Re: [PATCH v4 29/39] unwind_user/deferred: Add unwind cache

[Peter Zijlstra]

On Tue, Jan 21, 2025 at 06:31:21PM -0800, Josh Poimboeuf wrote:
> Cache the results of the unwind to ensure the unwind is only performed
> once, even when called by multiple tracers.
> 
> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> ---
>  include/linux/unwind_deferred_types.h |  8 +++++++-
>  kernel/unwind/deferred.c              | 26 ++++++++++++++++++++------
>  2 files changed, 27 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/unwind_deferred_types.h b/include/linux/unwind_deferred_types.h
> index 9749824aea09..6f71a06329fb 100644
> --- a/include/linux/unwind_deferred_types.h
> +++ b/include/linux/unwind_deferred_types.h
> @@ -2,8 +2,14 @@
>  #ifndef _LINUX_UNWIND_USER_DEFERRED_TYPES_H
>  #define _LINUX_UNWIND_USER_DEFERRED_TYPES_H
>  
> -struct unwind_task_info {
> +struct unwind_cache {
>  	unsigned long		*entries;
> +	unsigned int		nr_entries;
> +	u64			cookie;
> +};

If you make the return to user path clear nr_entries you don't need a
second cookie field I think.