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

Barry Song
2022-07-20T11:18:45Z

On Tue, Jul 19, 2022 at 1:28 AM Yicong Yang <yangyicong@huawei.com> 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.
>
> Thanks.

I guess that is because "./Run -c 1 -i 1 shell1" isn't an application
stressed on
memory. Hi Xin, in what kinds of configurations can we reproduce your test
result?

As I suppose tlbbatch will mainly affect the performance of user scenarios
which require memory page-out/page-in like reclaiming file/anon pages.
"./Run -c 1 -i 1 shell1" on a system with sufficient free memory won't be
affected by tlbbatch at all, I believe.

Thanks
Barry