Re: [GIT pull] perf/urgent for 5.7-rc2

[Ingo Molnar <mingo@kernel.org>]

* Peter Zijlstra <peterz@infradead.org> wrote:

> On Mon, Apr 20, 2020 at 09:48:45AM +0200, Ingo Molnar wrote:
> > Fortunately, much of what objtool does against vmlinux.o can be 
> > parallelized in a rather straightforward fashion I believe, if we build 
> > with -ffunction-sections.
> 
> So that FGKASLR is going to get us -ffunction-sections, but
> parallelizing objtool isn't going to be trivial, it's data structures
> aren't really build for that, esp. decode_instructions() which actively
> generates data.
> 
> Still, it's probably doable.

So AFAICS in the narrow code section I identified as the main overhead, 
only the instruction hash needs threading, i.e. this step:

                        hash_add(file->insn_hash, &insn->hash, insn->offset);
                        list_add_tail(&insn->list, &file->insn_list);

Objtool can still be single-threaded before and after generating the 
instruction hash.

99% of the overhead within decode_instructions() is in 
arch_decode_instruction(), which is fully thread-safe AFAICS.

So the run time of objtool could be cut in ~third on most systems.

Thanks,

	Ingo