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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

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))

Maybe we should also take CPU frequency into account, but that seems
too complex for the purpose.

Thanks, Roger.