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Date:   Tue, 16 Jul 2019 16:20:05 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Changbin Du <changbin.du@gmail.com>
Cc:     Will Deacon <will@kernel.org>, rostedt@goodmis.org,
        mingo@redhat.com, corbet@lwn.net, linux@armlinux.org.uk,
        catalin.marinas@arm.com, tglx@linutronix.de, bp@alien8.de,
        hpa@zytor.com, x86@kernel.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] tracing/fgraph: support recording function return values
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On Tue, Jul 16, 2019 at 10:08:18PM +0800, Changbin Du wrote:
> On Mon, Jul 15, 2019 at 12:12:31PM +0200, Peter Zijlstra wrote:

> > Alternatively, we can have recordmcount (or objtool) mark all functions
> > with a return value when the build has DEBUG_INFO on. The dwarves know
> > the function signature.
> >
> We can extend the recordmcount tool to search 'subprogram' tag in the DIE tree.
> In below example, the 'DW_AT_type' is the type of function pidfd_create().
> 
> $ readelf -w kernel/pid.o
>  [...]
>  <1><1b914>: Abbrev Number: 232 (DW_TAG_subprogram)
>     <1b916>   DW_AT_name        : (indirect string, offset: 0x415e): pidfd_create
>     <1b91a>   DW_AT_decl_file   : 1
>     <1b91b>   DW_AT_decl_line   : 471
>     <1b91d>   DW_AT_decl_column : 12
>     <1b91e>   DW_AT_prototyped  : 1
>     <1b91e>   DW_AT_type        : <0xcc>
>     <1b922>   DW_AT_low_pc      : 0x450
>     <1b92a>   DW_AT_high_pc     : 0x50
>     <1b932>   DW_AT_frame_base  : 1 byte block: 9c 	(DW_OP_call_frame_cfa)
>     <1b934>   DW_AT_GNU_all_call_sites: 1
>     <1b934>   DW_AT_sibling     : <0x1b9d9>
>  [...]
> 
> To that end, we need to introduce libdw library for recordmcount. I will have a
> try this week.

Right; but only when this config option is set.

> And probably, we can also record the parameters?

The 'fun' part is where to store all this information in the kernel and
how fast you can find it while tracing.