Serial console and interrupts latency.

Serial console and interrupts latency.

[Sergey Organov]

Hello,

[Extended CC list to try to get some attention]

I was investigating random serial overruns on my embedded board and
figured it strongly correlates with serial output (to another serial
port) from kernel printk() calls, that forced me to dig into the kernel
sources, and now I'm very confused.

I'm reading drivers/tty/serial/8250/8250_port.c, and
serial8250_console_write() function in particular (being on tty-next
branch).

What I see is that it locks interrupts

3141:		spin_lock_irqsave(&port->lock, flags);

and then calls wait_for_xmitr() both indirectly here:

3159:	uart_console_write(port, s, count, serial8250_console_putchar);

and then directly as well:

3165:	wait_for_xmitr(up, BOTH_EMPTY);

before re-enabling interrupts at:

3179:		spin_unlock_irqrestore(&port->lock, flags);

Now, wait_for_xmitr(), even according to comments, could busy-wait for
up to 10+1000 milliseconds, and in this case this huge delay will happen
at interrupts disabled?

Does it mean any serial console output out of printk() could cause 10
milliseconds or even 1 second interrupts latency? Somehow I can't
believe it.

What do I miss?

Thanks,
-- Sergey

Re: Serial console and interrupts latency.

[Jiri Slaby]

On 24. 03. 20, 10:04, Sergey Organov wrote:
> Hello,
> 
> [Extended CC list to try to get some attention]
> 
> I was investigating random serial overruns on my embedded board and
> figured it strongly correlates with serial output (to another serial
> port) from kernel printk() calls, that forced me to dig into the kernel
> sources, and now I'm very confused.
> 
> I'm reading drivers/tty/serial/8250/8250_port.c, and
> serial8250_console_write() function in particular (being on tty-next
> branch).
> 
> What I see is that it locks interrupts
> 
> 3141:		spin_lock_irqsave(&port->lock, flags);
> 
> and then calls wait_for_xmitr() both indirectly here:
> 
> 3159:	uart_console_write(port, s, count, serial8250_console_putchar);
> 
> and then directly as well:
> 
> 3165:	wait_for_xmitr(up, BOTH_EMPTY);
> 
> before re-enabling interrupts at:
> 
> 3179:		spin_unlock_irqrestore(&port->lock, flags);
> 
> Now, wait_for_xmitr(), even according to comments, could busy-wait for
> up to 10+1000 milliseconds, and in this case this huge delay will happen
> at interrupts disabled?
> 
> Does it mean any serial console output out of printk() could cause 10
> milliseconds or even 1 second interrupts latency? Somehow I can't
> believe it.
> 
> What do I miss?

1 second _timeout_ is for flow-control-enabled consoles.

10 ms is _timeout_ for a character. With slow 9600 baud console, sending
one character takes 0.8 ms. With 115200, it is 70 us.

If you send one line (80 chars), it is really 66 and 5.5 ms, respectively.

So yes, serial consoles can slow down the boot and add latency. Use
faster speeds or faster devices for consoles, if you mind. And do not
enable flow control. Serial is serial.

You can also try to eliminate the interrupts disablement, but that would
be a bit tough task, IMO.

thanks,
-- 
js
suse labs

Re: Serial console and interrupts latency.

[Russell King - ARM Linux admin]

On Fri, Mar 27, 2020 at 02:13:12PM +0100, Jiri Slaby wrote:
> On 24. 03. 20, 10:04, Sergey Organov wrote:
> > Hello,
> > 
> > [Extended CC list to try to get some attention]
> > 
> > I was investigating random serial overruns on my embedded board and
> > figured it strongly correlates with serial output (to another serial
> > port) from kernel printk() calls, that forced me to dig into the kernel
> > sources, and now I'm very confused.
> > 
> > I'm reading drivers/tty/serial/8250/8250_port.c, and
> > serial8250_console_write() function in particular (being on tty-next
> > branch).
> > 
> > What I see is that it locks interrupts
> > 
> > 3141:		spin_lock_irqsave(&port->lock, flags);
> > 
> > and then calls wait_for_xmitr() both indirectly here:
> > 
> > 3159:	uart_console_write(port, s, count, serial8250_console_putchar);
> > 
> > and then directly as well:
> > 
> > 3165:	wait_for_xmitr(up, BOTH_EMPTY);
> > 
> > before re-enabling interrupts at:
> > 
> > 3179:		spin_unlock_irqrestore(&port->lock, flags);
> > 
> > Now, wait_for_xmitr(), even according to comments, could busy-wait for
> > up to 10+1000 milliseconds, and in this case this huge delay will happen
> > at interrupts disabled?
> > 
> > Does it mean any serial console output out of printk() could cause 10
> > milliseconds or even 1 second interrupts latency? Somehow I can't
> > believe it.
> > 
> > What do I miss?
> 
> 1 second _timeout_ is for flow-control-enabled consoles.
> 
> 10 ms is _timeout_ for a character. With slow 9600 baud console, sending
> one character takes 0.8 ms. With 115200, it is 70 us.
> 
> If you send one line (80 chars), it is really 66 and 5.5 ms, respectively.
> 
> So yes, serial consoles can slow down the boot and add latency. Use
> faster speeds or faster devices for consoles, if you mind. And do not
> enable flow control. Serial is serial.
> 
> You can also try to eliminate the interrupts disablement, but that would
> be a bit tough task, IMO.

