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

Barry Song
2022-07-11T03:46:41Z

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

--
2.25.1