On Wed, Mar 16, 2022 at 12:27 PM David Laight <David.Laight@xxxxxxxxxx> 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).
