Subject: Re: [RFC PATCH 1/8] trace2: log fsync stats in trace2 rather than wrapper

From: Victoria Dye <[email-masked]>
Date: Tue, 27 Jun 2023 19:05:47 -0700

Calvin Wan wrote:
> As a library boundary, wrapper.c should not directly log trace2
> statistics, but instead provide those statistics upon
> request. Therefore, move the trace2 logging code to trace2.[ch.]. This
> also allows wrapper.c to not be dependent on trace2.h and repository.h.
> 

...

> diff --git a/trace2.h b/trace2.h
> index 4ced30c0db..689e9a4027 100644
> --- a/trace2.h
> +++ b/trace2.h
> @@ -581,4 +581,9 @@ void trace2_collect_process_info(enum trace2_process_info_reason reason);
>  
>  const char *trace2_session_id(void);
>  
> +/*
> + * Writes out trace statistics for fsync
> + */
> +void trace_git_fsync_stats(void);
> +

This function does not belong in 'trace2.h', IMO. The purpose of that file
is to contain the generic API for Trace2 (e.g., 'trace2_printf()',
'trace2_region_(enter|exit)'), whereas this function is effectively a
wrapper around a specific invocation of that API. 

You note in the commit message that "wrapper.c should not directly log
trace2 statistics" with the reasoning of "[it's] a library boundary," but I
suspect the unstated underlying reason is "because it tracks 'count_fsync_*'
in static variables." This case would be better handled, then, by replacing
the usage in 'wrapper.c' with a new Trace2 counter (API introduced in [1]).
That keeps this usage consistent with the API already established for
Trace2, rather than starting an unsustainable trend of creating ad-hoc,
per-metric wrappers in 'trace2.[c|h]'.

An added note re: the commit message - it's extremely important that
functions _anywhere in Git_ are able to use the Trace2 API directly. A
developer could reasonably want to measure performance, keep track of an
interesting metric, log when a region is entered in the larger trace,
capture error information, etc. for any function, regardless of where in
falls in the internal library organization. To that end, I think either the
commit message should be rephrased to remove that statement (if the issue is
really "we're using a static variable and we want to avoid that"), or the
libification effort should be updated to accommodate use of Trace2 anywhere
in Git. 

[1] https://lore.kernel.org/git/[email-masked]/