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

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Date:   Fri, 1 Apr 2022 17:20:27 -0300
From:   Wander Costa <wcosta@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Jiri Slaby <jirislaby@...nel.org>,
        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>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        André Goddard Rosa <andre.goddard@...il.com>,
        Sudip Mukherjee <sudipm.mukherjee@...il.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Jon Hunter <jonathanh@...dia.com>, phil@...pberrypi.com
Subject: Re: [PATCH v5 1/1] serial/8250: Use fifo in 8250 console driver

On Fri, Apr 1, 2022 at 1:12 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> 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.
>

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

> -- Steve
>