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

Barry Song
2022-07-14T04:59:10Z

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.

/*
> * 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