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SubjectRe: [PATCH] tracing/fgraph: support recording function return values
On Tue, 16 Jul 2019 16:20:05 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> 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.

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

>
> > 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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