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Date: Tue, 17 Nov 2020 18:49:39 +0800
From: Muchun Song <songmuchun@...edance.com>
To: "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>
Cc: "corbet@....net" <corbet@....net>,
"mike.kravetz@...cle.com" <mike.kravetz@...cle.com>,
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Subject: Re: [External] RE: [PATCH v4 00/21] Free some vmemmap pages of
hugetlb page
On Tue, Nov 17, 2020 at 6:16 PM Song Bao Hua (Barry Song)
<song.bao.hua@...ilicon.com> wrote:
>
>
>
> > -----Original Message-----
> > From: owner-linux-mm@...ck.org [mailto:owner-linux-mm@...ck.org] On
> > Behalf Of Muchun Song
> > Sent: Saturday, November 14, 2020 12:00 AM
> > To: corbet@....net; mike.kravetz@...cle.com; tglx@...utronix.de;
> > mingo@...hat.com; bp@...en8.de; x86@...nel.org; hpa@...or.com;
> > dave.hansen@...ux.intel.com; luto@...nel.org; peterz@...radead.org;
> > viro@...iv.linux.org.uk; akpm@...ux-foundation.org; paulmck@...nel.org;
> > mchehab+huawei@...nel.org; pawan.kumar.gupta@...ux.intel.com;
> > rdunlap@...radead.org; oneukum@...e.com; anshuman.khandual@....com;
> > jroedel@...e.de; almasrymina@...gle.com; rientjes@...gle.com;
> > willy@...radead.org; osalvador@...e.de; mhocko@...e.com
> > Cc: duanxiongchun@...edance.com; linux-doc@...r.kernel.org;
> > linux-kernel@...r.kernel.org; linux-mm@...ck.org;
> > linux-fsdevel@...r.kernel.org; Muchun Song <songmuchun@...edance.com>
> > Subject: [PATCH v4 00/21] Free some vmemmap pages of hugetlb page
> >
> > Hi all,
> >
> > This patch series will free some vmemmap pages(struct page structures)
> > associated with each hugetlbpage when preallocated to save memory.
> >
> > Nowadays we track the status of physical page frames using struct page
> > structures arranged in one or more arrays. And here exists one-to-one
> > mapping between the physical page frame and the corresponding struct page
> > structure.
> >
> > The HugeTLB support is built on top of multiple page size support that
> > is provided by most modern architectures. For example, x86 CPUs normally
> > support 4K and 2M (1G if architecturally supported) page sizes. Every
> > HugeTLB has more than one struct page structure. The 2M HugeTLB has 512
> > struct page structure and 1G HugeTLB has 4096 struct page structures. But
> > in the core of HugeTLB only uses the first 4 (Use of first 4 struct page
> > structures comes from HUGETLB_CGROUP_MIN_ORDER.) struct page
> > structures to
> > store metadata associated with each HugeTLB. The rest of the struct page
> > structures are usually read the compound_head field which are all the same
> > value. If we can free some struct page memory to buddy system so that we
> > can save a lot of memory.
> >
> > When the system boot up, every 2M HugeTLB has 512 struct page structures
> > which size is 8 pages(sizeof(struct page) * 512 / PAGE_SIZE).
> >
> > hugetlbpage struct pages(8 pages) page
> > frame(8 pages)
> > +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
> > | | | 0 | -------------> | 0
> > |
> > | | | 1 | -------------> | 1
> > |
> > | | | 2 | -------------> | 2
> > |
> > | | | 3 | -------------> | 3
> > |
> > | | | 4 | -------------> | 4
> > |
> > | 2M | | 5 | -------------> |
> > 5 |
> > | | | 6 | -------------> | 6
> > |
> > | | | 7 | -------------> | 7
> > |
> > | | +-----------+
> > +-----------+
> > | |
> > | |
> > +-----------+
> >
> >
> > When a hugetlbpage is preallocated, we can change the mapping from above
> > to
> > bellow.
> >
> > hugetlbpage struct pages(8 pages) page
> > frame(8 pages)
> > +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
> > | | | 0 | -------------> | 0
> > |
> > | | | 1 | -------------> | 1
> > |
> > | | | 2 | ------------->
> > +-----------+
> > | | | 3 | -----------------^ ^ ^ ^
> > ^
> > | | | 4 | -------------------+ | |
> > |
> > | 2M | | 5 | ---------------------+ |
> > |
> > | | | 6 | -----------------------+ |
> > | | | 7 | -------------------------+
> > | | +-----------+
> > | |
> > | |
> > +-----------+
> >
> > For tail pages, the value of compound_head is the same. So we can reuse
> > first page of tail page structs. We map the virtual addresses of the
> > remaining 6 pages of tail page structs to the first tail page struct,
> > and then free these 6 pages. Therefore, we need to reserve at least 2
> > pages as vmemmap areas.
> >
> > When a hugetlbpage is freed to the buddy system, we should allocate six
> > pages for vmemmap pages and restore the previous mapping relationship.
