Re: [PATCH] xen/console: do not drop serial output from the hardware domain

["Roger Pau Monné" <roger.pau@citrix.com>]

On Wed, Jun 22, 2022 at 10:04:19AM +0200, Jan Beulich wrote:
> On 16.06.2022 13:31, Roger Pau Monné wrote:
> > On Tue, Jun 14, 2022 at 11:45:54AM +0200, Jan Beulich wrote:
> >> On 14.06.2022 11:38, Roger Pau Monné wrote:
> >>> On Tue, Jun 14, 2022 at 11:13:07AM +0200, Jan Beulich wrote:
> >>>> On 14.06.2022 10:32, Roger Pau Monné wrote:
> >>>>> On Tue, Jun 14, 2022 at 10:10:03AM +0200, Jan Beulich wrote:
> >>>>>> On 14.06.2022 08:52, Roger Pau Monné wrote:
> >>>>>>> On Mon, Jun 13, 2022 at 03:56:54PM +0200, Jan Beulich wrote:
> >>>>>>>> On 13.06.2022 14:32, Roger Pau Monné wrote:
> >>>>>>>>> On Mon, Jun 13, 2022 at 11:18:49AM +0200, Jan Beulich wrote:
> >>>>>>>>>> On 13.06.2022 11:04, Roger Pau Monné wrote:
> >>>>>>>>>>> On Mon, Jun 13, 2022 at 10:29:43AM +0200, Jan Beulich wrote:
> >>>>>>>>>>>> On 13.06.2022 10:21, Roger Pau Monné wrote:
> >>>>>>>>>>>>> On Mon, Jun 13, 2022 at 09:30:06AM +0200, Jan Beulich wrote:
> >>>>>>>>>>>>>> On 10.06.2022 17:06, Roger Pau Monne wrote:
> >>>>>>>>>>>>>>> Prevent dropping console output from the hardware domain, since it's
> >>>>>>>>>>>>>>> likely important to have all the output if the boot fails without
> >>>>>>>>>>>>>>> having to resort to sync_console (which also affects the output from
> >>>>>>>>>>>>>>> other guests).
> >>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>> Do so by pairing the console_serial_puts() with
> >>>>>>>>>>>>>>> serial_{start,end}_log_everything(), so that no output is dropped.
> >>>>>>>>>>>>>>
> >>>>>>>>>>>>>> While I can see the goal, why would Dom0 output be (effectively) more
> >>>>>>>>>>>>>> important than Xen's own one (which isn't "forced")? And with this
> >>>>>>>>>>>>>> aiming at boot output only, wouldn't you want to stop the overriding
> >>>>>>>>>>>>>> once boot has completed (of which, if I'm not mistaken, we don't
> >>>>>>>>>>>>>> really have any signal coming from Dom0)? And even during boot I'm
> >>>>>>>>>>>>>> not convinced we'd want to let through everything, but perhaps just
> >>>>>>>>>>>>>> Dom0's kernel messages?
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> I normally use sync_console on all the boxes I'm doing dev work, so
> >>>>>>>>>>>>> this request is something that come up internally.
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> Didn't realize Xen output wasn't forced, since we already have rate
> >>>>>>>>>>>>> limiting based on log levels I was assuming that non-ratelimited
> >>>>>>>>>>>>> messages wouldn't be dropped.  But yes, I agree that Xen (non-guest
> >>>>>>>>>>>>> triggered) output shouldn't be rate limited either.
> >>>>>>>>>>>>
> >>>>>>>>>>>> Which would raise the question of why we have log levels for non-guest
> >>>>>>>>>>>> messages.
> >>>>>>>>>>>
> >>>>>>>>>>> Hm, maybe I'm confused, but I don't see a direct relation between log
> >>>>>>>>>>> levels and rate limiting.  If I set log level to WARNING I would
> >>>>>>>>>>> expect to not loose _any_ non-guest log messages with level WARNING or
> >>>>>>>>>>> above.  It's still useful to have log levels for non-guest messages,
> >>>>>>>>>>> since user might want to filter out DEBUG non-guest messages for
> >>>>>>>>>>> example.
> >>>>>>>>>>
> >>>>>>>>>> It was me who was confused, because of the two log-everything variants
> >>>>>>>>>> we have (console and serial). You're right that your change is unrelated
> >>>>>>>>>> to log levels. However, when there are e.g. many warnings or when an
> >>>>>>>>>> admin has lowered the log level, what you (would) do is effectively
> >>>>>>>>>> force sync_console mode transiently (for a subset of messages, but
> >>>>>>>>>> that's secondary, especially because the "forced" output would still
> >>>>>>>>>> be waiting for earlier output to make it out).
> >>>>>>>>>
> >>>>>>>>> Right, it would have to wait for any previous output on the buffer to
