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

Peter Zijlstra
2025-01-22T13:57:15Z

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@kernel.org>
> ---
> 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.