Re: [External] RE: [PATCH v4 00/21] Free some vmemmap pages of hugetlb page

[Muchun Song <songmuchun@bytedance.com>]

On Tue, Nov 17, 2020 at 6:16 PM Song Bao Hua (Barry Song)
<song.bao.hua@hisilicon.com> wrote:
>
>
>
> > -----Original Message-----
> > From: owner-linux-mm@kvack.org [mailto:owner-linux-mm@kvack.org] On
> > Behalf Of Muchun Song
> > Sent: Saturday, November 14, 2020 12:00 AM
> > To: corbet@lwn.net; mike.kravetz@oracle.com; tglx@linutronix.de;
> > mingo@redhat.com; bp@alien8.de; x86@kernel.org; hpa@zytor.com;
> > dave.hansen@linux.intel.com; luto@kernel.org; peterz@infradead.org;
> > viro@zeniv.linux.org.uk; akpm@linux-foundation.org; paulmck@kernel.org;
> > mchehab+huawei@kernel.org; pawan.kumar.gupta@linux.intel.com;
> > rdunlap@infradead.org; oneukum@suse.com; anshuman.khandual@arm.com;
> > jroedel@suse.de; almasrymina@google.com; rientjes@google.com;
> > willy@infradead.org; osalvador@suse.de; mhocko@suse.com
> > Cc: duanxiongchun@bytedance.com; linux-doc@vger.kernel.org;
> > linux-kernel@vger.kernel.org; linux-mm@kvack.org;
> > linux-fsdevel@vger.kernel.org; Muchun Song <songmuchun@bytedance.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 /@start[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 /@start[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 /@start[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 /@start[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