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

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Message-ID: <ZDTpGsT15s0iOrTJ@li-a450e7cc-27df-11b2-a85c-b5a9ac31e8ef.ibm.com>
Date:   Tue, 11 Apr 2023 10:29:06 +0530
From:   Kautuk Consul <kconsul@...ux.vnet.ibm.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Bagas Sanjaya <bagasdotme@...il.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Fabiano Rosas <farosas@...ux.ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Chao Peng <chao.p.peng@...ux.intel.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: PPC: BOOK3S: book3s_hv_nested.c: improve branch
 prediction for k.alloc

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 ?