... and likely difficult to implement, since the spinlock is there to
prevent coincident usage from userspace and kernel space to the port.
If userspace is outputting a message in unison with a kernel console
message, there must be exclusivity, otherwise the two messages will
either be intermingled, or completely corrupted due to TX FIFO
overflow, making the console quite useless.

Higher latency is something that IMHO has to be accepted as a result
of using serial console.

As Jiri says, the way around that is to use as fast a baud rate as
possible on the serial console.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line in suburbia: sync at 10.2Mbps down 587kbps up

Re: Serial console and interrupts latency.

[Sergey Organov]

Jiri Slaby <jslaby@suse.cz> writes:

> On 24. 03. 20, 10:04, Sergey Organov wrote:
>> Hello,
>> 
>> [Extended CC list to try to get some attention]
>> 
>> I was investigating random serial overruns on my embedded board and
>> figured it strongly correlates with serial output (to another serial
>> port) from kernel printk() calls, that forced me to dig into the kernel
>> sources, and now I'm very confused.
>> 
>> I'm reading drivers/tty/serial/8250/8250_port.c, and
>> serial8250_console_write() function in particular (being on tty-next
>> branch).
>> 
>> What I see is that it locks interrupts
>> 
>> 3141:		spin_lock_irqsave(&port->lock, flags);
>> 
>> and then calls wait_for_xmitr() both indirectly here:
>> 
>> 3159:	uart_console_write(port, s, count, serial8250_console_putchar);
>> 
>> and then directly as well:
>> 
>> 3165:	wait_for_xmitr(up, BOTH_EMPTY);
>> 
>> before re-enabling interrupts at:
>> 
>> 3179:		spin_unlock_irqrestore(&port->lock, flags);
>> 
>> Now, wait_for_xmitr(), even according to comments, could busy-wait for
>> up to 10+1000 milliseconds, and in this case this huge delay will happen
>> at interrupts disabled?
>> 
>> Does it mean any serial console output out of printk() could cause 10
>> milliseconds or even 1 second interrupts latency? Somehow I can't
>> believe it.
>> 
>> What do I miss?
>
> 1 second _timeout_ is for flow-control-enabled consoles.

Yeah, sure. So it does mean interrupts could be disabled for up to 1
second, on already up and running system. Too bad.

Actually, I use 8250 only as a reference implementation, my actual
chip is handled by imx.c, and the latter even has no timeouts on this
path, so apparently may block (the entire kernel) indefinitely!

> 10 ms is _timeout_ for a character. With slow 9600 baud console, sending
> one character takes 0.8 ms. With 115200, it is 70 us.

70us of disabled interrupts is a huge number, and for FIFO-enabled chips
the estimate should be multiplied by FIFO size (say, x16) that brings us
close to 1ms even on 115200, right?

Anyway, it must cause receiving overruns on another port running at
higher or the same baud rate and no DMA, sooner or later, as it does
for me.

>
> If you send one line (80 chars), it is really 66 and 5.5 ms, respectively.
>
> So yes, serial consoles can slow down the boot and add latency. Use
> faster speeds or faster devices for consoles, if you mind. And do not
> enable flow control. Serial is serial.

I don't care about slowing-down boot. I care about huge interrupt
latency on up and running system, causing loss of characters  (overruns)
on other serial ports.

To be sure, it is this code that works on already running system as
well, not only on boot-time, right? Or is my system somehow
misconfigured?

