Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Tue, 11 Apr 2023 10:29:06 +0530
From:   Kautuk Consul <kconsul@linux.vnet.ibm.com>
To:     Sean Christopherson <seanjc@google.com>
Cc:     Bagas Sanjaya <bagasdotme@gmail.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Nicholas Piggin <npiggin@gmail.com>,
        Christophe Leroy <christophe.leroy@csgroup.eu>,
        Fabiano Rosas <farosas@linux.ibm.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Chao Peng <chao.p.peng@linux.intel.com>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] KVM: PPC: BOOK3S: book3s_hv_nested.c: improve branch
 prediction for k.alloc
Message-ID: <ZDTpGsT15s0iOrTJ@li-a450e7cc-27df-11b2-a85c-b5a9ac31e8ef.ibm.com>
References: <20230407093147.3646597-1-kconsul@linux.vnet.ibm.com>
 <ZDAeuL2fz1aEW6rz@debian.me>
 <ZDA+WdiqB2931xHB@google.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <ZDA+WdiqB2931xHB@google.com>
Precedence: bulk

On 2023-04-07 09:01:29, Sean Christopherson wrote:
> On Fri, Apr 07, 2023, Bagas Sanjaya wrote:
> > On Fri, Apr 07, 2023 at 05:31:47AM -0400, Kautuk Consul wrote:
> > > I used the unlikely() macro on the return values of the k.alloc
> > > calls and found that it changes the code generation a bit.
> > > Optimize all return paths of k.alloc calls by improving
> > > branch prediction on return value of k.alloc.
> 
> Nit, this is improving code generation, not branch prediction.
Sorry my mistake.
> 
> > What about below?
> > 
> > "Improve branch prediction on kmalloc() and kzalloc() call by using
> > unlikely() macro to optimize their return paths."
> 
> Another nit, using unlikely() doesn't necessarily provide a measurable optimization.
> As above, it does often improve code generation for the happy path, but that doesn't
> always equate to improved performance, e.g. if the CPU can easily predict the branch
> and/or there is no impact on the cache footprint.
I see. I will submit a v2 of the patch with a better and more accurate
description. Does anyone else have any comments before I do so ?