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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 7 Mar 2019 09:45:35 -0800
Message-ID: <CAHk-=wi_vuD9fKk36=_DNG9=+S9rbTRu2FTFe9tECunC8oOtfg@mail.gmail.com>
Subject: Re: [PATCH 00/20] objtool: UACCESS validation v3
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Josh Poimboeuf <jpoimboe@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Anvin <hpa@zytor.com>,
        Julien Thierry <julien.thierry@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Ingo Molnar <mingo@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        James Morse <james.morse@arm.com>, valentin.schneider@arm.com,
        Brian Gerst <brgerst@gmail.com>,
        Andrew Lutomirski <luto@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Dmitry Vyukov <dvyukov@google.com>,
        Steven Rostedt <rostedt@goodmis.org>
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On Thu, Mar 7, 2019 at 9:38 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> Also; it seems to me that something PT, or maybe even simply:
>
>   perf -e branches -e branch-misses
>
> would get you similar or sufficient information.

Yeah, I'm not really seeing a lot of upside to PROFILE_ALL_BRANCHES.

Particularly since it doesn't actually profile all branches at all. It
only basically profiles "if ()" statements, which obviously misses
loops etc, but then also _does_ hit things where people turned loops
into "if (unlikely()) loop()", which happens in (for example)
low-level locking code etc that often has a fast-case "first try"
thing followed by a slow-case "ok, let's loop for it" thing.

So I think PROFILE_ALL_BRANCHES tends to have very random coverage.
I'd love to get rid of it, because it seems so random.

               Linus