On Tue, 16 Jul 2019 16:20:05 +0200
Peter Zijlstra [off-list ref] wrote:

On Tue, Jul 16, 2019 at 10:08:18PM +0800, Changbin Du wrote:

quoted

On Mon, Jul 15, 2019 at 12:12:31PM +0200, Peter Zijlstra wrote:  

quoted

quoted

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.

Sure, and we can have fgraph support of return values depend on that
option ;-)

quoted

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.

This has been on my TODO list for a long time (I'm really happy if
someone else would do it!). My thought is that this information would
need to be able to be a module and loaded (like config.gz can be). And
then you can load the info, do the tracing, and then unload it.

For the speed part, we can add a way to hook the function with the
parameters, which shouldn't be an issue, as we already do that when
filtering for function graph. There's a function hash table that fgraph
users have that is tested to see if it should trace the function or
not. And the functions themselves are recorded in a mostly binary array
that can be looked up via a binary search from the ip address.

-- Steve

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