> -----Original Message----- > From: owner-linux-mm@xxxxxxxxx [mailto:owner-linux-mm@xxxxxxxxx] On > Behalf Of Muchun Song > Sent: Saturday, November 14, 2020 12:00 AM > To: corbet@xxxxxxx; mike.kravetz@xxxxxxxxxx; tglx@xxxxxxxxxxxxx; > mingo@xxxxxxxxxx; bp@xxxxxxxxx; x86@xxxxxxxxxx; hpa@xxxxxxxxx; > dave.hansen@xxxxxxxxxxxxxxx; luto@xxxxxxxxxx; peterz@xxxxxxxxxxxxx; > viro@xxxxxxxxxxxxxxxxxx; akpm@xxxxxxxxxxxxxxxxxxxx; paulmck@xxxxxxxxxx; > mchehab+huawei@xxxxxxxxxx; pawan.kumar.gupta@xxxxxxxxxxxxxxx; > rdunlap@xxxxxxxxxxxxx; oneukum@xxxxxxxx; anshuman.khandual@xxxxxxx; > jroedel@xxxxxxx; almasrymina@xxxxxxxxxx; rientjes@xxxxxxxxxx; > willy@xxxxxxxxxxxxx; osalvador@xxxxxxx; mhocko@xxxxxxxx > Cc: duanxiongchun@xxxxxxxxxxxxx; linux-doc@xxxxxxxxxxxxxxx; > linux-kernel@xxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx; > linux-fsdevel@xxxxxxxxxxxxxxx; Muchun Song <songmuchun@xxxxxxxxxxxxx> > 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? > 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 > # 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
