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

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Oct 24, 2025 at 03:58:20PM +0200, Jens Remus wrote:
> Hello Peter!
> 
> On 10/24/2025 12:41 PM, Peter Zijlstra wrote:
> > On Fri, Oct 24, 2025 at 11:29:26AM +0200, Peter Zijlstra wrote:
> >> On Thu, Oct 23, 2025 at 05:00:02PM +0200, Peter Zijlstra wrote:
> >>
> >>> Trouble is, pretty much every unwind is 510 entries long -- this cannot
> >>> be right. I'm sure there's a silly mistake in unwind/user.c but I'm too
> >>> tired to find it just now. I'll try again tomorrow.
> >>
> >> PEBKAC
> > 
> > Anyway, while staring at this, I noted that the perf userspace unwind
> > code has a few bits that are missing from the new shiny thing.
> > 
> > How about something like so? This add an optional arch specific unwinder
> > at the very highest priority (bit 0) and uses that to do a few extra
> > bits before disabling itself and falling back to whatever lower prio
> > unwinder to do the actual unwinding.
> 
> unwind user sframe does not need any of this special handling, because
> it knows for each IP whether the SP or FP is the CFA base register
> and whether the FP and RA have been saved.

It still can't unwind VM86 stacks. But yes, it should do lots better
with that start of function hack.

> Isn't this actually specific to unwind user fp?  If the IP is at
> function entry, then the FP has not been setup yet.  I think unwind user
> fp could handle this using an arch specific is_uprobe_at_func_entry() to
> determine whether to use a new frame_fp_entry instead of frame_fp.  For
> x86 the following frame_fp_entry should work, if I am not wrong:
> 
> #define ARCH_INIT_USER_FP_ENTRY_FRAME(ws)	\
> 	.cfa_off	=  1*(ws),		\
> 	.ra_off		= -1*(ws),		\
> 	.fp_off		= 0,			\
> 	.use_fp		= false,
> 
> Following roughly outlines the required changes:
> 
> diff --git a/kernel/unwind/user.c b/kernel/unwind/user.c
> 
> -static int unwind_user_next_fp(struct unwind_user_state *state)
> +static int unwind_user_next_common(struct unwind_user_state *state,
> +                                  const struct unwind_user_frame *frame,
> +                                  struct pt_regs *regs)
> 
> @@ -71,6 +83,7 @@ static int unwind_user_next_common(struct unwind_user_state *state,
>         state->sp = sp;
>         if (frame->fp_off)
>                 state->fp = fp;
> +       state->topmost = false;
>         return 0;
>  }
> @@ -154,6 +167,7 @@ static int unwind_user_start(struct unwind_user_state *state)
>         state->sp = user_stack_pointer(regs);
>         state->fp = frame_pointer(regs);
>         state->ws = compat_user_mode(regs) ? sizeof(int) : sizeof(long);
> +       state->topmost = true;
> 
>         return 0;
>  }
> 
> static int unwind_user_next_fp(struct unwind_user_state *state)
> {
> 	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)
> 	};
> 	struct pt_regs *regs = task_pt_regs(current);
> 
> 	if (state->topmost && is_uprobe_at_func_entry(regs))
> 		return unwind_user_next_common(state, &fp_entry_frame, regs);
> 	else
> 		return unwind_user_next_common(state, &fp_frame, regs);
> }
> 
> diff --git a/include/linux/unwind_user_types.h b/include/linux/unwind_user_types.h
> @@ -43,6 +43,7 @@ struct unwind_user_state {
>         unsigned int                            ws;
>         enum unwind_user_type                   current_type;
>         unsigned int                            available_types;
> +       bool                                    topmost;
>         bool                                    done;
>  };
> 
> What do you think?

Yeah, I suppose that should work. Let me rework things accordingly.