Re: [PATCH 0/2] Simplify RCU freeing of shadow pages

[Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>]

On 04/22/2012 07:39 PM, Avi Kivity wrote:

> On 04/21/2012 05:31 AM, Marcelo Tosatti wrote:
>> On Thu, Apr 19, 2012 at 07:26:35PM +0300, Avi Kivity wrote:
>>> This patchset simplifies the freeing by RCU of mmu pages.
>>>
>>> Xiao, I'm sure you thought of always freeing by RCU.  Why didn't you choose
>>> this way?  I saves a couple of atomics in the fast path.
>>>
>>> Avi Kivity (2):
>>>   KVM: MMU: Always free shadow pages using RCU
>>>   KVM: MMU: Recover space used by rcu_head in struct kvm_mmu_page
>>>
>>>  arch/x86/include/asm/kvm_host.h |    9 +++---
>>>  arch/x86/kvm/mmu.c              |   58 ++++++++-------------------------------
>>>  2 files changed, 15 insertions(+), 52 deletions(-)
>>
>> Check Documentation/RCU/checklist.txt item 8.
>>
>>         a.      Keeping a count of the number of data-structure elements
>>                 used by the RCU-protected data structure, including
>>                 those waiting for a grace period to elapse.  Enforce a
>>                 limit on this number, stalling updates as needed to allow
>>                 previously deferred frees to complete.  Alternatively,
>>                 limit only the number awaiting deferred free rather than
>>                 the total number of elements.
>>
> 
> That's true before and after the patch.  Currently the amount of memory
> that depends on rcu for freeing is unbounded.
> 
> Maybe we should protect the fast path using local_irq_disable() instead
> of rcu_read_lock(), like x86 page tables.  That means that
> kvm_flush_remote_tlbs() needs to ignore OUTSIDE_GUEST_MODE when IPIing
> vcpu threads.

So, IPI is unconditionally sent, do we need introduce a new MODE (say
STOP_SP_FREE) to reduce IPI?