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

[Wander Costa]

On Fri, Apr 1, 2022 at 1:12 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> On Fri, 1 Apr 2022 06:35:58 +0200
> Jiri Slaby <jirislaby@xxxxxxxxxx> 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
>