Messages in this thread | | | Subject | Re: [PATCH v11 0/3] remain and optimize memblock_next_valid_pfn on arm and arm64 | From | Jia He <> | Date | Wed, 12 Jun 2019 09:05:59 +0800 |
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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)
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