From: Wander Lairson Costa
> Sent: 16 March 2022 14:37
>
> Note: I am using a small test app + driver located at [0] for the
> problem description. serco is a driver whose write function dispatches
> to the serial controller. sertest is a user-mode app that writes n bytes
> to the serial console using the serco driver.
>
> While investigating a bug in the RHEL kernel, I noticed that the serial
> console throughput is way below the configured speed of 115200 bps in
> a HP Proliant DL380 Gen9. I was expecting something above 10KB/s, but
> I got 2.5KB/s.
>
> $ time ./sertest -n 2500 /tmp/serco
>
> real 0m0.997s
> user 0m0.000s
> sys 0m0.997s
>
> With the help of the function tracer, I then noticed the serial
> controller was taking around 410us seconds to dispatch one single byte:
Did you verify the baud rate?
Or is there some horrid serial redirection going on.
It is even possible there is a bios smm interrupt
chugging through on another cpu core.
David
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On Wed, Mar 16, 2022 at 12:27 PM David Laight <[email protected]> wrote:
>
> From: Wander Lairson Costa
> > Sent: 16 March 2022 14:37
> >
> > Note: I am using a small test app + driver located at [0] for the
> > problem description. serco is a driver whose write function dispatches
> > to the serial controller. sertest is a user-mode app that writes n bytes
> > to the serial console using the serco driver.
> >
> > While investigating a bug in the RHEL kernel, I noticed that the serial
> > console throughput is way below the configured speed of 115200 bps in
> > a HP Proliant DL380 Gen9. I was expecting something above 10KB/s, but
> > I got 2.5KB/s.
> >
> > $ time ./sertest -n 2500 /tmp/serco
> >
> > real 0m0.997s
> > user 0m0.000s
> > sys 0m0.997s
> >
> > With the help of the function tracer, I then noticed the serial
> > controller was taking around 410us seconds to dispatch one single byte:
>
> Did you verify the baud rate?
>
Yes.
> Or is there some horrid serial redirection going on.
> It is even possible there is a bios smm interrupt
> chugging through on another cpu core.
>
I would be surprised if that were the case because I see the problem
even in low system activity. Someone in a previous patch-revision
theorized this might be due to a bad serial controller emulator
implemented in hardware (or something in this line of thought).