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

Peter Zijlstra
2025-10-24T14:08:30Z

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.