Re: [PATCH] pahole: Apply CU-level filters early in loading

[Alan Maguire <alan.maguire@oracle.com>]

On 30/07/2024 23:43, Matthew Maurer wrote:
> Without this, even with `--lang_exclude=rust` set, running on `vmlinux`
> with `CONFIG_RUST` enabled will lead to errors like:
> die__process_function: tag not supported 0x2f (template_type_parameter)!
> because the filtering doesn't happen until finalization, but unsupported
> tags are reported during loading.
> 
> As an added bonus, this should speed up processing of large objects with
> filtered CUs, as their details will no longer be walked.
>

One question on this; if we are always doing early filtering like this,
should the explicit cu__filter() call be removed from pahole_stealer()?

> Signed-off-by: Matthew Maurer <mmaurer@google.com>
> ---
>  dwarf_loader.c | 10 ++++++++++
>  dwarves.h      |  1 +
>  pahole.c       |  1 +
>  3 files changed, 12 insertions(+)
> 
> diff --git a/dwarf_loader.c b/dwarf_loader.c
> index b832c93..c48dfef 100644
> --- a/dwarf_loader.c
> +++ b/dwarf_loader.c
> @@ -2854,6 +2854,16 @@ static int die__process(Dwarf_Die *die, struct cu *cu, struct conf_load *conf)
>  
>  	cu->language = attr_numeric(die, DW_AT_language);
>  
> +	if (conf->early_cu_filter)
> +		cu = (conf->early_cu_filter)(cu);
> +
> +	/*
> +	 * If we filtered this CU out, we still want to keep iterating, but
> +	 * there's no need to walk the rest of the CU info.
> +	 */
> +	if (cu == NULL)
> +		return DWARF_CB_OK;
> +
>  	if (dwarf_child(die, &child) == 0) {
>  		int err = die__process_unit(&child, cu, conf);
>  		if (err)
> diff --git a/dwarves.h b/dwarves.h
> index f5ae79f..92d102b 100644
> --- a/dwarves.h
> +++ b/dwarves.h
> @@ -72,6 +72,7 @@ struct conf_load {
>  	enum load_steal_kind	(*steal)(struct cu *cu,
>  					 struct conf_load *conf,
>  					 void *thr_data);
> +	struct cu *		(*early_cu_filter)(struct cu *cu);
>  	int			(*thread_exit)(struct conf_load *conf, void *thr_data);
>  	void			*cookie;
>  	char			*format_path;
> diff --git a/pahole.c b/pahole.c
> index 954498d..937b0a1 100644
> --- a/pahole.c
> +++ b/pahole.c
> @@ -3765,6 +3765,7 @@ int main(int argc, char *argv[])
>  	memset(tab, ' ', sizeof(tab) - 1);
>  
>  	conf_load.steal = pahole_stealer;
> +	conf_load.early_cu_filter = cu__filter;
>  	conf_load.thread_exit = pahole_thread_exit;
>  
>  	if (conf_load.reproducible_build) {