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SubjectRe: [PATCH] tracing/fgraph: support recording function return values
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.

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