Re: [PATCH v2 0/4] mm: arm64: bring up BATCHED_UNMAP_TLB_FLUSH

Yicong Yang
2022-07-18T13:28:36Z

On 2022/7/14 12:51, Barry Song wrote:
> On Thu, Jul 14, 2022 at 3:29 PM Xin Hao <xhao@linux.alibaba.com> wrote:
>>
>> Hi barry.
>>
>> I do some test on Kunpeng arm64 machine use Unixbench.
>>
>> The test result as below.
>>
>> One core, we can see the performance improvement above +30%.
>
> I am really pleased to see the 30%+ improvement on unixbench on single core.
>
>> ./Run -c 1 -i 1 shell1
>> w/o
>> System Benchmarks Partial Index BASELINE RESULT INDEX
>> Shell Scripts (1 concurrent) 42.4 5481.0 1292.7
>> ========
>> System Benchmarks Index Score (Partial Only) 1292.7
>>
>> w/
>> System Benchmarks Partial Index BASELINE RESULT INDEX
>> Shell Scripts (1 concurrent) 42.4 6974.6 1645.0
>> ========
>> System Benchmarks Index Score (Partial Only) 1645.0
>>
>>
>> But with whole cores, there have little performance degradation above -5%
>
> That is sad as we might get more concurrency between mprotect(), madvise(),
> mremap(), zap_pte_range() and the deferred tlbi.
>
>>
>> ./Run -c 96 -i 1 shell1
>> w/o
>> Shell Scripts (1 concurrent) 80765.5 lpm (60.0 s, 1
>> samples)
>> System Benchmarks Partial Index BASELINE RESULT INDEX
>> Shell Scripts (1 concurrent) 42.4 80765.5 19048.5
>> ========
>> System Benchmarks Index Score (Partial Only) 19048.5
>>
>> w
>> Shell Scripts (1 concurrent) 76333.6 lpm (60.0 s, 1
>> samples)
>> System Benchmarks Partial Index BASELINE RESULT INDEX
>> Shell Scripts (1 concurrent) 42.4 76333.6 18003.2
>> ========
>> System Benchmarks Index Score (Partial Only) 18003.2
>>
>> ----------------------------------------------------------------------------------------------
>>
>>
>> After discuss with you, and do some changes in the patch.
>>
>> ndex a52381a680db..1ecba81f1277 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -727,7 +727,11 @@ void flush_tlb_batched_pending(struct mm_struct *mm)
>> int flushed = batch >> TLB_FLUSH_BATCH_FLUSHED_SHIFT;
>>
>> if (pending != flushed) {
>> +#ifdef CONFIG_ARCH_HAS_MM_CPUMASK
>> flush_tlb_mm(mm);
>> +#else
>> + dsb(ish);
>> +#endif
>>
>
> i was guessing the problem might be flush_tlb_batched_pending()
> so i asked you to change this to verify my guess.
>

flush_tlb_batched_pending() looks like the critical path for this issue then the code
above can mitigate this.

I cannot reproduce this on a 2P 128C Kunpeng920 server. The kernel is based on the
v5.19-rc6 and unixbench of version 5.1.3. The result of `./Run -c 128 -i 1 shell1` is:
iter-1 iter-2 iter-3
w/o 17708.1 17637.1 17630.1
w 17766.0 17752.3 17861.7

And flush_tlb_batched_pending()isn't the hot spot with the patch:
7.00% sh [kernel.kallsyms] [k] ptep_clear_flush
4.17% sh [kernel.kallsyms] [k] ptep_set_access_flags
2.43% multi.sh [kernel.kallsyms] [k] ptep_clear_flush
1.98% sh [kernel.kallsyms] [k] _raw_spin_unlock_irqrestore
1.69% sh [kernel.kallsyms] [k] next_uptodate_page
1.66% sort [kernel.kallsyms] [k] ptep_clear_flush
1.56% multi.sh [kernel.kallsyms] [k] ptep_set_access_flags
1.27% sh [kernel.kallsyms] [k] page_counter_cancel
1.11% sh [kernel.kallsyms] [k] page_remove_rmap
1.06% sh [kernel.kallsyms] [k] perf_event_alloc

Hi Xin Hao,

I'm not sure the test setup as well as the config is same with yours. (96C vs 128C
should not be the reason I think). Did you check that the 5% is a fluctuation or
not? It'll be helpful if more information provided for reproducing this issue.

Thanks.

