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

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Date:   Wed, 22 Apr 2020 09:45:12 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [GIT pull] perf/urgent for 5.7-rc2


* Peter Zijlstra <peterz@...radead.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