Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Mon, 5 Feb 2018 09:51:24 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     David Woodhouse <dwmw2@infradead.org>
Cc:     Alan Cox <gnomes@lxorguk.ukuu.org.uk>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        KarimAllah Ahmed <karahmed@amazon.de>, sironi@amazon.de,
        the arch/x86 maintainers <x86@kernel.org>,
        KVM list <kvm@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Borislav Petkov <bp@alien8.de>, Paul Turner <pjt@google.com>
Subject: Re: [PATCH] KVM: x86: Reduce retpoline performance impact in
 slot_handle_level_range()
Message-ID: <20180205085124.GF2249@hirez.programming.kicks-ass.net>
References: <1517583559-424-1-git-send-email-dwmw@amazon.co.uk>
 <CA+55aFwg88mxuOiJSh+xLnAfqEwAC-ReB9rdNx+r8fy=FTSHJw@mail.gmail.com>
 <CA+55aFwqCc1UOkdBjEgiuL9fMkRj7RPrGFYQ376YhrqTUpLwrg@mail.gmail.com>
 <1517599033.31953.71.camel@infradead.org>
 <20180202212321.5186c13c@alans-desktop>
 <1517669207.31953.120.camel@infradead.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <1517669207.31953.120.camel@infradead.org>
User-Agent: Mutt/1.9.2 (2017-12-15)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Sat, Feb 03, 2018 at 02:46:47PM +0000, David Woodhouse wrote:
> > For the simple case how about wrapping the if into
> > 
> > ????????????????call_likely(foo->bar, usualfunction, args)
> > 
> > as a companion to?
> > 
> > ?????????????????foo->bar(args)
> > 
> > that can resolve to nothing special on architectures that don't need it,
> > an if/else case on platforms with spectre, and potentially clever
> > stuff on any platform where you can beat the compiler by knowing
> > probabilities it can't infer ?
> 
> Yeah. I'm keen on being able to use something like alternatives to
> *change* 'usualfunction' at runtime though. I suspect it'll be a win
> for stuff like dma_ops.
> 
> But I'm also keen to actually base such things on real data, not just
> go randomly "optimising" stuff just because we can. Let's try to make
> sure we fix up the real bottlenecks, and not just go crazy.

Google has a fairly long history of using feedback driven optimization
compiles for the kernel. They were also the ones that developed perf
autofdo tooling IIRC.

https://gcc.gnu.org/wiki/AutoFDO/Tutorial

One of the things pjt promised was a series of patches doing the
proposed optimization for the scheduler code based on their results.