Re: [PATCH v7 00/24] Speculative page faults

[Laurent Dufour]

On 08/02/2018 21:53, Andrew Morton wrote:
> On Tue,  6 Feb 2018 17:49:46 +0100 Laurent Dufour <ldufour@xxxxxxxxxxxxxxxxxx> wrote:
> 
>> This is a port on kernel 4.15 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.
>>
>> 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
>> populate or if the underlying PTE are in the way to be collapsed. So we
>> cannot safely allocate a PMD if pmd_none() is true.
>>
>> This series builds on top of v4.15-mmotm-2018-01-31-16-51 and is
>> functional on x86 and PowerPC.
> 
> One question which people will want to answer is "is this thing
> working".  ie, how frequently does the code fall back to the regular
> heavyweight fault path.
> 
> I see that trace events have been added for this, but the overall
> changelog doesn't describe them.  I think this material is important
> enough to justify including it here.

Got it, I'll detail the new perf and trace events here.

> Also, a few words to help people figure out how to gather these stats
> would be nice.  And maybe helper scripts if appropriate?

I'll provide some command line examples detailing how to capture those events.
 
> I'm wondering if this info should even be presented via
> /proc/self/something, dunno.

My understanding is that this is part of the kernel ABI, so I was not comfortable 
to touch it but if needed I could probably put some numbers there.
 
> And it would be interesting to present the fallback frequency in the
> benchmark results.

Yes these numbers are missing.

Here are numbers I captured during a kernbench run on a 80 CPUs Power node:

          87549520      faults                                                      
                 0      spf                                                         

Which is expected as the kernbench's processes are not multithreaded.

When running ebizzy on the same node:

            711589      faults                                                      
            692649      spf                                                         
             10579      pagefault:spf_pte_lock                                      
              7815      pagefault:spf_vma_changed                                   
                 0      pagefault:spf_vma_noanon                                    
               417      pagefault:spf_vma_notsup                                    
                 0      pagefault:spf_vma_access                                    
                 0      pagefault:spf_pmd_changed                                   

Here about 98% of the page faults where managed in a speculative way.

> 
>> ------------------
>> Benchmarks results
>>
>> There is no functional change compared to the v6 so benchmark results are
>> the same.
>> Please see https://lkml.org/lkml/2018/1/12/515 for details.
> 
> Please include this vitally important info in the [0/n], don't make
> people chase links.

Sorry, will do next time.

> 
> And I'd really like to see some quantitative testing results for real
> workloads, not just a bunch of microbenchmarks.  Help us understand how
> useful this patchset is to our users.

We did non official runs using a "popular in memory multithreaded database product" on 
176 cores SMT8 Power system which showed a 30% improvements in the number of transaction
processed per second.
Here are the perf data captured during 2 of these runs :
		vanilla		spf
faults		89.418		101.364
spf                n/a		 97.989

With the SPF kernel, most of the page fault were processed in a speculative way.

Laurent.