Re: [PATCH v11 0/3] remain and optimize memblock_next_valid_pfn on arm and arm64

[Jia He <hejianet@gmail.com>]

Hi Hanjun

On 2019/6/11 23:18, Hanjun Guo wrote:
> Hello Ard,
>
> Thanks for the reply, please see my comments inline.
>
> On 2019/6/10 21:16, Ard Biesheuvel wrote:
>> On Sat, 8 Jun 2019 at 06:22, Hanjun Guo <guohanjun@huawei.com> wrote:
>>> Hi Ard, Will,
>>>
>>> This week we were trying to debug an issue of time consuming in mem_init(),
>>> and leading to this similar solution form Jia He, so I would like to bring this
>>> thread back, please see my detail test result below.
>>>
>>> On 2018/9/7 22:44, Will Deacon wrote:
>>>> On Thu, Sep 06, 2018 at 01:24:22PM +0200, Ard Biesheuvel wrote:
>>>>> On 22 August 2018 at 05:07, Jia He <hejianet@gmail.com> wrote:
>>>>>> Commit b92df1de5d28 ("mm: page_alloc: skip over regions of invalid pfns
>>>>>> where possible") optimized the loop in memmap_init_zone(). But it causes
>>>>>> possible panic bug. So Daniel Vacek reverted it later.
>>>>>>
>>>>>> But as suggested by Daniel Vacek, it is fine to using memblock to skip
>>>>>> gaps and finding next valid frame with CONFIG_HAVE_ARCH_PFN_VALID.
>>>>>>
>>>>>> More from what Daniel said:
>>>>>> "On arm and arm64, memblock is used by default. But generic version of
>>>>>> pfn_valid() is based on mem sections and memblock_next_valid_pfn() does
>>>>>> not always return the next valid one but skips more resulting in some
>>>>>> valid frames to be skipped (as if they were invalid). And that's why
>>>>>> kernel was eventually crashing on some !arm machines."
>>>>>>
>>>>>> About the performance consideration:
>>>>>> As said by James in b92df1de5,
>>>>>> "I have tested this patch on a virtual model of a Samurai CPU with a
>>>>>> sparse memory map.  The kernel boot time drops from 109 to 62 seconds."
>>>>>> Thus it would be better if we remain memblock_next_valid_pfn on arm/arm64.
>>>>>>
>>>>>> Besides we can remain memblock_next_valid_pfn, there is still some room
>>>>>> for improvement. After this set, I can see the time overhead of memmap_init
>>>>>> is reduced from 27956us to 13537us in my armv8a server(QDF2400 with 96G
>>>>>> memory, pagesize 64k). I believe arm server will benefit more if memory is
>>>>>> larger than TBs
>>>>>>
>>>>> OK so we can summarize the benefits of this series as follows:
>>>>> - boot time on a virtual model of a Samurai CPU drops from 109 to 62 seconds
>>>>> - boot time on a QDF2400 arm64 server with 96 GB of RAM drops by ~15
>>>>> *milliseconds*
>>>>>
>>>>> Google was not very helpful in figuring out what a Samurai CPU is and
>>>>> why we should care about the boot time of Linux running on a virtual
>>>>> model of it, and the 15 ms speedup is not that compelling either.
>>> Testing this patch set on top of Kunpeng 920 based ARM64 server, with
>>> 384G memory in total, we got the time consuming below
>>>
>>>               without this patch set      with this patch set
>>> mem_init()        13310ms                      1415ms
>>>
>>> So we got about 8x speedup on this machine, which is very impressive.
>>>
>> Yes, this is impressive. But does it matter in the grand scheme of
>> things?
> It matters for this machine, because it's for storage and there is
> a watchdog and the time consuming triggers the watchdog.
>
>> How much time does this system take to arrive at this point
>> from power on?
> Sorry, I don't have such data, as the arch timer is not initialized
> and I didn't see the time stamp at this point, but I read the cycles
> from arch timer before and after the time consuming function to get
> how much time consumed.
>
>>> The time consuming is related the memory DIMM size and where to locate those
>>> memory DIMMs in the slots. In above case, we are using 16G memory DIMM.
>>> We also tested 1T memory with 64G size for each memory DIMM on another ARM64
>>> machine, the time consuming reduced from 20s to 2s (I think it's related to
>>> firmware implementations).
>>>
>> I agree that this optimization looks good in isolation, but the fact
>> that you spotted a bug justifies my skepticism at the time. On the
>> other hand, now that we have several independent reports (from you,
>> but also from the Renesas folks) that the speedup is worthwhile for
>> real world use cases, I think it does make sense to revisit it.
> Thank you very much for taking care of this :)
>
>> So what I would like to see is the patch set being proposed again,
>> with the new data points added for documentation. Also, the commit
>> logs need to crystal clear about how the meaning of PFN validity
>> differs between ARM and other architectures, and why the assumptions
>> that the optimization is based on are guaranteed to hold.
> I think Jia He no longer works for HXT, if don't mind, I can repost
> this patch set with Jia He's authority unchanged.
Ok, I don't mind that, thanks for your followup :)

---
Cheers,
Justin (Jia He)