Subject: [PATCH v3 00/11] Introduces new count-based method for monitoring lockless pagetable walks

From: Leonardo Bras <[email-masked]>
Date: Tue, 24 Sep 2019 18:24:17 -0300

If a process (qemu) with a lot of CPUs (128) try to munmap() a large
chunk of memory (496GB) mapped with THP, it takes an average of 275
seconds, which can cause a lot of problems to the load (in qemu case,
the guest will lock for this time).

Trying to find the source of this bug, I found out most of this time is
spent on serialize_against_pte_lookup(). This function will take a lot
of time in smp_call_function_many() if there is more than a couple CPUs
running the user process. Since it has to happen to all THP mapped, it
will take a very long time for large amounts of memory.

By the docs, serialize_against_pte_lookup() is needed in order to avoid
pmd_t to pte_t casting inside find_current_mm_pte(), or any lockless
pagetable walk, to happen concurrently with THP splitting/collapsing.

It does so by calling a do_nothing() on each CPU in mm->cpu_bitmap[],
after interrupts are re-enabled.
Since, interrupts are (usually) disabled during lockless pagetable
walk, and serialize_against_pte_lookup will only return after
interrupts are enabled, it is protected.

So, by what I could understand, if there is no lockless pagetable walk
running, there is no need to call serialize_against_pte_lookup().

So, to avoid the cost of running serialize_against_pte_lookup(), I
propose a counter that keeps track of how many find_current_mm_pte()
are currently running, and if there is none, just skip
smp_call_function_many().

The related functions are:
start_lockless_pgtbl_walk(mm)
	Insert before starting any lockless pgtable walk
end_lockless_pgtbl_walk(mm)
	Insert after the end of any lockless pgtable walk
	(Mostly after the ptep is last used)
running_lockless_pgtbl_walk(mm)
	Returns the number of lockless pgtable walks running

On my workload (qemu), I could see munmap's time reduction from 275
seconds to 418ms.

Changes since v2:
 Rebased to v5.3
 Adds support on __get_user_pages_fast
 Adds usage decription to *_lockless_pgtbl_walk()
 Better style to dummy functions
 Link: http://patchwork.ozlabs.org/project/linuxppc-dev/list/?series=131839
 
Changes since v1:
 Isolated atomic operations in functions *_lockless_pgtbl_walk()
 Fixed behavior of decrementing before last ptep was used
 Link: http://patchwork.ozlabs.org/patch/1163093/
 
Leonardo Bras (11):
  powerpc/mm: Adds counting method to monitor lockless pgtable walks
  asm-generic/pgtable: Adds dummy functions to monitor lockless pgtable
    walks
  mm/gup: Applies counting method to monitor gup_pgd_range
  powerpc/mce_power: Applies counting method to monitor lockless pgtbl
    walks
  powerpc/perf: Applies counting method to monitor lockless pgtbl walks
  powerpc/mm/book3s64/hash: Applies counting method to monitor lockless
    pgtbl walks
  powerpc/kvm/e500: Applies counting method to monitor lockless pgtbl
    walks
  powerpc/kvm/book3s_hv: Applies counting method to monitor lockless
    pgtbl walks
  powerpc/kvm/book3s_64: Applies counting method to monitor lockless
    pgtbl walks
  powerpc/book3s_64: Enables counting method to monitor lockless pgtbl
    walk
  powerpc/mm/book3s64/pgtable: Uses counting method to skip serializing

 arch/powerpc/include/asm/book3s/64/mmu.h     |  3 ++
 arch/powerpc/include/asm/book3s/64/pgtable.h |  5 +++
 arch/powerpc/kernel/mce_power.c              | 13 +++++--
 arch/powerpc/kvm/book3s_64_mmu_hv.c          |  2 +
 arch/powerpc/kvm/book3s_64_mmu_radix.c       | 20 +++++++++-
 arch/powerpc/kvm/book3s_64_vio_hv.c          |  4 ++
 arch/powerpc/kvm/book3s_hv_nested.c          |  8 ++++
 arch/powerpc/kvm/book3s_hv_rm_mmu.c          |  9 ++++-
 arch/powerpc/kvm/e500_mmu_host.c             |  4 ++
 arch/powerpc/mm/book3s64/hash_tlb.c          |  2 +
 arch/powerpc/mm/book3s64/hash_utils.c        |  7 ++++
 arch/powerpc/mm/book3s64/mmu_context.c       |  1 +
 arch/powerpc/mm/book3s64/pgtable.c           | 40 +++++++++++++++++++-
 arch/powerpc/perf/callchain.c                |  5 ++-
 include/asm-generic/pgtable.h                | 15 ++++++++
 mm/gup.c                                     |  8 ++++
 16 files changed, 138 insertions(+), 8 deletions(-)

-- 
2.20.1