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

Peter Zijlstra
2025-10-24T14:57:44Z

On Fri, Oct 24, 2025 at 04:54:02PM +0200, Peter Zijlstra wrote:
> 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?

Urgh, that makes us call that weird hack for sframe too, which isn't
needed. Oh well, ignore this.