Hi Barry,
On 2022/7/7 20:52, Barry Song wrote:
> From: Barry Song <v-songbaohua@xxxxxxxx>
>
> 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
>
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
Feel free to add:
Tested-by: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
> Cc: Jonathan Corbet <corbet@xxxxxxx>
> Cc: Nadav Amit <namit@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
> ---
> Documentation/features/vm/TLB/arch-support.txt | 2 +-
> arch/arm64/Kconfig | 1 +
> arch/arm64/include/asm/tlbbatch.h | 12 ++++++++++++
> arch/arm64/include/asm/tlbflush.h | 13 +++++++++++++
> 4 files changed, 27 insertions(+), 1 deletion(-)
> 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 1c009312b9c1..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: | TODO |
> + | arm64: | ok |
> | csky: | TODO |
> | hexagon: | TODO |
> | ia64: | TODO |
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 1652a9800ebe..e94913a0b040 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
> select ARCH_WANT_COMPAT_IPC_PARSE_VERSION if COMPAT
> select ARCH_WANT_DEFAULT_BPF_JIT
> select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
> 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..b3ed163267ca 100644
> --- a/arch/arm64/include/asm/tlbflush.h
> +++ b/arch/arm64/include/asm/tlbflush.h
> @@ -272,6 +272,19 @@ static inline void flush_tlb_page(struct vm_area_struct *vma,
> dsb(ish);
> }
>
> +static inline void arch_tlbbatch_add_mm(struct arch_tlbflush_unmap_batch *batch,
> + struct mm_struct *mm,
> + struct vm_area_struct *vma,
> + unsigned long uaddr)
> +{
> + flush_tlb_page_nosync(vma, uaddr);
> +}
> +
> +static inline void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch)
> +{
> + dsb(ish);
> +}
> +
> /*
> * This is meant to avoid soft lock-ups on large TLB flushing ranges and not
> * necessarily a performance improvement.
>
