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From:   Wander Costa <wcosta@redhat.com>
Date:   Fri, 1 Apr 2022 17:20:27 -0300
Message-ID: <CAAq0SUnFnbo_ABbbAMJRZsa1+30T=-cojgncvnnpCmfrz77U6g@mail.gmail.com>
Subject: Re: [PATCH v5 1/1] serial/8250: Use fifo in 8250 console driver
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Jiri Slaby <jirislaby@kernel.org>,
        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>,
        =?UTF-8?Q?Pali_Roh=C3=A1r?= <pali@kernel.org>,
        "open list:SERIAL DRIVERS" <linux-serial@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>,
        Sergey Senozhatsky <senozhatsky@chromium.org>,
        =?UTF-8?Q?Andr=C3=A9_Goddard_Rosa?= <andre.goddard@gmail.com>,
        Sudip Mukherjee <sudipm.mukherjee@gmail.com>,
        Andy Shevchenko <andy.shevchenko@gmail.com>,
        David Laight <David.Laight@aculab.com>,
        Jon Hunter <jonathanh@nvidia.com>, phil@raspberrypi.com
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On Fri, Apr 1, 2022 at 1:12 PM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> 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.
>

Thanks for the tip. I used bpftrace as a low overhead tracer. Then I
used the function-graph tracer to get the details.

> -- Steve
>