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

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Date:   Sun, 27 Mar 2022 12:40:59 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vipin Sharma <vipinsh@...gle.com>,
        David Matlack <dmatlack@...gle.com>
Cc:     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 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