Re: [PATCH v6 2/2] arm64: support batched/deferred tlb shootdown during page reclamation

haoxin
2022-11-15T06:36:41Z

在 2022/11/15 上午11:14, Yicong Yang 写道:
> From: Barry Song <v-songbaohua@oppo.com>
>
> on x86, batched and deferred tlb shootdown has lead to 90%
> performance increase on tlb shootdown. on arm64, HW can do
> tlb shootdown without software IPI. But sync tlbi is still
> quite expensive.
>
> Even running a simplest program which requires swapout can
> prove this is true,
> #include <sys/types.h>
> #include <unistd.h>
> #include <sys/mman.h>
> #include <string.h>
>
> int main()
> {
> #define SIZE (1 * 1024 * 1024)
> volatile unsigned char *p = mmap(NULL, SIZE, PROT_READ | PROT_WRITE,
> MAP_SHARED | MAP_ANONYMOUS, -1, 0);
>
> memset(p, 0x88, SIZE);
>
> for (int k = 0; k < 10000; k++) {
> /* swap in */
> for (int i = 0; i < SIZE; i += 4096) {
> (void)p[i];
> }
>
> /* swap out */
> madvise(p, SIZE, MADV_PAGEOUT);
> }
> }
>
> Perf result on snapdragon 888 with 8 cores by using zRAM
> as the swap block device.
>
> ~ # perf record taskset -c 4 ./a.out
> [ perf record: Woken up 10 times to write data ]
> [ perf record: Captured and wrote 2.297 MB perf.data (60084 samples) ]
> ~ # perf report
> # To display the perf.data header info, please use --header/--header-only options.
> # To display the perf.data header info, please use --header/--header-only options.
> #
> #
> # Total Lost Samples: 0
> #
> # Samples: 60K of event 'cycles'
> # Event count (approx.): 35706225414
> #
> # Overhead Command Shared Object Symbol
> # ........ ....... ................. .............................................................................
> #
> 21.07% a.out [kernel.kallsyms] [k] _raw_spin_unlock_irq
> 8.23% a.out [kernel.kallsyms] [k] _raw_spin_unlock_irqrestore
> 6.67% a.out [kernel.kallsyms] [k] filemap_map_pages
> 6.16% a.out [kernel.kallsyms] [k] __zram_bvec_write
> 5.36% a.out [kernel.kallsyms] [k] ptep_clear_flush
> 3.71% a.out [kernel.kallsyms] [k] _raw_spin_lock
> 3.49% a.out [kernel.kallsyms] [k] memset64
> 1.63% a.out [kernel.kallsyms] [k] clear_page
> 1.42% a.out [kernel.kallsyms] [k] _raw_spin_unlock
> 1.26% a.out [kernel.kallsyms] [k] mod_zone_state.llvm.8525150236079521930
> 1.23% a.out [kernel.kallsyms] [k] xas_load
> 1.15% a.out [kernel.kallsyms] [k] zram_slot_lock
>
> ptep_clear_flush() takes 5.36% CPU in the micro-benchmark
> swapping in/out a page mapped by only one process. If the
> page is mapped by multiple processes, typically, like more
> than 100 on a phone, the overhead would be much higher as
> we have to run tlb flush 100 times for one single page.
> Plus, tlb flush overhead will increase with the number
> of CPU cores due to the bad scalability of tlb shootdown
> in HW, so those ARM64 servers should expect much higher
> overhead.
>
> Further perf annonate shows 95% cpu time of ptep_clear_flush
> is actually used by the final dsb() to wait for the completion
> of tlb flush. This provides us a very good chance to leverage
> the existing batched tlb in kernel. The minimum modification
> is that we only send async tlbi in the first stage and we send
> dsb while we have to sync in the second stage.
>
> With the above simplest micro benchmark, collapsed time to
> finish the program decreases around 5%.
>
> Typical collapsed time w/o patch:
> ~ # time taskset -c 4 ./a.out
> 0.21user 14.34system 0:14.69elapsed
> w/ patch:
> ~ # time taskset -c 4 ./a.out
> 0.22user 13.45system 0:13.80elapsed
>
> Also, Yicong Yang added the following observation.
> Tested with benchmark in the commit on Kunpeng920 arm64 server,
> observed an improvement around 12.5% with command
> `time ./swap_bench`.
> w/o w/
> real 0m13.460s 0m11.771s
> user 0m0.248s 0m0.279s
> sys 0m12.039s 0m11.458s
>
> Originally it's noticed a 16.99% overhead of ptep_clear_flush()
> which has been eliminated by this patch:
>
> [root@localhost yang]# perf record -- ./swap_bench && perf report
> [...]
