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.
> 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 | 20 +++++++++++++++++++-
> arch/powerpc/perf/callchain.c | 5 ++++-
> include/asm-generic/pgtable.h | 9 +++++++++
> mm/gup.c | 4 ++++
> 16 files changed, 108 insertions(+), 8 deletions(-)
>