> /*
>> * If the new TLB flushing is pending during flushing, leave
>> * mm->tlb_flush_batched as is, to avoid losing flushing.
>>
>> there have a performance improvement with whole cores, above +30%
>
> But I don't think it is a proper patch. There is no guarantee the cpu calling
> flush_tlb_batched_pending is exactly the cpu sending the deferred
> tlbi. so the solution is unsafe. But since this temporary code can bring the
> 30%+ performance improvement back for high concurrency, we have huge
> potential to finally make it.
>
> Unfortunately I don't have an arm64 server to debug on this. I only have
> 8 cores which are unlikely to reproduce regression which happens in
> high concurrency with 96 parallel tasks.
>
> So I'd ask if @yicong or someone else working on kunpeng or other
> arm64 servers is able to actually debug and figure out a proper
> patch for this, then add the patch as 5/5 into this series?
>
>>
>> ./Run -c 96 -i 1 shell1
>> 96 CPUs in system; running 96 parallel copies of tests
>>
>> Shell Scripts (1 concurrent) 109229.0 lpm (60.0 s, 1 samples)
>> System Benchmarks Partial Index BASELINE RESULT INDEX
>> Shell Scripts (1 concurrent) 42.4 109229.0 25761.6
>> ========
>> System Benchmarks Index Score (Partial Only) 25761.6
>>
>>
>> Tested-by: Xin Hao<xhao@linux.alibaba.com>
>
> Thanks for your testing!
>
>>
>> Looking forward to your next version patch.
>>
>> On 7/11/22 11:46 AM, Barry Song wrote:
>>> Though ARM64 has the hardware to do tlb shootdown, the hardware
>>> broadcasting is not free.
>>> A simplest micro benchmark shows even on snapdragon 888 with only
>>> 8 cores, the overhead for ptep_clear_flush is huge even for paging
>>> out one page mapped by only one process:
>>> 5.36% a.out [kernel.kallsyms] [k] ptep_clear_flush
>>>
>>> While pages are mapped by multiple processes or HW has more CPUs,
>>> the cost should become even higher due to the bad scalability of
>>> tlb shootdown.
>>>
>>> The same benchmark can result in 16.99% CPU consumption on ARM64
>>> server with around 100 cores according to Yicong's test on patch
>>> 4/4.
>>>
>>> This patchset leverages the existing BATCHED_UNMAP_TLB_FLUSH by
>>> 1. only send tlbi instructions in the first stage -
>>> arch_tlbbatch_add_mm()
>>> 2. wait for the completion of tlbi by dsb while doing tlbbatch
>>> sync in arch_tlbbatch_flush()
>>> My testing on snapdragon shows the overhead of ptep_clear_flush
>>> is removed by the patchset. The micro benchmark becomes 5% faster
>>> even for one page mapped by single process on snapdragon 888.
>>>
>>>
>>> -v2:
>>> 1. Collected Yicong's test result on kunpeng920 ARM64 server;
>>> 2. Removed the redundant vma parameter in arch_tlbbatch_add_mm()
>>> according to the comments of Peter Zijlstra and Dave Hansen
>>> 3. Added ARCH_HAS_MM_CPUMASK rather than checking if mm_cpumask
>>> is empty according to the comments of Nadav Amit
>>>
>>> Thanks, Yicong, Peter, Dave and Nadav for your testing or reviewing
>>> , and comments.
>>>
>>> -v1:
>>> https://lore.kernel.org/lkml/20220707125242.425242-1-21cnbao@gmail.com/
>>>
>>> Barry Song (4):
>>> Revert "Documentation/features: mark BATCHED_UNMAP_TLB_FLUSH doesn't
>>> apply to ARM64"
>>> mm: rmap: Allow platforms without mm_cpumask to defer TLB flush
>>> mm: rmap: Extend tlbbatch APIs to fit new platforms
>>> arm64: support batched/deferred tlb shootdown during page reclamation
>>>
>>> Documentation/features/arch-support.txt | 1 -
>>> .../features/vm/TLB/arch-support.txt | 2 +-
>>> arch/arm/Kconfig | 1 +
>>> arch/arm64/Kconfig | 1 +
>>> arch/arm64/include/asm/tlbbatch.h | 12 ++++++++++
>>> arch/arm64/include/asm/tlbflush.h | 23 +++++++++++++++++--
>>> arch/loongarch/Kconfig | 1 +
>>> arch/mips/Kconfig | 1 +
>>> arch/openrisc/Kconfig | 1 +
>>> arch/powerpc/Kconfig | 1 +
>>> arch/riscv/Kconfig | 1 +
>>> arch/s390/Kconfig | 1 +
>>> arch/um/Kconfig | 1 +
>>> arch/x86/Kconfig | 1 +
>>> arch/x86/include/asm/tlbflush.h | 3 ++-
>>> mm/Kconfig | 3 +++
>>> mm/rmap.c | 14 +++++++----
>>> 17 files changed, 59 insertions(+), 9 deletions(-)
>>> create mode 100644 arch/arm64/include/asm/tlbbatch.h
>>>
>> --
>> Best Regards!
>> Xin Hao
>>
>
> Thanks
> Barry
> .
>