[PATCH 0/4] powernv: kvm: numa fault improvement

Liu ping fan kernelfans at gmail.com
Tue Jan 21 20:11:31 EST 2014

On Tue, Jan 21, 2014 at 5:07 PM, Liu ping fan <kernelfans at gmail.com> wrote:
> On Tue, Jan 21, 2014 at 11:40 AM, Aneesh Kumar K.V
> <aneesh.kumar at linux.vnet.ibm.com> wrote:
>> Liu ping fan <kernelfans at gmail.com> writes:
>>> On Mon, Jan 20, 2014 at 11:45 PM, Aneesh Kumar K.V
>>> <aneesh.kumar at linux.vnet.ibm.com> wrote:
>>>> Liu ping fan <kernelfans at gmail.com> writes:
>>>>> On Thu, Jan 9, 2014 at 8:08 PM, Alexander Graf <agraf at suse.de> wrote:
>>>>>> On 11.12.2013, at 09:47, Liu Ping Fan <kernelfans at gmail.com> wrote:
>>>>>>> This series is based on Aneesh's series  "[PATCH -V2 0/5] powerpc: mm: Numa faults support for ppc64"
>>>>>>> For this series, I apply the same idea from the previous thread "[PATCH 0/3] optimize for powerpc _PAGE_NUMA"
>>>>>>> (for which, I still try to get a machine to show nums)
>>>>>>> But for this series, I think that I have a good justification -- the fact of heavy cost when switching context between guest and host,
>>>>>>> which is  well known.
>>>>>> This cover letter isn't really telling me anything. Please put a proper description of what you're trying to achieve, why you're trying to achieve what you're trying and convince your readers that it's a good idea to do it the way you do it.
>>>>> Sorry for the unclear message. After introducing the _PAGE_NUMA,
>>>>> kvmppc_do_h_enter() can not fill up the hpte for guest. Instead, it
>>>>> should rely on host's kvmppc_book3s_hv_page_fault() to call
>>>>> do_numa_page() to do the numa fault check. This incurs the overhead
>>>>> when exiting from rmode to vmode.  My idea is that in
>>>>> kvmppc_do_h_enter(), we do a quick check, if the page is right placed,
>>>>> there is no need to exit to vmode (i.e saving htab, slab switching)
>>>> Can you explain more. Are we looking at hcall from guest  and
>>>> hypervisor handling them in real mode ? If so why would guest issue a
>>>> hcall on a pte entry that have PAGE_NUMA set. Or is this about
>>>> hypervisor handling a missing hpte, because of host swapping this page
>>>> out ? In that case how we end up in h_enter ? IIUC for that case we
>>>> should get to kvmppc_hpte_hv_fault.
>>> After setting _PAGE_NUMA, we should flush out all hptes both in host's
>>> htab and guest's. So when guest tries to access memory, host finds
>>> that there is not hpte ready for guest in guest's htab. And host
>>> should raise dsi to guest.
>> Now guest receive that fault, removes the PAGE_NUMA bit and do an
>> hpte_insert. So before we do an hpte_insert (or H_ENTER) we should have
>> cleared PAGE_NUMA bit.
>>>This incurs that guest ends up in h_enter.
>>> And you can see in current code, we also try this quick path firstly.
>>> Only if fail, we will resort to slow path --  kvmppc_hpte_hv_fault.
>> hmm ? hpte_hv_fault is the hypervisor handling the fault.
> After we discuss in irc. I think we should also do the fast check in
> kvmppc_hpte_hv_fault() for the case of HPTE_V_ABSENT,
> and let H_ENTER take care of the rest case i.e. no hpte when pte_mknuma. Right?
Or we can delay the quick fix in H_ENTER, and let the host fault
again, so do the fix in kvmppc_hpte_hv_fault()

> Thanks and regards,
> Fan
>> -aneesh

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