I'm confused as this seems to be a major issue and nobody but me seems
to care or to suffer from it, and I can't figure why.

For reference, I figured this locking was introduced by:

commit d8a5a8d7cc32e4474326e0ecc1b959063490efc9
Author: Russell King <rmk@dyn-67.arm.linux.org.uk>
Date:   Tue May 2 16:04:29 2006 +0100

    [SERIAL] 8250: add locking to console write function
    
    x86 SMP breaks as a result of the previous change, we have no real
    option other than to add locking to the 8250 console write function.
    If an oops is in progress, try to acquire the lock.  If we fail to
    do so, continue anyway.


> You can also try to eliminate the interrupts disablement, but that would
> be a bit tough task, IMO.

It seems like I need to, and yeah, it'd be a somewhat tough task indeed,
but then there is one simple question: why isn't console output handled
through usual buffer/ISR paths?

I have an idea to investigate the netconsole module to see how it does
similar job for networks. Hopefully it has no such problems and will
give some useful tips. 

Thanks,
-- Sergey

Re: Serial console and interrupts latency.

[Sergey Organov]

Russell King - ARM Linux admin <linux@armlinux.org.uk> writes:

> On Fri, Mar 27, 2020 at 02:13:12PM +0100, Jiri Slaby wrote:
>> On 24. 03. 20, 10:04, Sergey Organov wrote:
>> > Hello,
>> > 
>> > [Extended CC list to try to get some attention]
>> > 
>> > I was investigating random serial overruns on my embedded board and
>> > figured it strongly correlates with serial output (to another serial
>> > port) from kernel printk() calls, that forced me to dig into the kernel
>> > sources, and now I'm very confused.
>> > 
>> > I'm reading drivers/tty/serial/8250/8250_port.c, and
>> > serial8250_console_write() function in particular (being on tty-next
>> > branch).
>> > 
>> > What I see is that it locks interrupts
>> > 
>> > 3141:		spin_lock_irqsave(&port->lock, flags);
>> > 
>> > and then calls wait_for_xmitr() both indirectly here:
>> > 
>> > 3159:	uart_console_write(port, s, count, serial8250_console_putchar);
>> > 
>> > and then directly as well:
>> > 
>> > 3165:	wait_for_xmitr(up, BOTH_EMPTY);
>> > 
>> > before re-enabling interrupts at:
>> > 
>> > 3179:		spin_unlock_irqrestore(&port->lock, flags);
>> > 
>> > Now, wait_for_xmitr(), even according to comments, could busy-wait for
>> > up to 10+1000 milliseconds, and in this case this huge delay will happen
>> > at interrupts disabled?
>> > 
>> > Does it mean any serial console output out of printk() could cause 10
>> > milliseconds or even 1 second interrupts latency? Somehow I can't
>> > believe it.
>> > 
>> > What do I miss?
>> 
>> 1 second _timeout_ is for flow-control-enabled consoles.
>> 
>> 10 ms is _timeout_ for a character. With slow 9600 baud console, sending
>> one character takes 0.8 ms. With 115200, it is 70 us.
>> 
>> If you send one line (80 chars), it is really 66 and 5.5 ms, respectively.
>> 
>> So yes, serial consoles can slow down the boot and add latency. Use
>> faster speeds or faster devices for consoles, if you mind. And do not
>> enable flow control. Serial is serial.
>> 
>> You can also try to eliminate the interrupts disablement, but that would
>> be a bit tough task, IMO.
>
> ... and likely difficult to implement, since the spinlock is there to
> prevent coincident usage from userspace and kernel space to the port.
> If userspace is outputting a message in unison with a kernel console
> message, there must be exclusivity, otherwise the two messages will
> either be intermingled, or completely corrupted due to TX FIFO
> overflow, making the console quite useless.
>
> Higher latency is something that IMHO has to be accepted as a result
> of using serial console.

This (local) interrupts disabling might be not a big problem on SMP, I
dunno, as another core could still handle IRQs, but for non-SMP case
this is too bad as it causes missed data on other ports.

Can we disable this locking on non-SMP configs as a work-around?

> As Jiri says, the way around that is to use as fast a baud rate as
> possible on the serial console.

Unfortunately, this is entirely impractical for an application that
utilizes maximum baud rates on other serial ports.

Thanks,
-- Sergey

Re: Serial console and interrupts latency.