> 16.99% swap_bench [kernel.kallsyms] [k] ptep_clear_flush
>
> It is tested on 4,8,128 CPU platforms and shows to be beneficial on
> large systems but may not have improvement on small systems like on
> a 4 CPU platform. So make ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH depends
> on CONFIG_EXPERT for this stage and only make this enabled on systems
> with more than 8 CPUs. User can modify this threshold according to
> their own platforms by CONFIG_NR_CPUS_FOR_BATCHED_TLB.
>
> Cc: Anshuman Khandual <anshuman.khandual@arm.com>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: Nadav Amit <namit@vmware.com>
> Cc: Mel Gorman <mgorman@suse.de>
> Tested-by: Yicong Yang <yangyicong@hisilicon.com>
> Tested-by: Xin Hao <xhao@linux.alibaba.com>
> Tested-by: Punit Agrawal <punit.agrawal@bytedance.com>
> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
> Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
> ---
> .../features/vm/TLB/arch-support.txt | 2 +-
> arch/arm64/Kconfig | 6 +++
> arch/arm64/include/asm/tlbbatch.h | 12 +++++
> arch/arm64/include/asm/tlbflush.h | 52 ++++++++++++++++++-
> arch/x86/include/asm/tlbflush.h | 3 +-
> mm/rmap.c | 10 ++--
> 6 files changed, 77 insertions(+), 8 deletions(-)
> create mode 100644 arch/arm64/include/asm/tlbbatch.h
>
> diff --git a/Documentation/features/vm/TLB/arch-support.txt b/Documentation/features/vm/TLB/arch-support.txt
> index 039e4e91ada3..2caf815d7c6c 100644
> --- a/Documentation/features/vm/TLB/arch-support.txt
> +++ b/Documentation/features/vm/TLB/arch-support.txt
> @@ -9,7 +9,7 @@
> | alpha: | TODO |
> | arc: | TODO |
> | arm: | TODO |
> - | arm64: | N/A |
> + | arm64: | ok |
> | csky: | TODO |
> | hexagon: | TODO |
> | ia64: | TODO |
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 505c8a1ccbe0..72975e82c7d7 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -93,6 +93,7 @@ config ARM64
> select ARCH_SUPPORTS_INT128 if CC_HAS_INT128
> select ARCH_SUPPORTS_NUMA_BALANCING
> select ARCH_SUPPORTS_PAGE_TABLE_CHECK
> + select ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH if EXPERT
> select ARCH_WANT_COMPAT_IPC_PARSE_VERSION if COMPAT
> select ARCH_WANT_DEFAULT_BPF_JIT
> select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
> @@ -268,6 +269,11 @@ config ARM64_CONT_PMD_SHIFT
> default 5 if ARM64_16K_PAGES
> default 4
>
> +config ARM64_NR_CPUS_FOR_BATCHED_TLB
> + int "Threshold to enable batched TLB flush"
> + default 8
> + depends on ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
> +
> config ARCH_MMAP_RND_BITS_MIN
> default 14 if ARM64_64K_PAGES
> default 16 if ARM64_16K_PAGES
> diff --git a/arch/arm64/include/asm/tlbbatch.h b/arch/arm64/include/asm/tlbbatch.h
> new file mode 100644
> index 000000000000..fedb0b87b8db
> --- /dev/null
> +++ b/arch/arm64/include/asm/tlbbatch.h
> @@ -0,0 +1,12 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _ARCH_ARM64_TLBBATCH_H
> +#define _ARCH_ARM64_TLBBATCH_H
> +
> +struct arch_tlbflush_unmap_batch {
> + /*
> + * For arm64, HW can do tlb shootdown, so we don't
> + * need to record cpumask for sending IPI
> + */
> +};
> +
> +#endif /* _ARCH_ARM64_TLBBATCH_H */
> diff --git a/arch/arm64/include/asm/tlbflush.h b/arch/arm64/include/asm/tlbflush.h
> index 412a3b9a3c25..20d12de0d231 100644
> --- a/arch/arm64/include/asm/tlbflush.h
> +++ b/arch/arm64/include/asm/tlbflush.h
> @@ -254,17 +254,23 @@ static inline void flush_tlb_mm(struct mm_struct *mm)
> dsb(ish);
> }
>
> -static inline void flush_tlb_page_nosync(struct vm_area_struct *vma,
> +static inline void __flush_tlb_page_nosync(struct mm_struct *mm,
> unsigned long uaddr)
> {
> unsigned long addr;
>
> dsb(ishst);
> - addr = __TLBI_VADDR(uaddr, ASID(vma->vm_mm));
> + addr = __TLBI_VADDR(uaddr, ASID(mm));
> __tlbi(vale1is, addr);
> __tlbi_user(vale1is, addr);
> }
>
> +static inline void flush_tlb_page_nosync(struct vm_area_struct *vma,
> + unsigned long uaddr)
> +{
> + return __flush_tlb_page_nosync(vma->vm_mm, uaddr);
> +}
> +
> static inline void flush_tlb_page(struct vm_area_struct *vma,
> unsigned long uaddr)
> {
> @@ -272,6 +278,48 @@ static inline void flush_tlb_page(struct vm_area_struct *vma,
> dsb(ish);
> }
>
> +#ifdef CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
> +
> +static inline bool arch_tlbbatch_should_defer(struct mm_struct *mm)
> +{
> + /*
> + * TLB batched flush is proved to be beneficial for systems with large
> + * number of CPUs, especially system with more than 8 CPUs. TLB shutdown
> + * is cheap on small systems which may not need this feature. So use
> + * a threshold for enabling this to avoid potential side effects on
> + * these platforms.
