On Tue, Mar 13, 2018 at 06:59:30PM +0100, Laurent Dufour wrote:

This is a port on kernel 4.16 of the work done by Peter Zijlstra to
handle page fault without holding the mm semaphore [1].

The idea is to try to handle user space page faults without holding the
mmap_sem. This should allow better concurrency for massively threaded
process since the page fault handler will not wait for other threads memory
layout change to be done, assuming that this change is done in another part
of the process's memory space. This type page fault is named speculative
page fault. If the speculative page fault fails because of a concurrency is
detected or because underlying PMD or PTE tables are not yet allocating, it
is failing its processing and a classic page fault is then tried.

The speculative page fault (SPF) has to look for the VMA matching the fault
address without holding the mmap_sem, this is done by introducing a rwlock
which protects the access to the mm_rb tree. Previously this was done using
SRCU but it was introducing a lot of scheduling to process the VMA's
freeing
operation which was hitting the performance by 20% as reported by Kemi Wang
[2].Using a rwlock to protect access to the mm_rb tree is limiting the
locking contention to these operations which are expected to be in a O(log
n)
order. In addition to ensure that the VMA is not freed in our back a
reference count is added and 2 services (get_vma() and put_vma()) are
introduced to handle the reference count. When a VMA is fetch from the RB
tree using get_vma() is must be later freeed using put_vma(). Furthermore,
to allow the VMA to be used again by the classic page fault handler a
service is introduced can_reuse_spf_vma(). This service is expected to be
called with the mmap_sem hold. It checked that the VMA is still matching
the specified address and is releasing its reference count as the mmap_sem
is hold it is ensure that it will not be freed in our back. In general, the
VMA's reference count could be decremented when holding the mmap_sem but it
should not be increased as holding the mmap_sem is ensuring that the VMA is
stable. I can't see anymore the overhead I got while will-it-scale
benchmark anymore.

The VMA's attributes checked during the speculative page fault processing
have to be protected against parallel changes. This is done by using a per
VMA sequence lock. This sequence lock allows the speculative page fault
handler to fast check for parallel changes in progress and to abort the
speculative page fault in that case.

Once the VMA is found, the speculative page fault handler would check for
the VMA's attributes to verify that the page fault has to be handled
correctly or not. Thus the VMA is protected through a sequence lock which
allows fast detection of concurrent VMA changes. If such a change is
detected, the speculative page fault is aborted and a *classic* page fault
is tried.  VMA sequence lockings are added when VMA attributes which are
checked during the page fault are modified.

When the PTE is fetched, the VMA is checked to see if it has been changed,
so once the page table is locked, the VMA is valid, so any other changes
leading to touching this PTE will need to lock the page table, so no
parallel change is possible at this time.

What would have been nice is some pseudo highlevel code before all the
above detailed description. Something like:
  speculative_fault(addr) {
    mm_lock_for_vma_snapshot()
    vma_snapshot = snapshot_vma_infos(addr)
    mm_unlock_for_vma_snapshot()
    ...
    if (!vma_can_speculatively_fault(vma_snapshot, addr))
        return;
    ...
    /* Do fault ie alloc memory, read from file ... */
    page = ...;

    preempt_disable();
    if (vma_snapshot_still_valid(vma_snapshot, addr) &&
        vma_pte_map_lock(vma_snapshot, addr)) {
        if (pte_same(ptep, orig_pte)) {
            /* Setup new pte */
            page = NULL;
        }
    }
    preempt_enable();
    if (page)
        put(page)
  }

I just find pseudo code easier for grasping the highlevel view of the
expected code flow.

The locking of the PTE is done with interrupts disabled, this allows to
check for the PMD to ensure that there is not an ongoing collapsing
operation. Since khugepaged is firstly set the PMD to pmd_none and then is
waiting for the other CPU to have catch the IPI interrupt, if the pmd is
valid at the time the PTE is locked, we have the guarantee that the
collapsing opertion will have to wait on the PTE lock to move foward. This
allows the SPF handler to map the PTE safely. If the PMD value is different
than the one recorded at the beginning of the SPF operation, the classic
page fault handler will be called to handle the operation while holding the
mmap_sem. As the PTE lock is done with the interrupts disabled, the lock is
done using spin_trylock() to avoid dead lock when handling a page fault
while a TLB invalidate is requested by an other CPU holding the PTE.

Support for THP is not done because when checking for the PMD, we can be
confused by an in progress collapsing operation done by khugepaged. The
issue is that pmd_none() could be true either if the PMD is not already
populated or if the underlying PTE are in the way to be collapsed. So we
cannot safely allocate a PMD if pmd_none() is true.

Might be a good topic fo LSF/MM, should we set the pmd to something
else then 0 when collapsing pmd (apply to pud too) ? This would allow
support THP.

[...]

Ebizzy:
-------
The test is counting the number of records per second it can manage, the
higher is the best. I run it like this 'ebizzy -mTRp'. To get consistent
result I repeated the test 100 times and measure the average result. The
number is the record processes per second, the higher is the best.

  		BASE		SPF		delta	
16 CPUs x86 VM	14902.6		95905.16	543.55%
80 CPUs P8 node	37240.24	78185.67	109.95%

I find those results interesting as it seems that SPF do not scale well
on big configuration. Note that it still have a sizeable improvement so
it is still a very interesting feature i believe.

Still understanding what is happening here might a good idea. From the
numbers below it seems there is 2 causes to the scaling issue. First
pte lock contention (kind of expected i guess). Second changes to vma
while faulting.

Have you thought about this ? Do i read those numbers in the wrong way ?

Here are the performance counter read during a run on a 16 CPUs x86 VM:
 Performance counter stats for './ebizzy -mRTp':
            888157      faults
            884773      spf
                92      pagefault:spf_pte_lock
              2379      pagefault:spf_vma_changed
                 0      pagefault:spf_vma_noanon
                80      pagefault:spf_vma_notsup
                 0      pagefault:spf_vma_access
                 0      pagefault:spf_pmd_changed

And the ones captured during a run on a 80 CPUs Power node:
 Performance counter stats for './ebizzy -mRTp':
            762134      faults
            728663      spf
             19101      pagefault:spf_pte_lock
             13969      pagefault:spf_vma_changed
                 0      pagefault:spf_vma_noanon
               272      pagefault:spf_vma_notsup
                 0      pagefault:spf_vma_access
                 0      pagefault:spf_pmd_changed

There is one aspect that i would like to see cover. Maybe i am not
understanding something fundamental, but it seems to me that SPF can
trigger OOM or at very least over stress page allocation.

Assume you have a lot of concurrent SPF to anonymous vma and they all
allocate new pages, then you might overallocate for a single address
by a factor correlated with the number of CPUs in your system. Now,
multiply this for several distinc address and you might be allocating
a lot of memory transiently ie just for a short period time. While
the fact that you quickly free when you fail should prevent the OOM
reaper. But still this might severly stress the memory allocation
path.

Am i missing something in how this all work ? Or is the above some-
thing that might be of concern ? Should there be some boundary on the
maximum number of concurrent SPF (and thus boundary on maximum page
temporary page allocation) ?

Cheers,
Jérôme