[PATCH v5 00/11] Introduces new count-based method for tracking lockless pagetable walks

[Leonardo Bras]

Hello Peter, thanks for the feedback!

On Thu, 2019-10-03 at 09:29 +0200, Peter Zijlstra wrote:
> On Wed, Oct 02, 2019 at 10:33:14PM -0300, 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.
> 
> This is something entirely specific to Power, you shouldn't be touching
> generic code at all.

Up to v4, I was declaring dummy functions so it would not mess up with
other archs: http://patchwork.ozlabs.org/patch/1168779/

But I was recommended to create a generic function that could guide the
way to archs: http://patchwork.ozlabs.org/patch/1168775/

The idea was to concentrate all routines of beginning/ending lockless
pagetable walks on these functions, and call them instead of
irq_disable/irq_enable.
Then it was easy to place the refcount-based tracking in these
functions. It should only be enabled in case the config chooses to do
so. 

> 
> Also, I'm not sure I understand things properly.
> 
> So serialize_against_pte_lookup() wants to wait for all currently
> out-standing __find_linux_pte() instances (which are very similar to
> gup_fast).
> 
> It seems to want to do this before flushing the THP TLB for some reason;
> why? Should not THP observe the normal page table freeing rules which
> includes a RCU-like grace period like this already.
> 
> Why is THP special here? This doesn't seem adequately explained.

"It's necessary to monitor lockless pagetable walks, in order to avoid
doing THP splitting/collapsing during them."

If a there is a THP split/collapse during the lockless pagetable walk,
the returned ptep can be a pointing to an invalid pte. 

To avoid that, the pmd is updated, then serialize_against_pte_lookup is
ran. Serialize runs a do_nothing in all cpu in cpu_mask. 

So, after all cpus finish running do_nothing(), there is a guarantee
that if there is any 'lockless pagetable walk' it is running on top of
a updated version of this pmd, and so, collapsing/splitting THP is
safe.

> 
> Also, specifically to munmap(), this seems entirely superfluous,
> munmap() uses the normal page-table freeing code and should be entirely
> fine without additional waiting.

To be honest, I remember it being needed in munmap case, but I really
don't remember the details. I will take a deeper look and come back
with this answer. 

> Furthermore, Power never accurately tracks mm_cpumask(), so using that
> makes the whole thing more expensive than it needs to be. Also, I
> suppose that is buggered vs file backed THP.

That accuracy of mm_cpumask is above my knowledge right now. =)

I agree that it's to expensive to do that. That's why I suggested this
method, that can check if there is any 'lockless pagetable walk'
running before trying to serialize. It reduced the waiting time a lot
for large amounts of memory. (more details on cover letter)

Best regards,

Leonardo Brás
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