> + */
> + if (num_online_cpus() < CONFIG_ARM64_NR_CPUS_FOR_BATCHED_TLB)
> + return false;
> +
> + /*
> + * TLB flush deferral is not required on systems, which are affected with
> + * ARM64_WORKAROUND_REPEAT_TLBI, as __tlbi()/__tlbi_user() implementation
> + * will have two consecutive TLBI instructions with a dsb(ish) in between
> + * defeating the purpose (i.e save overall 'dsb ish' cost).
> + */
> +#ifdef CONFIG_ARM64_WORKAROUND_REPEAT_TLBI
> + if (unlikely(cpus_have_const_cap(ARM64_WORKAROUND_REPEAT_TLBI)))
> + return false;
> +#endif
> +
> + return true;
> +}
> +
> +static inline void arch_tlbbatch_add_mm(struct arch_tlbflush_unmap_batch *batch,
> + struct mm_struct *mm,
> + unsigned long uaddr)
> +{
> + __flush_tlb_page_nosync(mm, uaddr);
> +}
> +
> +static inline void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch)
> +{
> + dsb(ish);
> +}
> +
> +#endif /* CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH */
> +
> /*
> * This is meant to avoid soft lock-ups on large TLB flushing ranges and not
> * necessarily a performance improvement.
> diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
> index 8a497d902c16..5bd78ae55cd4 100644
> --- a/arch/x86/include/asm/tlbflush.h
> +++ b/arch/x86/include/asm/tlbflush.h
> @@ -264,7 +264,8 @@ static inline u64 inc_mm_tlb_gen(struct mm_struct *mm)
> }
>
> static inline void arch_tlbbatch_add_mm(struct arch_tlbflush_unmap_batch *batch,
> - struct mm_struct *mm)
> + struct mm_struct *mm,
> + unsigned long uaddr)
> {
> inc_mm_tlb_gen(mm);
> cpumask_or(&batch->cpumask, &batch->cpumask, mm_cpumask(mm));
> diff --git a/mm/rmap.c b/mm/rmap.c
> index a9ab10bc0144..a1b408ff44e5 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -640,12 +640,13 @@ void try_to_unmap_flush_dirty(void)
> #define TLB_FLUSH_BATCH_PENDING_LARGE \
> (TLB_FLUSH_BATCH_PENDING_MASK / 2)
>
> -static void set_tlb_ubc_flush_pending(struct mm_struct *mm, bool writable)
> +static void set_tlb_ubc_flush_pending(struct mm_struct *mm, bool writable,
> + unsigned long uaddr)
> {
> struct tlbflush_unmap_batch *tlb_ubc = &current->tlb_ubc;
> int batch, nbatch;
>
> - arch_tlbbatch_add_mm(&tlb_ubc->arch, mm);
> + arch_tlbbatch_add_mm(&tlb_ubc->arch, mm, uaddr);
> tlb_ubc->flush_required = true;
>
> /*
> @@ -723,7 +724,8 @@ void flush_tlb_batched_pending(struct mm_struct *mm)
> }
> }
> #else
> -static void set_tlb_ubc_flush_pending(struct mm_struct *mm, bool writable)
> +static void set_tlb_ubc_flush_pending(struct mm_struct *mm, bool writable,
> + unsigned long uaddr)
> {
> }
>
> @@ -1596,7 +1598,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> */
> pteval = ptep_get_and_clear(mm, address, pvmw.pte);
>
> - set_tlb_ubc_flush_pending(mm, pte_dirty(pteval));
> + set_tlb_ubc_flush_pending(mm, pte_dirty(pteval), address);
> } else {
> pteval = ptep_clear_flush(vma, address, pvmw.pte);
> }

Nice work, thanks.

Reviewed-by: Xin Hao <xhao@linux.alibaba.com>