Re: [PATCH v16 0/4] perf: Support the deferred unwinding infrastructure

Peter Zijlstra
2025-10-24T14:54:12Z

On Fri, Oct 24, 2025 at 04:51:56PM +0200, Peter Zijlstra wrote:

> --- a/arch/x86/include/asm/unwind_user.h
> +++ b/arch/x86/include/asm/unwind_user.h
> @@ -3,6 +3,7 @@
> #define _ASM_X86_UNWIND_USER_H
>
> #include <asm/ptrace.h>
> +#include <asm/uprobes.h>
>
> #define ARCH_INIT_USER_FP_FRAME(ws) \
> .cfa_off = 2*(ws), \
> @@ -10,6 +11,12 @@
> .fp_off = -2*(ws), \
> .use_fp = true,
>
> +#define ARCH_INIT_USER_FP_ENTRY_FRAME(ws) \
> + .cfa_off = 1*(ws), \
> + .ra_off = -1*(ws), \
> + .fp_off = 0, \
> + .use_fp = false,
> +
> static inline int unwind_user_word_size(struct pt_regs *regs)
> {
> /* We can't unwind VM86 stacks */
> @@ -22,4 +29,9 @@ static inline int unwind_user_word_size(
> return sizeof(long);
> }
>
> +static inline bool unwind_user_at_function_start(struct pt_regs *regs)
> +{
> + return is_uprobe_at_func_entry(regs);
> +}
> +
> #endif /* _ASM_X86_UNWIND_USER_H */

> --- a/include/linux/unwind_user_types.h
> +++ b/include/linux/unwind_user_types.h
> @@ -39,6 +39,7 @@ struct unwind_user_state {
> unsigned int ws;
> enum unwind_user_type current_type;
> unsigned int available_types;
> + bool topmost;
> bool done;
> };
>
> --- a/kernel/unwind/user.c
> +++ b/kernel/unwind/user.c

>
> +static int unwind_user_next_fp(struct unwind_user_state *state)
> +{
> + struct pt_regs *regs = task_pt_regs(current);
> +
> + const struct unwind_user_frame fp_frame = {
> + ARCH_INIT_USER_FP_FRAME(state->ws)
> + };
> + const struct unwind_user_frame fp_entry_frame = {
> + ARCH_INIT_USER_FP_ENTRY_FRAME(state->ws)
> + };
> +
> + if (state->topmost && unwind_user_at_function_start(regs))
> + return unwind_user_next_common(state, &fp_entry_frame);
> +
> + return unwind_user_next_common(state, &fp_frame);
> +}
> +
> static int unwind_user_next(struct unwind_user_state *state)
> {
> unsigned long iter_mask = state->available_types;
> @@ -118,6 +134,7 @@ static int unwind_user_start(struct unwi
> state->done = true;
> return -EINVAL;
> }
> + state->topmost = true;
>
> return 0;
> }

And right before sending this; I realized we could do the
unwind_user_at_function_start() in unwind_user_start() and set something
like state->entry = true instead of topmost.

That saves having to do task_pt_regs() in unwind_user_next_fp().

Does that make sense?