Re: [PATCH v2] Use the POWER8 Micro Partition Prefetch Engine in KVM HV on POWER8

[Alexander Graf <agraf@suse.de>]

On 10.07.14 15:30, Mel Gorman wrote:
> On Thu, Jul 10, 2014 at 03:17:16PM +0200, Alexander Graf wrote:
>> On 10.07.14 15:07, Mel Gorman wrote:
>>> On Thu, Jul 10, 2014 at 01:05:47PM +0200, Alexander Graf wrote:
>>>> On 09.07.14 00:59, Stewart Smith wrote:
>>>>> Hi!
>>>>>
>>>>> Thanks for review, much appreciated!
>>>>>
>>>>> Alexander Graf <agraf@suse.de> writes:
>>>>>> On 08.07.14 07:06, Stewart Smith wrote:
>>>>>>> @@ -1528,6 +1535,7 @@ static void kvmppc_run_core(struct kvmppc_vcore *vc)
>>>>>>>    	int i, need_vpa_update;
>>>>>>>    	int srcu_idx;
>>>>>>>    	struct kvm_vcpu *vcpus_to_update[threads_per_core];
>>>>>>> +	phys_addr_t phy_addr, tmp;
>>>>>> Please put the variable declarations into the if () branch so that the
>>>>>> compiler can catch potential leaks :)
>>>>> ack. will fix.
>>>>>
>>>>>>> @@ -1590,9 +1598,48 @@ static void kvmppc_run_core(struct kvmppc_vcore *vc)
>>>>>>>    	srcu_idx = srcu_read_lock(&vc->kvm->srcu);
>>>>>>> +	/* If we have a saved list of L2/L3, restore it */
>>>>>>> +	if (cpu_has_feature(CPU_FTR_ARCH_207S) && vc->mpp_buffer) {
>>>>>>> +		phy_addr = virt_to_phys((void *)vc->mpp_buffer);
>>>>>>> +#if defined(CONFIG_PPC_4K_PAGES)
>>>>>>> +		phy_addr = (phy_addr + 8*4096) & ~(8*4096);
>>>>>> get_free_pages() is automatically aligned to the order, no?
>>>>> That's what Paul reckoned too, and then we've attempted to find anywhere
>>>>> that documents that behaviour. Happen to be able to point to docs/source
>>>>> that say this is part of API?
>>>> Phew - it's probably buried somewhere. I could only find this
>>>> document saying that we always get order-aligned allocations:
>>>>
>>>> http://www.thehackademy.net/madchat/ebooks/Mem_virtuelle/linux-mm/zonealloc.html
>>>>
>>>> Mel, do you happen to have any pointer to something that explicitly
>>>> (or even properly implicitly) says that get_free_pages() returns
>>>> order-aligned memory?
>>>>
>>> I did not read the whole thread so I lack context and will just answer
>>> this part.
>>>
>>> There is no guarantee that pages are returned in PFN order for multiple
>>> requests to the page allocator. This is the relevant comment in
>>> rmqueue_bulk
>>>
>>>                  /*
>>>                   * Split buddy pages returned by expand() are received here
>>>                   * in physical page order. The page is added to the callers and
>>>                   * list and the list head then moves forward. From the callers
>>>                   * perspective, the linked list is ordered by page number in
>>>                   * some conditions. This is useful for IO devices that can
>>>                   * merge IO requests if the physical pages are ordered
>>>                   * properly.
>>>                   */
>>>
>>> It will probably be true early in the lifetime of the system but the milage
>>> will vary on systems with a lot of uptime. If you depend on this behaviour
>>> for correctness then you will have a bad day.
>>>
>>> High-order page requests to the page allocator are guaranteed to be in physical
>>> order. However, this does not apply to vmalloc() where allocations are
>>> only guaranteed to be virtually contiguous.
>> Hrm, ok to be very concrete:
>>
>>    Does __get_free_pages(..., 4); on a 4k page size system give me a
>> 64k aligned pointer? :)
>>
> Yes.

Awesome - thanks a lot! :)


Alex