Re: [PATCH v2 5/9] trace2: add thread-name override to perf target

[Jeff Hostetler <git@jeffhostetler.com>]

On 12/28/21 8:48 PM, Ævar Arnfjörð Bjarmason wrote:
> 
> On Tue, Dec 28 2021, Jeff Hostetler via GitGitGadget wrote:
> 
>> From: Jeff Hostetler <jeffhost@microsoft.com>
>>
>> Teach the message formatter in the Trace2 perf target to accept an
>> optional thread name argument.  This will override the thread name
>> inherited from the thread local storage data block.
>>
>> This will be used in a future commit for global events that should
>> not be tied to a particular thread, such as a global stopwatch timer.
> 
> We already have a "ctx", and that "ctx" has a "thread_name", but here
> and in the preceding commit we're adding a "thread_name" to every caller
> of these functions in case we'd like to override it.
> 
> Wouldn't it make more sense to just pass a "ctx" to these functions? One
> of them already takes it, here's an (obviously incomplete) fixup on top
> of your series to make the one that doesn't take a "ctx", and for the
> only non-NULL users of "thread_name" to just use a trivial helper to
> pass in a "ctx" with a new "thread_name", then to swap it back.
> 
> It would make for a smaller diffstat for this already large series, or
> we could do exactly what we're doing now, but avoid the churn of
> adjusting every caller by introducing a new sister function for those
> who want this parameter to be non-NULL.

I suppose it is possible to have a helper version of
`event_fmt_prepare()` that takes the extra argument and
fixup the existing function to call it with NULL.

I'll see if that makes sense.


[...]
>   
> +static void event_fmt_prepare_tn(const char *event_name, const char *file,
> +				 int line, const struct repository *repo,
> +				 struct json_writer *jw,
> +				 const char *thread_name)
> +{
> +	struct tr2tls_thread_ctx *ctx = tr2tls_get_self();
> +	const char *tmp;
> +
> +	tmp = ctx->thread_name;
> +	ctx->thread_name = thread_name;
> +	event_fmt_prepare(event_name, __FILE__, __LINE__, NULL, jw, ctx);
> +	ctx->thread_name = tmp;
> +}
[...]

This only works if we agree that thread name is a pointer inside
the structure and not a flex-array.

Personally, I think this is trying to do things backwards by
temporarily changing the ctx->thread_name field.  I think it
would be better to `event_fmt_prepare_tn()` do the actual
work with the supplied thread name and have the existing
`event_fmt_prepare()` just call it with ctx->thread_name.
Then we don't need to hack up the ctx.

I'll see if this makes the diffs a little cleaner.

Jeff