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

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Message-ID: <20251024145401.GN4068168@noisy.programming.kicks-ass.net>
Date: Fri, 24 Oct 2025 16:54:01 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Jens Remus <jremus@...ux.ibm.com>
Cc: Steven Rostedt <rostedt@...nel.org>, linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org, bpf@...r.kernel.org,
	x86@...nel.org, Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrii Nakryiko <andrii@...nel.org>,
	Indu Bhagat <indu.bhagat@...cle.com>,
	"Jose E. Marchesi" <jemarch@....org>,
	Beau Belgrave <beaub@...ux.microsoft.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Florian Weimer <fweimer@...hat.com>, Sam James <sam@...too.org>,
	Kees Cook <kees@...nel.org>, Carlos O'Donell <codonell@...hat.com>,
	Heiko Carstens <hca@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>
Subject: Re: [PATCH v16 0/4] perf: Support the deferred unwinding
 infrastructure

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?