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From: Marcelo Tosatti <mtosatti@redhat.com>
To: Gleb Natapov <gleb@redhat.com>
Cc: Xiao Guangrong <xiaoguangrong.eric@gmail.com>,
	Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>,
	avi.kivity@gmail.com, pbonzini@redhat.com,
	linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Subject: Re: [PATCH v2 12/15] KVM: MMU: allow locklessly access shadow page table out of vcpu thread
Date: Thu, 10 Oct 2013 13:42:22 -0300	[thread overview]
Message-ID: <20131010164222.GB3211@amt.cnet> (raw)
In-Reply-To: <20131010120845.GT3574@redhat.com>

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@eric-desktop tracing]# cat trace > ~/log | sleep 3
> > > [root@eric-desktop tracing]# cat ~/log | grep new | wc -l
> > > 10816
> > > [root@eric-desktop tracing]# cat ~/log | grep prepare | wc -l
> > > 10656
> > > [root@eric-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.


  reply	other threads:[~2013-10-10 16:42 UTC|newest]

Thread overview: 50+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2013-09-05 10:29 [PATCH v2 00/15] KVM: MMU: locklessly wirte-protect Xiao Guangrong
2013-09-05 10:29 ` [PATCH v2 01/15] KVM: MMU: fix the count of spte number Xiao Guangrong
2013-09-08 12:19   ` Gleb Natapov
2013-09-08 13:55     ` Xiao Guangrong
2013-09-08 14:01       ` Gleb Natapov
2013-09-08 14:24         ` Xiao Guangrong
2013-09-08 14:26           ` Gleb Natapov
2013-09-05 10:29 ` [PATCH v2 02/15] KVM: MMU: properly check last spte in fast_page_fault() Xiao Guangrong
2013-09-30 21:23   ` Marcelo Tosatti
2013-10-03  6:16     ` Xiao Guangrong
2013-09-05 10:29 ` [PATCH v2 03/15] KVM: MMU: lazily drop large spte Xiao Guangrong
2013-09-30 22:39   ` Marcelo Tosatti
2013-10-03  6:29     ` Xiao Guangrong
2013-09-05 10:29 ` [PATCH v2 04/15] KVM: MMU: flush tlb if the spte can be locklessly modified Xiao Guangrong
2013-09-05 10:29 ` [PATCH v2 05/15] KVM: MMU: flush tlb out of mmu lock when write-protect the sptes Xiao Guangrong
2013-09-30 23:05   ` Marcelo Tosatti
2013-10-03  6:46     ` Xiao Guangrong
2013-09-05 10:29 ` [PATCH v2 06/15] KVM: MMU: update spte and add it into rmap before dirty log Xiao Guangrong
2013-09-05 10:29 ` [PATCH v2 07/15] KVM: MMU: redesign the algorithm of pte_list Xiao Guangrong
2013-09-05 10:29 ` [PATCH v2 08/15] KVM: MMU: introduce nulls desc Xiao Guangrong
2013-09-05 10:29 ` [PATCH v2 09/15] KVM: MMU: introduce pte-list lockless walker Xiao Guangrong
2013-09-08 12:03   ` Xiao Guangrong
2013-09-16 12:42   ` Gleb Natapov
2013-09-16 13:52     ` Xiao Guangrong
2013-09-16 15:04       ` Gleb Natapov
2013-09-05 10:29 ` [PATCH v2 10/15] KVM: MMU: initialize the pointers in pte_list_desc properly Xiao Guangrong
2013-09-05 10:29 ` [PATCH v2 11/15] KVM: MMU: reintroduce kvm_mmu_isolate_page() Xiao Guangrong
2013-09-05 10:29 ` [PATCH v2 12/15] KVM: MMU: allow locklessly access shadow page table out of vcpu thread Xiao Guangrong
2013-10-08  1:23   ` Marcelo Tosatti
2013-10-08  4:02     ` Xiao Guangrong
2013-10-09  1:56       ` Marcelo Tosatti
2013-10-09 10:45         ` Xiao Guangrong
2013-10-10  1:47           ` Marcelo Tosatti
2013-10-10 12:08             ` Gleb Natapov
2013-10-10 16:42               ` Marcelo Tosatti [this message]
2013-10-10 19:16                 ` Gleb Natapov
2013-10-10 21:03                   ` Marcelo Tosatti
2013-10-11  5:38                     ` Gleb Natapov
2013-10-11 20:30                       ` Marcelo Tosatti
2013-10-12  5:53                         ` Gleb Natapov
2013-10-14 19:29                           ` Marcelo Tosatti
2013-10-15  3:57                             ` Gleb Natapov
2013-10-15 22:21                               ` Marcelo Tosatti
2013-10-16  0:41                                 ` Xiao Guangrong
2013-10-16  9:12                                 ` Gleb Natapov
2013-10-16 20:43                                   ` Marcelo Tosatti
2013-09-05 10:29 ` [PATCH v2 13/15] KVM: MMU: locklessly write-protect the page Xiao Guangrong
2013-09-05 10:29 ` [PATCH v2 14/15] KVM: MMU: clean up spte_write_protect Xiao Guangrong
2013-09-05 10:29 ` [PATCH v2 15/15] KVM: MMU: use rcu functions to access the pointer Xiao Guangrong
2013-09-15 10:26 ` [PATCH v2 00/15] KVM: MMU: locklessly wirte-protect Gleb Natapov

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