> >>>>>>>>> go out first.  In any case we can guarantee that no more output will
> >>>>>>>>> be added to the buffer while Xen waits for it to be flushed.
> >>>>>>>>>
> >>>>>>>>> So for the hardware domain it might make sense to wait for the TX
> >>>>>>>>> buffers to be half empty (the current tx_quench logic) by preempting
> >>>>>>>>> the hypercall.  That however could cause issues if guests manage to
> >>>>>>>>> keep filling the buffer while the hardware domain is being preempted.
> >>>>>>>>>
> >>>>>>>>> Alternatively we could always reserve half of the buffer for the
> >>>>>>>>> hardware domain, and allow it to be preempted while waiting for space
> >>>>>>>>> (since it's guaranteed non hardware domains won't be able to steal the
> >>>>>>>>> allocation from the hardware domain).
> >>>>>>>>
> >>>>>>>> Getting complicated it seems. I have to admit that I wonder whether we
> >>>>>>>> wouldn't be better off leaving the current logic as is.
> >>>>>>>
> >>>>>>> Another possible solution (more like a band aid) is to increase the
> >>>>>>> buffer size from 4 pages to 8 or 16.  That would likely allow to cope
> >>>>>>> fine with the high throughput of boot messages.
> >>>>>>
> >>>>>> You mean the buffer whose size is controlled by serial_tx_buffer?
> >>>>>
> >>>>> Yes.
> >>>>>
> >>>>>> On
> >>>>>> large systems one may want to simply make use of the command line
> >>>>>> option then; I don't think the built-in default needs changing. Or
> >>>>>> if so, then perhaps not statically at build time, but taking into
> >>>>>> account system properties (like CPU count).
> >>>>>
> >>>>> So how about we use:
> >>>>>
> >>>>> min(16384, ROUNDUP(1024 * num_possible_cpus(), 4096))
> >>>>
> >>>> That would _reduce_ size on small systems, wouldn't it? Originally
> >>>> you were after increasing the default size. But if you had meant
> >>>> max(), then I'd fear on very large systems this may grow a little
> >>>> too large.
> >>>
> >>> See previous followup about my mistake of using min() instead of
> >>> max().
> >>>
> >>> On a system with 512 CPUs that would be 512KB, I don't think that's a
> >>> lot of memory, specially taking into account that a system with 512
> >>> CPUs should have a matching amount of memory I would expect.
> >>>
> >>> It's true however that I very much doubt we would fill a 512K buffer,
> >>> so limiting to 64K might be a sensible starting point?
> >>
> >> Yeah, 64k could be a value to compromise on. What total size of
> >> output have you observed to trigger the making of this patch? Xen
> >> alone doesn't even manage to fill 16k on most of my systems ...
> > 
> > I've tried on one of the affected systems now, it's a 8 CPU Kaby Lake
> > at 3,5GHz, and manages to fill the buffer while booting Linux.
> > 
> > My proposed formula won't fix this use case, so what about just
> > bumping the buffer to 32K by default, which does fix it?
> 
> As said, suitably explained I could also agree with going to 64k. The
> question though is in how far 32k, 64k, or ...
> 
> > Or alternatively use the proposed formula, but adjust the buffer to be
> > between [32K,64K].
> 
> ... this formula would cover a wide range of contemporary systems.
> Without such I can't really see what good a bump would do, as then
> many people may still find themselves in need of using the command
> line option to put in place a larger buffer.

I'm afraid I don't know how to make progress with this.

The current value is clearly too low for at least one of my systems.
I don't think it's feasible for me to propose a value or formula that
I can confirm will be suitable for all systems, hence I would suggest
increasing the buffer value to 32K as that does fix the problem on
that specific system (without claiming it's a value that would suit
all setups).

I agree that many people could still find themselves in the need of
using the command line option, but I can assure that new buffer value
would fix the issue on at least one system, which should be enough as
a justification.

Thanks, Roger.