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 10:32:53AM +0200, 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))

Er, sorry, that should be max(...) instead.

Thanks, Roger.