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

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

[Leonardo Bras]

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John Hubbard <jhubbard@nvidia.com> writes:

> Hi Leonardo,
>
> Thanks for adding linux-mm to CC for this next round of reviews. For the benefit
> of any new reviewers, I'd like to add that there are some issues that were discovered
> while reviewing the v2 patchset, that are not (yet) addressed in this v3 series.

> Since those issues are not listed in the cover letter above, I'll list them here

Thanks for bringing that.
The cover letter is a great place to put this info, I will keep that in
mind for future patchsets.

>
> 1. The locking model requires a combination of disabling interrupts and
> atomic counting and memory barriers, but
>
> 	a) some memory barriers are missing
> 	(start/end_lockless_pgtbl_walk), and

It seems that it works fine today because of the amount of intructions
executed between the irq_disable / start_lockless_pgtbl_walk and where
the THP collapse/split can happen. (It's very unlikely that it reorders
that much).

But I don't think it would be so bad to put a memory barrier after
irq_disable just in case.

> 	b) some cases (patch #8) fail to disable interrupts

I have done some looking into that, and it seems that some uses of
{start,end}_lockless_pgtbl_walk are unneeded, because they operate in
(nested) guest pgd and I was told it's safe against THP split/collapse.

In other uses, there is no interrupt disable because the function is
called in real mode, with MSR_EE=0, and there we have instructions
disabled, so there is no need to disable them again.

>
> ...so the synchronization appears to be inadequate. (And if it *is* adequate, then
> definitely we need the next item, to explain it.)


>
> 2. Documentation of the synchronization/locking model needs to exist, once we
> figure out the exact details of (1).

I will add the missing doc in the code, so it may be easier to
understand in the future.

>
> 3. Related to (1), I've asked to change things so that interrupt controls and 
> atomic inc/dec are in the same start/end calls--assuming, of course, that the
> caller can tolerate that. 

I am not sure if it would be ok to use irq_{save,restore} in real mode,
I will do some more reading of the docs before addressing this. 
>
> 4. Please see the v2 series for any other details I've missed.
>
> thanks,
> -- 
> John Hubbard
> NVIDIA
>

Thank you for helping, John!

Best regards,
Leonardo Bras

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

[Leonardo Bras]

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On Fri, 2019-09-27 at 11:46 -0300, Leonardo Bras wrote:
> I am not sure if it would be ok to use irq_{save,restore} in real mode,
> I will do some more reading of the docs before addressing this. 

It looks like it's unsafe to merge irq_{save,restore} in
{start,end}_lockless_pgtbl_walk(), due to a possible access of code
that is not accessible in real mode.

I am sending a v4 for the changes so far.
I will look forward for your feedback.

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

[Leonardo Bras]

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(-)

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