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

[Jan Beulich <jbeulich@xxxxxxxx>]

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? 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).

Jan