Re: [PATCH] serial: core: prevent softlockups on slow consoles

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Greg Kroah-Hartman <gregkh@linuxfoundation.org> writes:

> On Mon, Aug 31, 2015 at 04:34:16PM +0200, Vitaly Kuznetsov wrote:
>> Hyper-V serial port is very slow on multi-vCPU guest, this causes
>> soflockups on intensive console writes. Touch nmi watchdog after putting
>> every char on port to avoid the issue for all serial drivers, the overhead
>> should be small.
>> 
>> This is just a part of the fix: serial8250_console_write() disables irqs
>> for all its execution time (which on such slow consoles can be dozens of
>> seconds), it should be possible to observe devices being stuck on this
>> CPU. We need to find a better way, e.g. do output in batches enabling irqs
>> in between.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>> ---
>>  drivers/tty/serial/serial_core.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>> 
>> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
>> index f368520..cc05785 100644
>> --- a/drivers/tty/serial/serial_core.c
>> +++ b/drivers/tty/serial/serial_core.c
>> @@ -33,7 +33,7 @@
>>  #include <linux/serial.h> /* for serial_state and serial_icounter_struct */
>>  #include <linux/serial_core.h>
>>  #include <linux/delay.h>
>> -#include <linux/mutex.h>
>> +#include <linux/nmi.h>
>>  
>>  #include <asm/irq.h>
>>  #include <asm/uaccess.h>
>> @@ -1792,6 +1792,7 @@ void uart_console_write(struct uart_port *port, const char *s,
>>  		if (*s == '\n')
>>  			putchar(port, '\r');
>>  		putchar(port, *s);
>> +		touch_nmi_watchdog();
>
> I don't like this, please narrow this down to the real problem that your
> hardware has here, the putchar function should not be this slow.  If it
> is, something is wrong.

I'm afraid this is really the case:

3)               |                                          serial8250_console_putchar() {
3)               |                                            wait_for_xmitr() {
3) # 3111.189 us |                                              io_serial_in();
3) # 3115.334 us |                                            }
3) # 2234.099 us |                                            io_serial_out();
3) # 5353.883 us |                                          }

This is one char and I use local pipe for Hyper-V output. In case
something like remote pipe is in use ...

So I'm sorry, but I don't really understand the suggestion to 'narrow
this down' - this is how slow Hyper-V serial's implementation is,
io_serial_in() is just an inb() and io_serial_out() is an outb().

-- 
  Vitaly