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

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Message-ID: <20251024151142.GF3245006@noisy.programming.kicks-ass.net>
Date: Fri, 24 Oct 2025 17:11:42 +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 05:09:02PM +0200, Jens Remus wrote:
> Hello Peter,
> 
> very nice!
> 
> On 10/24/2025 4:51 PM, Peter Zijlstra wrote:
> 
> > Subject: unwind_user/x86: Teach FP unwind about start of function
> > From: Peter Zijlstra <peterz@...radead.org>
> > Date: Fri Oct 24 12:31:10 CEST 2025
> > 
> > When userspace is interrupted at the start of a function, before we
> > get a chance to complete the frame, unwind will miss one caller.
> > 
> > X86 has a uprobe specific fixup for this, add bits to the generic
> > unwinder to support this.
> > 
> > Suggested-by: Jens Remus <jremus@...ux.ibm.com>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> 
> > +++ 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);
> 
> IIUC this will cause kernel/unwind/user.c to fail compile on
> architectures that will support HAVE_UNWIND_USER_SFRAME but not
> HAVE_UNWIND_USER_FP (such as s390), and thus do not need to implement
> unwind_user_at_function_start().
> 
> Either s390 would need to supply a dummy unwind_user_at_function_start()
> or the unwind user sframe series needs to address this and supply
> a dummy one if FP is not enabled, so that the code compiles with only
> SFRAME enabled.
> 
> What do you think?

I'll make it conditional on HAVE_UNWIND_USER_FP -- but tomorrow or so.