Re: [PATCH v2 00/11] Introduces new count-based method for monitoring lockless pagetable wakls

[John Hubbard <jhubbard@nvidia.com>]

On 9/20/19 1:12 PM, Leonardo Bras wrote:
> 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().

Just noticed that this really should also include linux-mm, maybe
it's best to repost the patchset with them included?

In particular, there is likely to be some feedback about adding more
calls, in addition to local_irq_disable/enable, around the gup_fast() path,
separately from my questions about the synchronization cases in ppc.

thanks,
-- 
John Hubbard
NVIDIA

> 
> 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(-)
>>