On Tue, 16 Jul 2019 16:20:05 +0200 Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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
