RFR: 8340363: User-specified default decorators for UnifiedLogging

[David Holmes]

On Fri, 13 Sep 2024 09:03:55 GMT, Antón Seoane <duke at openjdk.org> wrote:

> Hi all,
> 
> Currently, the Unified Logging framework defaults to three decorators (uptime, level, tags) whenever the user does not specify otherwise through `-Xlog`. This can result in cumbersome input whenever a specific user that relies on a particular tag(s) has some predefined needs. For example, C2 developers rarely need decorations, and having to manually specify this every time results inconvenient.
> 
> To address this, this PR enables the possibility of adding tag-specific default decorators to UL. These defaults are in no way overriding user input -- they will only act whenever `-Xlog` has no decorators supplied and there is a positive match with the pre-specified defaults. Such a match is based on the following:
> 
> - Inclusion: if `-Xlog:jit+compilation` is provided, a default for `jit` may be applied.
> - Specificity: if, for the above line, there is a more specific default for `jit+compilation` the latter shall be applied. Upon equal specificity cases, both defaults will be applied.
> - Additionally, defaults may target a specific log level.
> 
> Decorators are also associated with an output file, so an output device may only have one set of decorators. For this reason, if different `LogSelection`s trigger defaults, none is to be applied.
> 
> In summary, these defaults may be seen as a "tailored" or "flavoured" version of the existing "uptime-level-tags" current defaults.
> 
> Please consider this PR, and thanks!

A few comments ...

First these are not "user-specified" decorators, but more developer-specified, but more simply they are "custom default decorators".

Second I'm not clear how tag combinations actually work (is this the "specificity" you refer to? I'm not sure what that means.). E.g. is `-Xlog:jit+inlining` treated the same as `-Xlog:inlining+jit`? And what about wildcards? Will `jit*` trigger this `jit+inlining` default?

> Decorators are also associated with an output file

Right this is key design point in UL: decorators are defined per output. But you are now trying to change that so that decorators are associated with tags instead. This seems a significant deviation from the design.

In any case I'm unclear exactly what happens in this PR - if we log to stdout we can have different decorators, but if we log to a real file they are all disabled? Or does any attempt to control decorators for tags going to the same output result in them all being ignored?

Finally, this is really subjective. You'd really need to socialise the actual proposed changes to the defaults independent of any mechanism to allow it.

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PR Review: https://git.openjdk.org/jdk/pull/20988#pullrequestreview-2314164523