[RFC PATCH 00/11] Avoid synchronous TLB invalidation for intermediate page-table entries on arm64

Will Deacon
2018-08-24T15:53:34Z

Hi all,

I hacked up this RFC on the back of the recent changes to the mmu_gather
stuff in mainline. It's had a bit of testing and it looks pretty good so
far.

The main changes in the series are:

- Avoid emitting a DSB barrier after clearing each page-table entry.
Instead, we can have a single DSB prior to the actual TLB invalidation.

- Batch last-level TLB invalidation until the end of the VMA, and use
last-level-only invalidation instructions

- Batch intermediate TLB invalidation until the end of the gather, and
use all-level invalidation instructions

- Adjust the stride of TLB invalidation based upon the smallest unflushed
granule in the gather

As a really stupid benchmark, unmapping a populated mapping of
0x4_3333_3000 bytes using munmap() takes around 20% of the time it took
before.

The core changes now track the levels of page-table that have been visited
by the mmu_gather since the last flush. It may be possible to use the
page_size field instead if we decide to resurrect that from its current
"debug" status, but I think I'd prefer to track the levels explicitly.

Anyway, I wanted to post this before disappearing for the long weekend
(Monday is a holiday in the UK) in the hope that it helps some of the
ongoing discussions.

Cheers,

Will

--->8

Peter Zijlstra (1):
asm-generic/tlb: Track freeing of page-table directories in struct
mmu_gather

Will Deacon (10):
arm64: tlb: Use last-level invalidation in flush_tlb_kernel_range()
arm64: tlb: Add DSB ISHST prior to TLBI in
__flush_tlb_[kernel_]pgtable()
arm64: pgtable: Implement p[mu]d_valid() and check in set_p[mu]d()
arm64: tlb: Justify non-leaf invalidation in flush_tlb_range()
arm64: tlbflush: Allow stride to be specified for __flush_tlb_range()
arm64: tlb: Remove redundant !CONFIG_HAVE_RCU_TABLE_FREE code
asm-generic/tlb: Guard with #ifdef CONFIG_MMU
asm-generic/tlb: Track which levels of the page tables have been
cleared
arm64: tlb: Adjust stride and type of TLBI according to mmu_gather
arm64: tlb: Avoid synchronous TLBIs when freeing page tables

arch/arm64/Kconfig | 1 +
arch/arm64/include/asm/pgtable.h | 10 ++++-
arch/arm64/include/asm/tlb.h | 34 +++++++----------
arch/arm64/include/asm/tlbflush.h | 28 +++++++-------
include/asm-generic/tlb.h | 79 +++++++++++++++++++++++++++++++++------
mm/memory.c | 4 +-
6 files changed, 105 insertions(+), 51 deletions(-)

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2.1.4