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

xhao
2022-07-23T09:17:18Z

On 7/18/22 9:28 PM, Yicong Yang wrote:
> 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.
Yes, not always the 5% reduce,  there exist a fluctuation.
>
> 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
>> .
>>
--
Best Regards!
Xin Hao