Re: [PATCH] KVM: PPC: BOOK3S: book3s_hv_nested.c: improve branch prediction for k.alloc

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
Date: Tue, 11 Apr 2023 10:29:06 +0530	[thread overview]
Message-ID: <ZDTpGsT15s0iOrTJ@li-a450e7cc-27df-11b2-a85c-b5a9ac31e8ef.ibm.com> (raw)
In-Reply-To: <ZDA+WdiqB2931xHB@google.com>

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 ?