Re: [RFC][PATCH v3 09/16] genirq/irqdesc: Have nr_irqs as non-static

[David Hildenbrand <david@redhat.com>]

> 
>>> this would go to a separate section called .tagged_memory.
> 
> That'd be confusing vs. actual memory tags, no?

Yeah, I came to the conclusion just after an upstream call we just had 
about that topic (bi-weekly MM alignment session).

I'm open for any suggestions that make it more generic. My first 
instinct was "named memory regions".

>   
>> Maybe just "tagged_memory.h" or sth. like that? I'm bad at naming, so I
>> would let others make better suggestions.
> 
> inspect.h :)
> 
> I'm going to use 'inspect' as prefix for the thoughts below, but that's
> obviously subject to s/inspect/$BETTERNAME/g :)
> 
>>> Then anyone can walk through the section and collect the data.
>>>
>>> I am just coming up with ideas here.
>>> Could it be even part of mm.h instead of having a new header perhaps ?
>>> Then we won't need to include one more.
>>
>> I don't really have something against a new include, just not one that
>> sounded like a very specific subsystem, not something more generic.
> 
> Right. We really don't want to have five different mechanisms for five
> infrastructures which all allow to inspect kernel memory (life or
> dead) in one way or the other. The difference between them is mostly:
> 
>     - Which subset of the information they expose for inspection
> 
>     - The actual exposure mechanism: crash dump, firmware storage,
>       run-time snapshots in a filesystem, ....
> 
> Having one shared core infrastructure to expose data to those mechanisms
> makes everyones life simpler.
> 
> That obviously needs to collect the superset of data, but that's just a
> bit more memory consumed. That's arguably significantly smaller than
> supporting a zoo of mechanisms to register data for different
> infrastructures.
> 
> I'm quite sure that at least a substantial amount of the required
> information can be collected at compile time in special section
> tables. The rest can be collected in runtime tables, which have the same
> format as the compile time section tables to avoid separate parsers.
> 
> Let me just float some ideas here, how that might look like. It might be
> completely inpractical, but then it might be at least fodder for
> thoughts.

Thanks a bunch for writing all that down!

> 
> As this is specific for the compiled kernel version you can define an
> extensible struct format for the table.
> 
> struct inspect_entry {
> 	unsigned long	properties;
>          unsigned int	type;
>          unsigned int	id;
>          const char	name[$MAX_NAME_LEN];
> 	unsigned long	address;
>          unsigned long	length;
>          ....
> };
> 
> @type
>         refers either to a table with type information, which describes
>         the struct in some way or just generate a detached compile time
>         description.
> 
> @id
>         a unique id created at compile time or via registration at
>         runtime. Might not be required

We discussed that maybe one would want some kind of a "class" 
description. For example we might have to register one pgdat area per 
node. Giving each one a unique name might be impractical / unreasonable.

Still, someone would want to select / filter out all entries of the same 
"class".

Just a thought.

> 
> @name:
>         Name of the memory region. That might go into a separate table
>         which is referenced by @id, but that's up for debate.

Jup.

> 
> @address:
> @length:
>         obvious :)
> 
> ...
>          Whatever a particular consumer might require
> 
> @properties:
> 
>          A "bitfield", which allows to mark this entry as (in)valid for a
>          particular consumer.
> 
>          That obviously requires to modify these properties when the
>          requirements of a consumer change, new consumers arrive or new
>          producers are added, but I think it's easier to do that at the
>          producer side than maintaining filters on all consumer ends
>          forever.

Question would be if that is not up to a consumer to decide ("allowlist" 
/ filter) by class or id, stored elsewhere.

> 
> Though I might be wrong as usual. IOW this needs some thoughts. :)
> 
> The interesting engineering challenge with such a scheme is to come up
> with a annotation mechanism which is extensible.
> 
>       Runtime is trivial as it just needs to fill in the new field in the
>       datastructure and all other runtime users have that zero
>       initialized automatically, if you get the mechanism correct in the
>       first place. Think in templates :)
> 
>       Compile time is a bit more effort, but that should be solvable with
>       key/value pairs.
> 
>       Don't even waste a thought about creating the final tables and
>       sections in macro magic. All the annotation macros have to do is to
>       emit the pairs in a structured way into discardable sections.
> 
>       Those section are then converted in post processing into the actual
>       section table formats and added to the kernel image. Not a
>       spectacular new concept. The kernel build does this already today.
> 
>       Just keep the compile time annotation macro magic simple and
>       stupid. It can waste 10k per entry at compile time and then let
>       postprocessing worry about downsizing and consolidation. Nothing to
>       see here :)

Sounds interesting!

-- 
Cheers

David / dhildenb