linux-kernel - Re: [PATCH v7] printk: hash addresses printed with %p

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Message-ID: <20171026091036.GI12341@eros>
Date:   Thu, 26 Oct 2017 20:10:36 +1100
From:   "Tobin C. Harding" <me@...in.cc>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        kernel-hardening@...ts.openwall.com, Theodore Ts'o <tytso@....edu>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Tycho Andersen <tycho@...ker.com>,
        "Roberts, William C" <william.c.roberts@...el.com>,
        Tejun Heo <tj@...nel.org>,
        Jordan Glover <Golden_Miller83@...tonmail.ch>,
        Petr Mladek <pmladek@...e.com>, Joe Perches <joe@...ches.com>,
        Ian Campbell <ijc@...lion.org.uk>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <wilal.deacon@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Chris Fries <cfries@...gle.com>,
        Dave Weinstein <olorin@...gle.com>,
        Daniel Micay <danielmicay@...il.com>,
        Djalal Harouni <tixxdz@...il.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v7] printk: hash addresses printed with %p

On Thu, Oct 26, 2017 at 09:00:03AM +0200, Greg KH wrote:
> On Thu, Oct 26, 2017 at 12:59:08AM +0200, Jason A. Donenfeld wrote:
> > On Thu, Oct 26, 2017 at 12:27 AM, Tobin C. Harding <me@...in.cc> wrote:
> > > How good is unlikely()?
> > 
> > It places that branch way at the bottom of the function so that it's
> > less likely to pollute the icache.
> 
> But always measure it.  Lots of times (old numbers were 90% or so), we
> get the marking wrong, so please, always benchmark the thing to verify
> it actually is doing what you think it should be doing, otherwise it
> could make the code worse.

Does this come under 'premature optimization is the root of all evil'?

Should we be leaving out things like unlikely() and __read_only until
the code has been profiled?

thanks,
Tobin.