[Russell King - ARM Linux admin]

On Fri, Mar 27, 2020 at 04:58:33PM +0300, Sergey Organov wrote:
> Jiri Slaby <jslaby@suse.cz> writes:
> 
> > On 24. 03. 20, 10:04, Sergey Organov wrote:
> >> Hello,
> >> 
> >> [Extended CC list to try to get some attention]
> >> 
> >> I was investigating random serial overruns on my embedded board and
> >> figured it strongly correlates with serial output (to another serial
> >> port) from kernel printk() calls, that forced me to dig into the kernel
> >> sources, and now I'm very confused.
> >> 
> >> I'm reading drivers/tty/serial/8250/8250_port.c, and
> >> serial8250_console_write() function in particular (being on tty-next
> >> branch).
> >> 
> >> What I see is that it locks interrupts
> >> 
> >> 3141:		spin_lock_irqsave(&port->lock, flags);
> >> 
> >> and then calls wait_for_xmitr() both indirectly here:
> >> 
> >> 3159:	uart_console_write(port, s, count, serial8250_console_putchar);
> >> 
> >> and then directly as well:
> >> 
> >> 3165:	wait_for_xmitr(up, BOTH_EMPTY);
> >> 
> >> before re-enabling interrupts at:
> >> 
> >> 3179:		spin_unlock_irqrestore(&port->lock, flags);
> >> 
> >> Now, wait_for_xmitr(), even according to comments, could busy-wait for
> >> up to 10+1000 milliseconds, and in this case this huge delay will happen
> >> at interrupts disabled?
> >> 
> >> Does it mean any serial console output out of printk() could cause 10
> >> milliseconds or even 1 second interrupts latency? Somehow I can't
> >> believe it.
> >> 
> >> What do I miss?
> >
> > 1 second _timeout_ is for flow-control-enabled consoles.
> 
> Yeah, sure. So it does mean interrupts could be disabled for up to 1
> second, on already up and running system. Too bad.
> 
> Actually, I use 8250 only as a reference implementation, my actual
> chip is handled by imx.c, and the latter even has no timeouts on this
> path, so apparently may block (the entire kernel) indefinitely!
> 
> > 10 ms is _timeout_ for a character. With slow 9600 baud console, sending
> > one character takes 0.8 ms. With 115200, it is 70 us.
> 
> 70us of disabled interrupts is a huge number, and for FIFO-enabled chips
> the estimate should be multiplied by FIFO size (say, x16) that brings us
> close to 1ms even on 115200, right?
> 
> Anyway, it must cause receiving overruns on another port running at
> higher or the same baud rate and no DMA, sooner or later, as it does
> for me.

So, don't use serial console then, it's unsuitable for your use case.

> > If you send one line (80 chars), it is really 66 and 5.5 ms, respectively.
> >
> > So yes, serial consoles can slow down the boot and add latency. Use
> > faster speeds or faster devices for consoles, if you mind. And do not
> > enable flow control. Serial is serial.
> 
> I don't care about slowing-down boot. I care about huge interrupt
> latency on up and running system, causing loss of characters  (overruns)
> on other serial ports.
> 
> To be sure, it is this code that works on already running system as
> well, not only on boot-time, right? Or is my system somehow
> misconfigured?
> 
> I'm confused as this seems to be a major issue and nobody but me seems
> to care or to suffer from it, and I can't figure why.
> 
> For reference, I figured this locking was introduced by:
> 
> commit d8a5a8d7cc32e4474326e0ecc1b959063490efc9
> Author: Russell King <rmk@dyn-67.arm.linux.org.uk>
> Date:   Tue May 2 16:04:29 2006 +0100
> 
>     [SERIAL] 8250: add locking to console write function
>     
>     x86 SMP breaks as a result of the previous change, we have no real
>     option other than to add locking to the 8250 console write function.
>     If an oops is in progress, try to acquire the lock.  If we fail to
>     do so, continue anyway.

Correct, and what I said back then still applies - and more.

> It seems like I need to, and yeah, it'd be a somewhat tough task indeed,
> but then there is one simple question: why isn't console output handled
> through usual buffer/ISR paths?

The "usual" paths may not be active, and, in the case of an oops, we
want to see the output, which we wouldn't be able to if the oops
occurred in interrupt context.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line in suburbia: sync at 10.2Mbps down 587kbps up

Re: Serial console and interrupts latency.

