linux-kernel - Re: [PATCH v2 12/15] KVM: MMU: allow locklessly access shadow page table out of vcpu thread

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Date:	Thu, 10 Oct 2013 13:42:22 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	Xiao Guangrong <xiaoguangrong.eric@...il.com>,
	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>,
	avi.kivity@...il.com, pbonzini@...hat.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v2 12/15] KVM: MMU: allow locklessly access shadow page
 table out of vcpu thread

On Thu, Oct 10, 2013 at 03:08:45PM +0300, Gleb Natapov wrote:
> On Wed, Oct 09, 2013 at 10:47:10PM -0300, Marcelo Tosatti wrote:
> > > >> Gleb has a idea that uses RCU_DESTORY to protect the shadow page table
> > > >> and encodes the page-level into the spte (since we need to check if the spte
> > > >> is the last-spte. ).  How about this?
> > > > 
> > > > Pointer please? Why is DESTROY_SLAB_RCU any safer than call_rcu with
> > > > regards to limitation? (maybe it is).
> > > 
> > > For my experience, freeing shadow page and allocing shadow page are balanced,
> > > we can check it by (make -j12 on a guest with 4 vcpus and):
> > > 
> > > # echo > trace
> > > [root@...c-desktop tracing]# cat trace > ~/log | sleep 3
> > > [root@...c-desktop tracing]# cat ~/log | grep new | wc -l
> > > 10816
> > > [root@...c-desktop tracing]# cat ~/log | grep prepare | wc -l
> > > 10656
> > > [root@...c-desktop tracing]# cat set_event
> > > kvmmmu:kvm_mmu_get_page
> > > kvmmmu:kvm_mmu_prepare_zap_page
> > > 
> > > alloc VS. free = 10816 : 10656
> > > 
> > > So that, mostly all allocing and freeing are done in the slab's
> > > cache and the slab frees shdadow pages very slowly, there is no rcu issue.
> > 
> > A more detailed test case would be:
> > 
> > - cpu0-vcpu0 releasing pages as fast as possible
> > - cpu1 executing get_dirty_log
> > 
> > Think of a very large guest.
> > 
> The number of shadow pages allocated from slab will be bounded by
> n_max_mmu_pages, 

Correct, but that limit is not suitable (maximum number of mmu pages
should be larger than number of mmu pages freeable in a rcu grace
period).

> and, in addition, page released to slab is immediately
> available for allocation, no need to wait for grace period. 

See SLAB_DESTROY_BY_RCU comment at include/linux/slab.h.

> RCU comes
> into play only when slab is shrunk, which should be almost never. If
> SLAB_DESTROY_BY_RCU slab does not rate limit how it frees its pages this
> is for slab to fix, not for its users.

Agree.

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