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

[Ard Biesheuvel]

On 22 August 2018 at 05:07, Jia He <hejianet@xxxxxxxxx> 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.

Apologies to Jia that it took 11 revisions to reach this conclusion,
but in /my/ opinion, tweaking the fragile memblock/pfn handling code
for this reason is totally unjustified, and we're better off
disregarding these patches.





> Patch 1 introduces new config to make codes more generic
> Patch 2 remains the memblock_next_valid_pfn on arm and arm64,this patch is
>         originated from b92df1de5
> Patch 3 optimizes the memblock_next_valid_pfn()
>
> Changelog:
> V11:- drop patch#4-6, refine the codes
> V10:- move codes to memblock.c, refine the performance consideration
> V9: - rebase to mmotm master, refine the log description. No major changes
> V8: - introduce new config and move generic code to early_pfn.h
>     - optimize memblock_next_valid_pfn as suggested by Matthew Wilcox
> V7: - fix i386 compilation error. refine the commit description
> V6: - simplify the codes, move arm/arm64 common codes to one file.
>     - refine patches as suggested by Danial Vacek and Ard Biesheuvel
> V5: - further refining as suggested by Danial Vacek. Make codes
>       arm/arm64 more arch specific
> V4: - refine patches as suggested by Danial Vacek and Wei Yang
>     - optimized on arm besides arm64
> V3: - fix 2 issues reported by kbuild test robot
> V2: - rebase to mmotm latest
>     - remain memblock_next_valid_pfn on arm64
>     - refine memblock_search_pfn_regions and pfn_valid_region
>
> Jia He (3):
>   arm: arm64: introduce CONFIG_HAVE_MEMBLOCK_PFN_VALID
>   mm: page_alloc: remain memblock_next_valid_pfn() on arm/arm64
>   mm: page_alloc: reduce unnecessary binary search in
>     memblock_next_valid_pfn
>
>  arch/arm/Kconfig       |  1 +
>  arch/arm64/Kconfig     |  1 +
>  include/linux/mmzone.h |  9 +++++++++
>  mm/Kconfig             |  3 +++
>  mm/memblock.c          | 51 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  mm/page_alloc.c        |  5 ++++-
>  6 files changed, 69 insertions(+), 1 deletion(-)
>
> --
> 1.8.3.1
>