linux-kernel - Re: [PATCH v2 0/7] KVM: MMU: fast zap all shadow pages

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Date:	Thu, 18 Apr 2013 11:01:18 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v2 0/7] KVM: MMU: fast zap all shadow pages

On Thu, Apr 18, 2013 at 12:42:39PM +0300, Gleb Natapov wrote:
> > > that, but if not then less code is better.
> > 
> > The number of sp->role.invalid=1 pages is small (only shadow roots). It
> > can grow but is bounded to a handful. No improvement visible there.
> > 
> > The number of shadow pages with old mmu_gen_number is potentially large.
> > 
> > Returning all shadow pages to the allocator is problematic because it
> > takes a long time (therefore the suggestion to postpone it).
> > 
> > Spreading the work to free (or reuse) those shadow pages to individual
> > page fault instances alleviates the mmu_lock hold time issue without
> > significant reduction to post kvm_mmu_zap_all operation (which has to
> > rebuilt all pagetables anyway).
> > 
> > You prefer to modify SLAB allocator to aggressively free these stale
> > shadow pages rather than kvm_mmu_get_page to reuse them?
> Are you saying that what makes kvm_mmu_zap_all() slow is that we return
> all the shadow pages to the SLAB allocator? As far as I understand what
> makes it slow is walking over huge number of shadow pages via various
> lists, actually releasing them to the SLAB is not an issue, otherwise
> the problem could have been solved by just moving
> kvm_mmu_commit_zap_page() out of the mmu_lock. If there is measurable
> SLAB overhead from not reusing the pages I am all for reusing them, but
> is this really the case or just premature optimization?

Actually releasing them is not a problem. Walking all pages, lists and
releasing in the process part of the problem ("returning them to the allocator"
would have been clearer with "freeing them").

Point is at some point you have to walk all pages and release their data
structures. With Xiaos scheme its possible to avoid this lengthy process
by either:

1) reusing the pages with stale generation number
or
2) releasing them via the SLAB shrinker more aggressively

(another typo, i meant "SLAB shrinker" not "SLAB allocator").

But you seem to be concerned for 1) due to code complexity issues?


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