Re: [PATCH 4/4] arm64: support batched/deferred tlb shootdown during page reclamation

[Yicong Yang]

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.
>