> >
> > If we uses the 1G hugetlbpage, we can save 4088 pages(There are 4096 pages
> > for
> > struct page structures, we reserve 2 pages for vmemmap and 8 pages for page
> > tables. So we can save 4088 pages). This is a very substantial gain. On our
> > server, run some SPDK/QEMU applications which will use 1024GB hugetlbpage.
> > With this feature enabled, we can save ~16GB(1G hugepage)/~11GB(2MB
> > hugepage)
>
> Hi Muchun,
>
> Do we really save 11GB for 2MB hugepage?
> How much do we save if we only get one 2MB hugetlb from one 128MB mem_section?
> It seems we need to get at least one page for the PTEs since we are splitting PMD of
> vmemmap into PTE?
There are 524288(1024GB/2MB) 2MB HugeTLB pages. We can save 6 pages for each
2MB HugeTLB page. So we can save 3145728 pages. But we need to split PMD page
table for every one 128MB mem_section and every section need one page
as PTE page
table. So we need 8192(1024GB/128MB) pages as PTE page tables.
Finally, we can save
3137536(3145728-8192) pages which is 11.97GB.
Thanks Barry.
>
> > memory.
> >
> > Because there are vmemmap page tables reconstruction on the
> > freeing/allocating
> > path, it increases some overhead. Here are some overhead analysis.
> >
> > 1) Allocating 10240 2MB hugetlb pages.
> >
> > a) With this patch series applied:
> > # time echo 10240 > /proc/sys/vm/nr_hugepages
> >
> > real 0m0.166s
> > user 0m0.000s
> > sys 0m0.166s
> >
> > # bpftrace -e 'kprobe:alloc_fresh_huge_page { @start[tid] = nsecs; }
> > kretprobe:alloc_fresh_huge_page /@...rt[tid]/ { @latency = hist(nsecs -
> > @start[tid]); delete(@start[tid]); }'
> > Attaching 2 probes...
> >
> > @latency:
> > [8K, 16K) 8360
> > |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
> > @@@@@@@@@@@@@@@@|
> > [16K, 32K) 1868 |@@@@@@@@@@@
> > |
> > [32K, 64K) 10 |
> > |
> > [64K, 128K) 2 |
> > |
> >
> > b) Without this patch series:
> > # time echo 10240 > /proc/sys/vm/nr_hugepages
> >
> > real 0m0.066s
> > user 0m0.000s
> > sys 0m0.066s
> >
> > # bpftrace -e 'kprobe:alloc_fresh_huge_page { @start[tid] = nsecs; }
> > kretprobe:alloc_fresh_huge_page /@...rt[tid]/ { @latency = hist(nsecs -
> > @start[tid]); delete(@start[tid]); }'
> > Attaching 2 probes...
> >
> > @latency:
> > [4K, 8K) 10176
> > |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
> > @@@@@@@@@@@@@@@@|
> > [8K, 16K) 62 |
> > |
> > [16K, 32K) 2 |
> > |
> >
> > Summarize: this feature is about ~2x slower than before.
> >
> > 2) Freeing 10240 @MB hugetlb pages.
> >
> > a) With this patch series applied:
> > # time echo 0 > /proc/sys/vm/nr_hugepages
> >
> > real 0m0.004s
> > user 0m0.000s
> > sys 0m0.002s
> >
>
> Something is wrong here, it is faster than the case without this patchset:
> 0.004s vs. 0m0.077s
Yeah, it is faster. And Why the 'real' time of patched is smaller than before?
Because in this patch series, the freeing HugeTLB is
asynchronous(through worker).
>
> > # bpftrace -e 'kprobe:__free_hugepage { @start[tid] = nsecs; }
> > kretprobe:__free_hugepage /@...rt[tid]/ { @latency = hist(nsecs - @start[tid]);
> > delete(@start[tid]); }'
> > Attaching 2 probes...
> >
> > @latency:
> > [16K, 32K) 10240
> > |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
> > @@@@@@@@@@@@@@@@|
> >
> > b) Without this patch series:
> > # time echo 0 > /proc/sys/vm/nr_hugepages
> >
> > real 0m0.077s
> > user 0m0.001s
> > sys 0m0.075s
> >
> > # bpftrace -e 'kprobe:__free_hugepage { @start[tid] = nsecs; }
> > kretprobe:__free_hugepage /@...rt[tid]/ { @latency = hist(nsecs - @start[tid]);
> > delete(@start[tid]); }'
> > Attaching 2 probes...
> >
> > @latency:
> > [4K, 8K) 9950
> > |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
> > @@@@@@@@@@@@@@@@|
> > [8K, 16K) 287 |@
> > |
> > [16K, 32K) 3 |
> > |
> >
> > Summarize: The overhead of __free_hugepage is about ~2-4x slower than
> > before.
> > But according to the allocation test above, I think that here is
> > also ~2x slower than before.
> >
> > But why the 'real' time of patched is smaller than before?
