Re: [PATCH v16 03/10] unwind_user/deferred: Add unwind cache

[Indu Bhagat <indu.bhagat@xxxxxxxxxx>]

On 7/29/25 11:23 AM, Steven Rostedt wrote:
> From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
>
> Cache the results of the unwind to ensure the unwind is only performed
> once, even when called by multiple tracers.
>
> The cache nr_entries gets cleared every time the task exits the kernel.
> When a stacktrace is requested, nr_entries gets set to the number of
> entries in the stacktrace. If another stacktrace is requested, if
> nr_entries is not zero, then it contains the same stacktrace that would be
> retrieved so it is not processed again and the entries is given to the
> caller.
>
> Reviewed-by: Jens Remus <jremus@xxxxxxxxxxxxx>
> Co-developed-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
> Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
> ---
>   include/linux/entry-common.h          |  2 ++
>   include/linux/unwind_deferred.h       |  8 +++++++
>   include/linux/unwind_deferred_types.h |  7 +++++-
>   kernel/unwind/deferred.c              | 31 +++++++++++++++++++++------
>   4 files changed, 40 insertions(+), 8 deletions(-)
>
> diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h
> index f94f3fdf15fc..8908b8eeb99b 100644
> --- a/include/linux/entry-common.h
> +++ b/include/linux/entry-common.h
> @@ -12,6 +12,7 @@
>   #include <linux/resume_user_mode.h>
>   #include <linux/tick.h>
>   #include <linux/kmsan.h>
> +#include <linux/unwind_deferred.h>
>   
>   #include <asm/entry-common.h>
>   #include <asm/syscall.h>
> @@ -362,6 +363,7 @@ static __always_inline void exit_to_user_mode(void)
>   	lockdep_hardirqs_on_prepare();
>   	instrumentation_end();
>   
> +	unwind_reset_info();
>   	user_enter_irqoff();
>   	arch_exit_to_user_mode();
>   	lockdep_hardirqs_on(CALLER_ADDR0);
> diff --git a/include/linux/unwind_deferred.h b/include/linux/unwind_deferred.h
> index a5f6e8f8a1a2..baacf4a1eb4c 100644
> --- a/include/linux/unwind_deferred.h
> +++ b/include/linux/unwind_deferred.h
> @@ -12,6 +12,12 @@ void unwind_task_free(struct task_struct *task);
>   
>   int unwind_user_faultable(struct unwind_stacktrace *trace);
>   
> +static __always_inline void unwind_reset_info(void)
> +{
> +	if (unlikely(current->unwind_info.cache))
> +		current->unwind_info.cache->nr_entries = 0;
> +}

Should the entries[] items upto nr_entries (stack trace info from the 
previous request) also be reset to 0 here ?

> +
>   #else /* !CONFIG_UNWIND_USER */
>   
>   static inline void unwind_task_init(struct task_struct *task) {}
> @@ -19,6 +25,8 @@ static inline void unwind_task_free(struct task_struct *task) {}
>   
>   static inline int unwind_user_faultable(struct unwind_stacktrace *trace) { return -ENOSYS; }
>   
> +static inline void unwind_reset_info(void) {}
> +
>   #endif /* !CONFIG_UNWIND_USER */
>   
>   #endif /* _LINUX_UNWIND_USER_DEFERRED_H */
> diff --git a/include/linux/unwind_deferred_types.h b/include/linux/unwind_deferred_types.h
> index aa32db574e43..db5b54b18828 100644
> --- a/include/linux/unwind_deferred_types.h
> +++ b/include/linux/unwind_deferred_types.h
> @@ -2,8 +2,13 @@
>   #ifndef _LINUX_UNWIND_USER_DEFERRED_TYPES_H
>   #define _LINUX_UNWIND_USER_DEFERRED_TYPES_H
>   
> +struct unwind_cache {
> +	unsigned int		nr_entries;
> +	unsigned long		entries[];
> +};
> +

Should we use __counted_by ?

>   struct unwind_task_info {
> -	unsigned long		*entries;
> +	struct unwind_cache	*cache;
>   };
>   
>   #endif /* _LINUX_UNWIND_USER_DEFERRED_TYPES_H */
> diff --git a/kernel/unwind/deferred.c b/kernel/unwind/deferred.c
> index a0badbeb3cc1..96368a5aa522 100644
> --- a/kernel/unwind/deferred.c
> +++ b/kernel/unwind/deferred.c
> @@ -4,10 +4,13 @@
>    */
>   #include <linux/kernel.h>
>   #include <linux/sched.h>
> +#include <linux/sizes.h>
>   #include <linux/slab.h>
>   #include <linux/unwind_deferred.h>
>   
> -#define UNWIND_MAX_ENTRIES 512
> +/* Make the cache fit in a 4K page */
> +#define UNWIND_MAX_ENTRIES					\
> +	((SZ_4K - sizeof(struct unwind_cache)) / sizeof(long))
>   
>   /**
>    * unwind_user_faultable - Produce a user stacktrace in faultable context
> @@ -24,6 +27,7 @@
>   int unwind_user_faultable(struct unwind_stacktrace *trace)
>   {
>   	struct unwind_task_info *info = &current->unwind_info;
> +	struct unwind_cache *cache;
>   
>   	/* Should always be called from faultable context */
>   	might_fault();
> @@ -31,17 +35,30 @@ int unwind_user_faultable(struct unwind_stacktrace *trace)
>   	if (current->flags & PF_EXITING)
>   		return -EINVAL;
>   
> -	if (!info->entries) {
> -		info->entries = kmalloc_array(UNWIND_MAX_ENTRIES, sizeof(long),
> -					      GFP_KERNEL);
> -		if (!info->entries)
> +	if (!info->cache) {
> +		info->cache = kzalloc(struct_size(cache, entries, UNWIND_MAX_ENTRIES),
> +				      GFP_KERNEL);
> +		if (!info->cache)
>   			return -ENOMEM;
>   	}
>   
> +	cache = info->cache;
> +	trace->entries = cache->entries;
> +
> +	if (cache->nr_entries) {
> +		/*
> +		 * The user stack has already been previously unwound in this
> +		 * entry context.  Skip the unwind and use the cache.
> +		 */
> +		trace->nr = cache->nr_entries;
> +		return 0;
> +	}
> +
>   	trace->nr = 0;
> -	trace->entries = info->entries;
>   	unwind_user(trace, UNWIND_MAX_ENTRIES);
>   
> +	cache->nr_entries = trace->nr;
> +
>   	return 0;
>   }
>   
> @@ -56,5 +73,5 @@ void unwind_task_free(struct task_struct *task)
>   {
>   	struct unwind_task_info *info = &task->unwind_info;
>   
> -	kfree(info->entries);
> +	kfree(info->cache);
>   }