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Date:   Fri, 1 Apr 2022 12:07:05 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Jiri Slaby <jirislaby@kernel.org>
Cc:     Wander Lairson Costa <wander@redhat.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Johan Hovold <johan@kernel.org>,
        "Maciej W. Rozycki" <macro@orcam.me.uk>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Lukas Wunner <lukas@wunner.de>,
        Pali =?UTF-8?B?Um9ow6Fy?= <pali@kernel.org>,
        "open list:SERIAL DRIVERS" <linux-serial@vger.kernel.org>,
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        senozhatsky@chromium.org, andre.goddard@gmail.com,
        sudipm.mukherjee@gmail.com, andy.shevchenko@gmail.com,
        David.Laight@aculab.com, jonathanh@nvidia.com, phil@raspberrypi.com
Subject: Re: [PATCH v5 1/1] serial/8250: Use fifo in 8250 console driver
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On Fri, 1 Apr 2022 06:35:58 +0200
Jiri Slaby <jirislaby@kernel.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