.com, andy.shevchenko@...il.com,
        David.Laight@...lab.com, jonathanh@...dia.com, phil@...pberrypi.com
Subject: Re: [PATCH v5 1/1] serial/8250: Use fifo in 8250 console driver

On Fri, 1 Apr 2022 06:35:58 +0200
Jiri Slaby <jirislaby@...nel.org> wrote:

> > 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
> > 

Note, the function graph tracer can add a noticeable amount of overhead to
these timings. If you want a more accurate time for a function, just trace
that one function:

	trace-cmd record -p function_graph -l serial8250_console_write ...

As that will only trace the serial8250_console_write() function (think of
-l as "limit"), and the overhead of function graph for tracing a single
function becomes negligible, and then you can see the true time of that
function.

-- Steve