> > Because
> > In this patch series, the freeing hugetlb is asynchronous(through
> > kwoker).
> >
> > Although the overhead has increased, the overhead is not significant. Like MIke
> > said, "However, remember that the majority of use cases create hugetlb pages
> > at
> > or shortly after boot time and add them to the pool. So, additional overhead is
> > at pool creation time. There is no change to 'normal run time' operations of
> > getting a page from or returning a page to the pool (think page fault/unmap)".
> >
>
> It seems it is true. At runtime, people normally don't change hugetlb.
>
> > changelog in v4:
> > 1. Move all the vmemmap functions to hugetlb_vmemmap.c.
> > 2. Make the CONFIG_HUGETLB_PAGE_FREE_VMEMMAP default to y, if we
> > want to
> > disable this feature, we should disable it by a boot/kernel command line.
> > 3. Remove vmemmap_pgtable_{init, deposit, withdraw}() helper functions.
> > 4. Initialize page table lock for vmemmap through core_initcall mechanism.
> >
> > Thanks for Mike and Oscar's suggestions.
> >
> > changelog in v3:
> > 1. Rename some helps function name. Thanks Mike.
> > 2. Rework some code. Thanks Mike and Oscar.
> > 3. Remap the tail vmemmap page with PAGE_KERNEL_RO instead of
> > PAGE_KERNEL. Thanks Matthew.
> > 4. Add some overhead analysis in the cover letter.
> > 5. Use vmemap pmd table lock instead of a hugetlb specific global lock.
> >
> > changelog in v2:
> > 1. Fix do not call dissolve_compound_page in alloc_huge_page_vmemmap().
> > 2. Fix some typo and code style problems.
> > 3. Remove unused handle_vmemmap_fault().
> > 4. Merge some commits to one commit suggested by Mike.
> >
> > Muchun Song (21):
> > mm/memory_hotplug: Move bootmem info registration API to
> > bootmem_info.c
> > mm/memory_hotplug: Move {get,put}_page_bootmem() to bootmem_info.c
> > mm/hugetlb: Introduce a new config HUGETLB_PAGE_FREE_VMEMMAP
> > mm/hugetlb: Introduce nr_free_vmemmap_pages in the struct hstate
> > mm/hugetlb: Introduce pgtable allocation/freeing helpers
> > mm/bootmem_info: Introduce {free,prepare}_vmemmap_page()
> > mm/bootmem_info: Combine bootmem info and type into page->freelist
> > mm/hugetlb: Initialize page table lock for vmemmap
> > mm/hugetlb: Free the vmemmap pages associated with each hugetlb page
> > mm/hugetlb: Defer freeing of hugetlb pages
> > mm/hugetlb: Allocate the vmemmap pages associated with each hugetlb
> > page
> > mm/hugetlb: Introduce remap_huge_page_pmd_vmemmap helper
> > mm/hugetlb: Use PG_slab to indicate split pmd
> > mm/hugetlb: Support freeing vmemmap pages of gigantic page
> > mm/hugetlb: Set the PageHWPoison to the raw error page
> > mm/hugetlb: Flush work when dissolving hugetlb page
> > mm/hugetlb: Add a kernel parameter hugetlb_free_vmemmap
> > mm/hugetlb: Merge pte to huge pmd only for gigantic page
> > mm/hugetlb: Gather discrete indexes of tail page
> > mm/hugetlb: Add BUILD_BUG_ON to catch invalid usage of tail struct
> > page
> > mm/hugetlb: Disable freeing vmemmap if struct page size is not power
> > of two
> >
> > Documentation/admin-guide/kernel-parameters.txt | 9 +
> > Documentation/admin-guide/mm/hugetlbpage.rst | 3 +
> > arch/x86/include/asm/hugetlb.h | 17 +
> > arch/x86/include/asm/pgtable_64_types.h | 8 +
> > arch/x86/mm/init_64.c | 7 +-
> > fs/Kconfig | 14 +
> > include/linux/bootmem_info.h | 78 +++
> > include/linux/hugetlb.h | 19 +
> > include/linux/hugetlb_cgroup.h | 15 +-
> > include/linux/memory_hotplug.h | 27 -
> > mm/Makefile | 2 +
> > mm/bootmem_info.c | 124 ++++
> > mm/hugetlb.c | 163 +++++-
> > mm/hugetlb_vmemmap.c | 732
> > ++++++++++++++++++++++++
> > mm/hugetlb_vmemmap.h | 104 ++++
> > mm/memory_hotplug.c | 116 ----
> > mm/sparse.c | 5 +-
> > 17 files changed, 1263 insertions(+), 180 deletions(-)
> > create mode 100644 include/linux/bootmem_info.h
> > create mode 100644 mm/bootmem_info.c
> > create mode 100644 mm/hugetlb_vmemmap.c
> > create mode 100644 mm/hugetlb_vmemmap.h
> >
>
> Thanks
> Barry
>
--
Yours,
Muchun
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