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

[Hanjun Guo <guohanjun@huawei.com>]

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.

Thanks
Hanjun


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