[PATCH v8 2/2] arm64: support batched/deferred tlb shootdown during page reclamation

[Punit Agrawal]

Hi Yicong,

Yicong Yang <yangyicong at huawei.com> writes:

> From: Barry Song <v-songbaohua at 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 at 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 make this disabled on systems
> with less than 8 CPUs. User can modify this threshold according to
> their own platforms by CONFIG_NR_CPUS_FOR_BATCHED_TLB.

The commit log and the patch disagree on the name of the config option
(CONFIG_NR_CPUS_FOR_BATCHED_TLB vs CONFIG_ARM64_NR_CPUS_FOR_BATCHED_TLB).

But more importantly, I was wondering why this posting doesn't address
Catalin's feedback [a] about using a runtime tunable. Maybe I missed the
follow-up discussion.

Thanks,
Punit

[a] https://lore.kernel.org/linux-mm/Y7xMhPTAwcUT4O6b@arm.com/

> Also this patch improve the performance of page migration. Using pmbench
> and tries to migrate the pages of pmbench between node 0 and node 1 for
> 20 times, this patch decrease the time used more than 50% and saved the
> time used by ptep_clear_flush().
>
> This patch extends arch_tlbbatch_add_mm() to take an address of the
> target page to support the feature on arm64. Also rename it to
> arch_tlbbatch_add_pending() to better match its function since we
> don't need to handle the mm on arm64 and add_mm is not proper.
> add_pending will make sense to both as on x86 we're pending the
> TLB flush operations while on arm64 we're pending the synchronize
> operations.
>
> Cc: Anshuman Khandual <anshuman.khandual at arm.com>
> Cc: Jonathan Corbet <corbet at lwn.net>
> Cc: Nadav Amit <namit at vmware.com>
> Cc: Mel Gorman <mgorman at suse.de>
> Tested-by: Yicong Yang <yangyicong at hisilicon.com>
> Tested-by: Xin Hao <xhao at linux.alibaba.com>
> Tested-by: Punit Agrawal <punit.agrawal at bytedance.com>
> Signed-off-by: Barry Song <v-songbaohua at oppo.com>
> Signed-off-by: Yicong Yang <yangyicong at hisilicon.com>
> Reviewed-by: Kefeng Wang <wangkefeng.wang at huawei.com>
> Reviewed-by: Xin Hao <xhao at linux.alibaba.com>
> Reviewed-by: Anshuman Khandual <anshuman.khandual at arm.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               |  5 +-
>  include/linux/mm_types_task.h                 |  4 +-
>  mm/rmap.c                                     | 12 +++--
>  7 files changed, 81 insertions(+), 12 deletions(-)
>  create mode 100644 arch/arm64/include/asm/tlbbatch.h


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