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

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Date:   Wed, 19 Apr 2023 15:38:52 +0530
From:   Kautuk Consul <kconsul@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Bagas Sanjaya <bagasdotme@...il.com>,
        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 2023-04-12 12:34:13, Kautuk Consul wrote:
> Hi,
> 
> On 2023-04-11 16:35:10, Michael Ellerman wrote:
> > Kautuk Consul <kconsul@...ux.vnet.ibm.com> writes:
> > > 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 ?
> >  
> > In general I think unlikely should be saved for cases where either the
> > compiler is generating terrible code, or the likelyness of the condition
> > might be surprising to a human reader.
> > 
> > eg. if you had some code that does a NULL check and it's *expected* that
> > the value is NULL, then wrapping that check in likely() actually adds
> > information for a human reader.
> >     
> > Also please don't use unlikely in init paths or other cold paths, it
> > clutters the code (only slightly but a little) and that's not worth the
> > possible tiny benefit for code that only runs once or infrequently.
> > 
> > I would expect the compilers to do the right thing in all
> > these cases without the unlikely. But if you can demonstrate that they
> > meaningfully improve the code generation with a before/after
> > dissassembly then I'd be interested.
> Just FYI, the last email by kautuk.consul.80@...il.com was by me.
> That last email contains a diff file attachment which compares 2 files:
> before my changes and after my changes.
> This diff file shows a lot of changes in code generation. Im assuming
> all those changes are made by the compiler towards optimizing all return
> paths to k.alloc calls.
> Kindly review and comment.
Any comments on the numerous code generation changes as shown by the
files I attached to this mail chain ? Sorry I don't have concrete
figures of any type to prove that this leads to any measurable performance
improvements. I am just assuming that the compiler's modified code
generation (due to the use of the unlikely macro) would be optimal.

Thanks.
> > cheers