linux-kernel - Re: [PATCH] tty: serial: Use fifo in 8250 console driver

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Date:   Tue, 25 Jan 2022 09:03:54 +0000
From:   Jon Hunter <jonathanh@...dia.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     wander@...hat.com, Jiri Slaby <jirislaby@...nel.org>,
        "Maciej W. Rozycki" <macro@...am.me.uk>,
        Johan Hovold <johan@...nel.org>,
        Andrew Jeffery <andrew@...id.au>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH] tty: serial: Use fifo in 8250 console driver


On 25/01/2022 08:50, Greg Kroah-Hartman wrote:
> On Tue, Jan 25, 2022 at 08:39:24AM +0000, Jon Hunter wrote:
>>
>> On 29/10/2021 21:14, wander@...hat.com wrote:
>>> From: Wander Lairson Costa <wander@...hat.com>
>>>
>>> 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:
>>>
>>> $ trace-cmd record -p function_graph -g serial8250_console_write \
>>>      ./sertest -n 1 /tmp/serco
>>>
>>> $ trace-cmd report
>>>
>>>               |  serial8250_console_write() {
>>>    0.384 us   |    _raw_spin_lock_irqsave();
>>>    1.836 us   |    io_serial_in();
>>>    1.667 us   |    io_serial_out();
>>>               |    uart_console_write() {
>>>               |      serial8250_console_putchar() {
>>>               |        wait_for_xmitr() {
>>>    1.870 us   |          io_serial_in();
>>>    2.238 us   |        }
>>>    1.737 us   |        io_serial_out();
>>>    4.318 us   |      }
>>>    4.675 us   |    }
>>>               |    wait_for_xmitr() {
>>>    1.635 us   |      io_serial_in();
>>>               |      __const_udelay() {
>>>    1.125 us   |        delay_tsc();
>>>    1.429 us   |      }
>>> ...
>>> ...
>>> ...
>>>    1.683 us   |      io_serial_in();
>>>               |      __const_udelay() {
>>>    1.248 us   |        delay_tsc();
>>>    1.486 us   |      }
>>>    1.671 us   |      io_serial_in();
>>>    411.342 us |    }
>>>
>>> In another machine, I measured a throughput of 11.5KB/s, with the serial
>>> controller taking between 80-90us to send each byte. That matches the
>>> expected throughput for a configuration of 115200 bps.
>>>
>>> This patch changes the serial8250_console_write to use the 16550 fifo
>>> if available. In my benchmarks I got around 25% improvement in the slow
>>> machine, and no performance penalty in the fast machine.
>>>
>>> Signed-off-by: Wander Lairson Costa <wander@...hat.com>
>>
>>
>> On the current mainline and -next branches, I have noticed that the
>> serial output on many of our Tegra boards is corrupted and so
>> parsing the serial output is failing.
>>
>> Before this change the serial console would appear as follows ...
>>
>> [    0.000000] Booting Linux on physical CPU 0x0000000000 [0x411fd071]
>> [    0.000000] Linux version 5.16.0-rc6-00091-gadbfddc757ae (jonathanh@...athanh-vm-01) (aarch64-linux-gnu-gcc (Linaro GCC 6.4-2017.08) 6.4.1 20170707, GNU ld (Linaro_Binutils-2017.08) 2.27.0.20161019) #15 SMP PREEMPT Tue Jan 25 00:15:25 PST 2022
>> [    0.000000] Machine model: NVIDIA Jetson TX1 Developer Kit
>>
>> And now I see ...
>>
>> [    0.000000] Booting Linux on physicalfd071]
>> [    0.000000] Linux version 5.16.0-rc6-athanh@...inux-g017.08) Linaro_B20161019n 25 00:[    0.000000] Machine model: NVIDIA Jet[    0.000000] efi: UEFI not found.
>> [    0.000000] NUMA: No NUMA configurati[    0.000000] NUMA: Faking a node at [m00000001[    0.000000] NUMA: NODE_DATA [mem 0x17[    0.000000] Zone ranges:
>>
>> Bisecting is pointing to this commit. Let me know if there are any
>> tests I can run. Otherwise we may need to disable this at least
>> for Tegra.
> 
> Ick.  Does this uart have any other quirks assigned to it that are
> somehow not getting assigned here?


Not that I know of, but I can have a look. I did check to see if there 
are any known issues that could be related but I have not found any so far.

Jon

-- 
nvpublic