[Sergey Organov]

Russell King - ARM Linux admin <linux@armlinux.org.uk>:

>
> On Fri, Mar 27, 2020 at 04:58:33PM +0300, Sergey Organov wrote:
> > Jiri Slaby <jslaby@suse.cz> writes:
> >
> > > On 24. 03. 20, 10:04, Sergey Organov wrote:
> > >> Hello,
> > >>
> > >> [Extended CC list to try to get some attention]
> > >>
> > >> I was investigating random serial overruns on my embedded board and
> > >> figured it strongly correlates with serial output (to another serial
> > >> port) from kernel printk() calls, that forced me to dig into the kernel
> > >> sources, and now I'm very confused.
> > >>
> > >> I'm reading drivers/tty/serial/8250/8250_port.c, and
> > >> serial8250_console_write() function in particular (being on tty-next
> > >> branch).
> > >>
> > >> What I see is that it locks interrupts
> > >>
> > >> 3141:              spin_lock_irqsave(&port->lock, flags);
> > >>
> > >> and then calls wait_for_xmitr() both indirectly here:
> > >>
> > >> 3159:      uart_console_write(port, s, count, serial8250_console_putchar);
> > >>
> > >> and then directly as well:
> > >>
> > >> 3165:      wait_for_xmitr(up, BOTH_EMPTY);
> > >>
> > >> before re-enabling interrupts at:
> > >>
> > >> 3179:              spin_unlock_irqrestore(&port->lock, flags);
> > >>
> > >> Now, wait_for_xmitr(), even according to comments, could busy-wait for
> > >> up to 10+1000 milliseconds, and in this case this huge delay will happen
> > >> at interrupts disabled?
> > >>
> > >> Does it mean any serial console output out of printk() could cause 10
> > >> milliseconds or even 1 second interrupts latency? Somehow I can't
> > >> believe it.
> > >>
> > >> What do I miss?
> > >
> > > 1 second _timeout_ is for flow-control-enabled consoles.
> >
> > Yeah, sure. So it does mean interrupts could be disabled for up to 1
> > second, on already up and running system. Too bad.
> >
> > Actually, I use 8250 only as a reference implementation, my actual
> > chip is handled by imx.c, and the latter even has no timeouts on this
> > path, so apparently may block (the entire kernel) indefinitely!
> >
> > > 10 ms is _timeout_ for a character. With slow 9600 baud console, sending
> > > one character takes 0.8 ms. With 115200, it is 70 us.
> >
> > 70us of disabled interrupts is a huge number, and for FIFO-enabled chips
> > the estimate should be multiplied by FIFO size (say, x16) that brings us
> > close to 1ms even on 115200, right?
> >
> > Anyway, it must cause receiving overruns on another port running at
> > higher or the same baud rate and no DMA, sooner or later, as it does
> > for me.
>
> So, don't use serial console then, it's unsuitable for your use case.

Well, I'd rather fix it, as serial console is otherwise very suitable
for my needs.

If nobody else interested, I'll simply disable the lock you've added
for non-smp builds
in my kernel version, rather than trying to fix the issue in general.

That said, finding generic solution would be an interesting quest.

>
> > > If you send one line (80 chars), it is really 66 and 5.5 ms, respectively.
> > >
> > > So yes, serial consoles can slow down the boot and add latency. Use
> > > faster speeds or faster devices for consoles, if you mind. And do not
> > > enable flow control. Serial is serial.
> >
> > I don't care about slowing-down boot. I care about huge interrupt
> > latency on up and running system, causing loss of characters  (overruns)
> > on other serial ports.
> >
> > To be sure, it is this code that works on already running system as
> > well, not only on boot-time, right? Or is my system somehow
> > misconfigured?
> >
> > I'm confused as this seems to be a major issue and nobody but me seems
> > to care or to suffer from it, and I can't figure why.
> >
> > For reference, I figured this locking was introduced by:
> >
> > commit d8a5a8d7cc32e4474326e0ecc1b959063490efc9
> > Author: Russell King <rmk@dyn-67.arm.linux.org.uk>
> > Date:   Tue May 2 16:04:29 2006 +0100
> >
> >     [SERIAL] 8250: add locking to console write function
> >
> >     x86 SMP breaks as a result of the previous change, we have no real
> >     option other than to add locking to the 8250 console write function.
> >     If an oops is in progress, try to acquire the lock.  If we fail to
> >     do so, continue anyway.
>
> Correct, and what I said back then still applies - and more.

