[PATCH v3 4/4] arm64: support batched/deferred tlb shootdown during page reclamation
Anshuman Khandual
anshuman.khandual at arm.com
Thu Sep 15 16:07:44 AEST 2022
On 9/9/22 11:05, Barry Song wrote:
> On Fri, Sep 9, 2022 at 5:24 PM Anshuman Khandual
> <anshuman.khandual at arm.com> wrote:
>>
>>
>>
>> On 8/22/22 13:51, Yicong Yang wrote:
>>> 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
>>>
>>> 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>
>>> Signed-off-by: Barry Song <v-songbaohua at oppo.com>
>>> Signed-off-by: Yicong Yang <yangyicong at hisilicon.com>
>>> ---
>>> .../features/vm/TLB/arch-support.txt | 2 +-
>>> arch/arm64/Kconfig | 1 +
>>> arch/arm64/include/asm/tlbbatch.h | 12 ++++++++
>>> arch/arm64/include/asm/tlbflush.h | 28 +++++++++++++++++--
>>> 4 files changed, 40 insertions(+), 3 deletions(-)
>>> create mode 100644 arch/arm64/include/asm/tlbbatch.h
>>>
>>> diff --git a/Documentation/features/vm/TLB/arch-support.txt b/Documentation/features/vm/TLB/arch-support.txt
>>> index 1c009312b9c1..2caf815d7c6c 100644
>>> --- a/Documentation/features/vm/TLB/arch-support.txt
>>> +++ b/Documentation/features/vm/TLB/arch-support.txt
>>> @@ -9,7 +9,7 @@
>>> | alpha: | TODO |
>>> | arc: | TODO |
>>> | arm: | TODO |
>>> - | arm64: | TODO |
>>> + | arm64: | ok |
>>> | csky: | TODO |
>>> | hexagon: | TODO |
>>> | ia64: | TODO |
>>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>>> index 571cc234d0b3..09d45cd6d665 100644
>>> --- a/arch/arm64/Kconfig
>>> +++ b/arch/arm64/Kconfig
>>> @@ -93,6 +93,7 @@ config ARM64
>>> select ARCH_SUPPORTS_INT128 if CC_HAS_INT128
>>> select ARCH_SUPPORTS_NUMA_BALANCING
>>> select ARCH_SUPPORTS_PAGE_TABLE_CHECK
>>> + select ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
>>> select ARCH_WANT_COMPAT_IPC_PARSE_VERSION if COMPAT
>>> select ARCH_WANT_DEFAULT_BPF_JIT
>>> select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
>>> diff --git a/arch/arm64/include/asm/tlbbatch.h b/arch/arm64/include/asm/tlbbatch.h
>>> new file mode 100644
>>> index 000000000000..fedb0b87b8db
>>> --- /dev/null
>>> +++ b/arch/arm64/include/asm/tlbbatch.h
>>> @@ -0,0 +1,12 @@
>>> +/* SPDX-License-Identifier: GPL-2.0 */
>>> +#ifndef _ARCH_ARM64_TLBBATCH_H
>>> +#define _ARCH_ARM64_TLBBATCH_H
>>> +
>>> +struct arch_tlbflush_unmap_batch {
>>> + /*
>>> + * For arm64, HW can do tlb shootdown, so we don't
>>> + * need to record cpumask for sending IPI
>>> + */
>>> +};
>>> +
>>> +#endif /* _ARCH_ARM64_TLBBATCH_H */
>>> diff --git a/arch/arm64/include/asm/tlbflush.h b/arch/arm64/include/asm/tlbflush.h
>>> index 412a3b9a3c25..23cbc987321a 100644
>>> --- a/arch/arm64/include/asm/tlbflush.h
>>> +++ b/arch/arm64/include/asm/tlbflush.h
>>> @@ -254,17 +254,24 @@ static inline void flush_tlb_mm(struct mm_struct *mm)
>>> dsb(ish);
>>> }
>>>
>>> -static inline void flush_tlb_page_nosync(struct vm_area_struct *vma,
>>> +
>>> +static inline void __flush_tlb_page_nosync(struct mm_struct *mm,
>>> unsigned long uaddr)
>>> {
>>> unsigned long addr;
>>>
>>> dsb(ishst);
>>> - addr = __TLBI_VADDR(uaddr, ASID(vma->vm_mm));
>>> + addr = __TLBI_VADDR(uaddr, ASID(mm));
>>> __tlbi(vale1is, addr);
>>> __tlbi_user(vale1is, addr);
>>> }
>>>
>>> +static inline void flush_tlb_page_nosync(struct vm_area_struct *vma,
>>> + unsigned long uaddr)
>>> +{
>>> + return __flush_tlb_page_nosync(vma->vm_mm, uaddr);
>>> +}
>>> +
>>> static inline void flush_tlb_page(struct vm_area_struct *vma,
>>> unsigned long uaddr)
>>> {
>>> @@ -272,6 +279,23 @@ static inline void flush_tlb_page(struct vm_area_struct *vma,
>>> dsb(ish);
>>> }
>>>
>>> +static inline bool arch_tlbbatch_should_defer(struct mm_struct *mm)
>>> +{
>>> + return true;
>>> +}
>>
>> Always defer and batch up TLB flush, unconditionally ?
>
> My understanding is we actually don't need tlbbatch for a machine with one
> or two cores as the tlb flush is not expensive. even for a system with four
> cortex-a55 cores, i didn't see obvious cost. it was less than 1%.
> when we have 8 cores, we see the obvious cost of tlb flush. for a server with
> 100 crores, the cost is incredibly huge.
Although dsb(ish) is deferred via arch_tlbbatch_flush(), there is still
one dsb(isht) instruction left in __flush_tlb_page_nosync(). Is not that
expensive as well, while queuing up individual TLB flushes ?
The very idea behind TLB deferral is the opportunity it (might) provide
to accumulate address ranges and cpu masks so that individual TLB flush
can be replaced with a more cost effective range based TLB flush. Hence
I guess unless address range or cpumask based cost effective TLB flush
is available, deferral does not improve the unmap performance as much.
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