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`
arranged in one or more arrays. And here exists one-to-one mapping between
the physical page frame and the corresponding `struct page`.
The hugetlbpage 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 hugetlbpage has more than one `struct page`. The 2M hugetlbpage
has 512 `struct page` and 1G hugetlbpage has 4096 `struct page`. But
in the core of hugetlbpage only uses the first 4 `struct page` to store
metadata associated with each hugetlbpage. The rest of the `struct page`
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 hugetlbpage has 512 `struct page` which
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 4095 pages. This is a very
substantial gain. On our server, run some SPDK/QEMU applications which
will use 1000GB hugetlbpage. With this feature enabled, we can save
~16GB(1G hugepage)/~11GB(2MB hugepage) 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
# 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. But the overhead is not on the
allocating/freeing of each hugetlb page, it is only once when we reserve
some hugetlb pages through /proc/sys/vm/nr_hugepages. Once the reservation
is successful, the subsequent allocating, freeing and using are the same
as before (not patched). So I think that the overhead is acceptable.
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/vmemmap: 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: Add a BUILD_BUG_ON to check if struct page size is a power
of two
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
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 | 16 +
include/linux/bootmem_info.h | 79 +++
include/linux/hugetlb.h | 45 ++
include/linux/hugetlb_cgroup.h | 15 +-
include/linux/memory_hotplug.h | 27 -
include/linux/mm.h | 49 ++
mm/Makefile | 1 +
mm/bootmem_info.c | 124 ++++
mm/hugetlb.c | 806 +++++++++++++++++++++++-
mm/memory_hotplug.c | 116 ----
mm/sparse-vmemmap.c | 31 +
mm/sparse.c | 5 +-
17 files changed, 1181 insertions(+), 177 deletions(-)
create mode 100644 include/linux/bootmem_info.h
create mode 100644 mm/bootmem_info.c
--
2.11.0
