linux-kernel - Re: [RFC PATCH v2] perf/x86: make perf callchain work without CONFIG_FRAME

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Date:   Mon, 15 Apr 2019 17:36:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kairui Song <kasong@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Josh Poimboeuf <jpoimboe@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, Dave Young <dyoung@...hat.com>
Subject: Re: [RFC PATCH v2] perf/x86: make perf callchain work without
 CONFIG_FRAME_POINTER


I'll mostly defer to Josh on unwinding, but a few comments below.

On Tue, Apr 09, 2019 at 12:59:42AM +0800, Kairui Song wrote:
> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> index e2b1447192a8..6075a4f94376 100644
> --- a/arch/x86/events/core.c
> +++ b/arch/x86/events/core.c
> @@ -2355,6 +2355,12 @@ void arch_perf_update_userpage(struct perf_event *event,
>  	cyc2ns_read_end();
>  }
>  
> +static inline int
> +valid_perf_registers(struct pt_regs *regs)
> +{
> +	return (regs->ip && regs->bp && regs->sp);
> +}

I'm unconvinced by this, with both guess and orc having !bp is perfectly
valid.

>  void
>  perf_callchain_kernel(struct perf_callchain_entry_ctx *entry, struct pt_regs *regs)
>  {
> @@ -2366,11 +2372,17 @@ perf_callchain_kernel(struct perf_callchain_entry_ctx *entry, struct pt_regs *re
>  		return;
>  	}
>  
> -	if (perf_callchain_store(entry, regs->ip))
> +	if (valid_perf_registers(regs)) {
> +		if (perf_callchain_store(entry, regs->ip))
> +			return;
> +		unwind_start(&state, current, regs, NULL);
> +	} else if (regs->sp) {
> +		unwind_start(&state, current, NULL, (unsigned long *)regs->sp);
> +	} else {
>  		return;
> +	}

AFAICT if we, by pure accident, end up with !bp for ORC, then we
initialize the unwind wrong.

Note that @regs is mostly trivially correct, except for that tracepoint
case. So I don't think we should magic here.

> -	for (unwind_start(&state, current, regs, NULL); !unwind_done(&state);
> -	     unwind_next_frame(&state)) {
> +	for (; !unwind_done(&state); unwind_next_frame(&state)) {
>  		addr = unwind_get_return_address(&state);
>  		if (!addr || perf_callchain_store(entry, addr))
>  			return;


> diff --git a/arch/x86/include/asm/stacktrace.h b/arch/x86/include/asm/stacktrace.h
> index f335aad404a4..226077e20412 100644
> --- a/arch/x86/include/asm/stacktrace.h
> +++ b/arch/x86/include/asm/stacktrace.h
> @@ -98,18 +98,23 @@ struct stack_frame_ia32 {
>      u32 return_address;
>  };
>  
> +#ifdef CONFIG_FRAME_POINTER
>  static inline unsigned long caller_frame_pointer(void)
>  {
>  	struct stack_frame *frame;
>  
>  	frame = __builtin_frame_address(0);
>  
> -#ifdef CONFIG_FRAME_POINTER
>  	frame = frame->next_frame;
> -#endif
>  
>  	return (unsigned long)frame;
>  }
> +#else
> +static inline unsigned long caller_frame_pointer(void)
> +{
> +	return 0;
> +}
> +#endif

OK, that makes sense I guess.