What bothers me is "we have no real option..." part of this, as it's rarely
happens to be the case.

>
> > It seems like I need to, and yeah, it'd be a somewhat tough task indeed,
> > but then there is one simple question: why isn't console output handled
> > through usual buffer/ISR paths?
>
> The "usual" paths may not be active, and, in the case of an oops, we
> want to see the output, which we wouldn't be able to if the oops
> occurred in interrupt context.

The oops part is already special-cased and could be left special-cased, one
way or another. What is important is not to keep interrupts disabled for long
during normal system operation.

If it could be achieved at all, it probably should then be implemented at the
upper level,  to save low-level drivers from these complexities.

Thanks,
-- Sergey

Re: Serial console and interrupts latency.

[Russell King - ARM Linux admin]

On Tue, Apr 14, 2020 at 12:50:49PM +0300, Sergey Organov wrote:
> Russell King - ARM Linux admin <linux@armlinux.org.uk>:
> > Correct, and what I said back then still applies - and more.
> 
> What bothers me is "we have no real option..." part of this, as it's rarely
> happens to be the case.

I don't see you coming forward with a solution beyond "let's revert
the commit" or "let's comment out the lock" - neither of which are
an option for mainline kernels.

Until *you* do, since you obviously have better ideas, "we have no
real option".

But, as I've said, one of the *important* characteristics of serial
console is that it is synchronous with the kernel, so it can be
relied upon to get complete messages out if the kernel crashes after
a printk() has been executed, and that must not be lost.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line in suburbia: sync at 10.2Mbps down 587kbps up

Re: Serial console and interrupts latency.

[Sergey Organov]

Russell King - ARM Linux admin <linux@armlinux.org.uk> writes:

> On Tue, Apr 14, 2020 at 12:50:49PM +0300, Sergey Organov wrote:
>> Russell King - ARM Linux admin <linux@armlinux.org.uk>:
>> > Correct, and what I said back then still applies - and more.
>> 
>> What bothers me is "we have no real option..." part of this, as it's rarely
>> happens to be the case.
>
> I don't see you coming forward with a solution beyond "let's revert
> the commit" or "let's comment out the lock" - neither of which are
> an option for mainline kernels.

I didn't suggest to revert the commit, as it obviously solves real
problem. I asked if and why the lock is needed on non-SMP targets, but
either I didn't get a reply or missed it, sorry.

I mean everything I said was to get some help rather than to spread
useless critics or even insults.

>
> Until *you* do, since you obviously have better ideas, "we have no
> real option".

I'm in the process of making myself familiar with the internals of
printk machinery to find generic solution, but if nobody else actually
cares, I'll stop bothering you guys with my questions.

Do I get it right that you still think there are no sensible options but
to disable interrupts for ages? If so, it would greatly reduce my drive
to find one... or maybe the other way round, but it'd be nice to know
either way.

> But, as I've said, one of the *important* characteristics of serial
> console is that it is synchronous with the kernel, so it can be
> relied upon to get complete messages out if the kernel crashes after
> a printk() has been executed, and that must not be lost.

Do I get it right that it's covered by the 'oops' case, or are there
hidden pitfalls as well?

Thanks,
-- Sergey

Serial console and interrupts latency.

[Sergey Organov]

Hello,

I was investigating random serial overruns on my embedded board and
figured it strongly correlates with serial output (to another serial
port) from kernel printk() calls, that forced me to dig into the kernel
sources, and now I'm very confused.

I'm reading drivers/tty/serial/8250/8250_port.c, and
serial8250_console_write() function in particular (being on tty-next
branch).

What I see is that it locks interrupts

3141:		spin_lock_irqsave(&port->lock, flags);

and then calls wait_for_xmitr() both indirectly here:

3159:	uart_console_write(port, s, count, serial8250_console_putchar);

and then directly as well:

3165:	wait_for_xmitr(up, BOTH_EMPTY);

before re-enabling interrupts at:

3179:		spin_unlock_irqrestore(&port->lock, flags);

Now, wait_for_xmitr(), even according to comments, could busy-wait for
up to 10+1000 milliseconds, and in this case this huge delay will happen
at interrupts disabled?

Does it mean any serial console output out of printk() could cause 10
milliseconds or even 1 second interrupts latency? Somehow I can't
believe it.

What do I miss?

Thanks,
-- Sergey