linux-kernel - Re: [PATCH] KVM: x86/mmu: Speed up slot_rmap_walk

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Date:   Mon, 18 Apr 2022 09:29:16 -0700
From:   Vipin Sharma <vipinsh@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     David Matlack <dmatlack@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86/mmu: Speed up slot_rmap_walk_next for sparsely
 populated rmaps

On Fri, Apr 8, 2022 at 12:31 PM Vipin Sharma <vipinsh@...gle.com> wrote:
>
> On Sun, Mar 27, 2022 at 3:41 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
> >
> > On 3/26/22 01:31, Vipin Sharma wrote:
> > >>> -static void slot_rmap_walk_next(struct slot_rmap_walk_iterator *iterator)
> > >>> +static noinline void
> > >>
> > >> What is the reason to add noinline?
> > >
> > > My understanding is that since this method is called from
> > > __always_inline methods, noinline will avoid gcc inlining the
> > > slot_rmap_walk_next in those functions and generate smaller code.
> > >
> >
> > Iterators are written in such a way that it's way more beneficial to
> > inline them.  After inlining, compilers replace the aggregates (in this
> > case, struct slot_rmap_walk_iterator) with one variable per field and
> > that in turn enables a lot of optimizations, so the iterators should
> > actually be always_inline if anything.
> >
> > For the same reason I'd guess the effect on the generated code should be
> > small (next time please include the output of "size mmu.o"), but should
> > still be there.  I'll do a quick check of the generated code and apply
> > the patch.
> >
> > Paolo
> >
>
> Let me know if you are still planning to modify the current patch by
> removing "noinline" and merge or if you prefer a v2 without noinline.

Hi Paolo,

Any update on this patch?

Thanks
Vipin