From: Gleb Natapov <gleb@redhat.com>
To: Marcelo Tosatti <mtosatti@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: Sat, 12 Oct 2013 08:53:56 +0300 [thread overview]
Message-ID: <20131012055356.GC14789@redhat.com> (raw)
In-Reply-To: <20131011203017.GA29576@amt.cnet>
On Fri, Oct 11, 2013 at 05:30:17PM -0300, Marcelo Tosatti wrote:
> On Fri, Oct 11, 2013 at 08:38:31AM +0300, Gleb Natapov wrote:
> > > n_max_mmu_pages is not a suitable limit to throttle freeing of pages via
> > > RCU (its too large). If the free memory watermarks are smaller than
> > > n_max_mmu_pages for all guests, OOM is possible.
> > >
> > Ah, yes. I am not saying n_max_mmu_pages will throttle RCU, just saying
> > that slab size will be bound, so hopefully shrinker will touch it
> > rarely.
> >
> > > > > > 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.
> > > > >
> > > > This comment is exactly what I was referring to in the code you quoted. Do
> > > > you see anything problematic in what comment describes?
> > >
> > > "This delays freeing the SLAB page by a grace period, it does _NOT_
> > > delay object freeing." The page is not available for allocation.
> > By "page" I mean "spt page" which is a slab object. So "spt page"
> > AKA slab object will be available fo allocation immediately.
>
> The object is reusable within that SLAB cache only, not the
> entire system (therefore it does not prevent OOM condition).
>
Since object is allocatable immediately by shadow paging code the number
of SLAB objects is bound by n_max_mmu_pages. If there is no enough
memory for n_max_mmu_pages OOM condition can happen anyway since shadow
paging code will usually have exactly n_max_mmu_pages allocated.
> OK, perhaps it is useful to use SLAB_DESTROY_BY_RCU, but throttling
> is still necessary, as described in the RCU documentation.
>
I do not see what should be throttled if we use SLAB_DESTROY_BY_RCU. RCU
comes into play only when SLAB cache is shrunk and it happens far from
kvm code.
--
Gleb.
next prev parent reply other threads:[~2013-10-12 5:54 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
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 [this message]
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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