Speculative page faults

This is a port on kernel 4.17 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. Once a VMA is fetched from the RB tree using get_vma(), it must be later freed using put_vma(). 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 has been 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 checking 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 caught the IPI interrupt, if the pmd is valid at the time the PTE is locked, we have the guarantee that the collapsing operation will have to wait on the PTE lock to move forward. This allows the SPF handler to map the PTE safely. If the PMD value is different from 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 another CPU holding the PTE.

In pseudo code, this could be seen as:

    speculative_page_fault()
    {
	    vma = get_vma()
	    check vma sequence count
	    check vma's support
	    disable interrupt
		  check pgd,p4d,...,pte
		  save pmd and pte in vmf
		  save vma sequence counter in vmf
	    enable interrupt
	    check vma sequence count
	    handle_pte_fault(vma)
		    ..
		    page = alloc_page()
		    pte_map_lock()
			    disable interrupt
				    abort if sequence counter has changed
				    abort if pmd or pte has changed
				    pte map and lock
			    enable interrupt
		    if abort
		       free page
		       abort
		    ...
    }
    
    arch_fault_handler()
    {
	    if (speculative_page_fault(&vma))
	       goto done
    again:
	    lock(mmap_sem)
	    vma = find_vma();
	    handle_pte_fault(vma);
	    if retry
	       unlock(mmap_sem)
	       goto again;
    done:
	    handle fault error
    }

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.

This series add a new software performance event named 'speculative-faults' or 'spf'. It counts the number of successful page fault event handled speculatively. When recording 'faults,spf' events, the faults one is counting the total number of page fault events while 'spf' is only counting the part of the faults processed speculatively.

There are some trace events introduced by this series. They allow identifying why the page faults were not processed speculatively. This doesn't take in account the faults generated by a monothreaded process which directly processed while holding the mmap_sem. This trace events are grouped in a system named 'pagefault', they are:
- pagefault:spf_vma_changed : if the VMA has been changed in our back
- pagefault:spf_vma_noanon : the vma->anon_vma field was not yet set.
- pagefault:spf_vma_notsup : the VMA's type is not supported
- pagefault:spf_vma_access : the VMA's access right are not respected
- pagefault:spf_pmd_changed : the upper PMD pointer has changed in our back.

To record all the related events, the easier is to run perf with the following arguments :

$ perf stat -e 'faults,spf,pagefault:*' <command>

There is also a dedicated vmstat counter showing the number of successful page fault handled speculatively. I can be seen this way:

$ grep speculative_pgfault /proc/vmstat

This series builds on top of v4.16-mmotm-2018-04-13-17-28 and is functional on x86, PowerPC and arm64.

---------------------
Real Workload results

As mentioned in previous email, 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. This run has been done on the v6 series, but changes introduced in this new version should not impact the performance boost seen.

Here are the perf data captured during 2 of these runs on top of the v8 series:
vanilla spf
faults 89.418 101.364 +13%
spf n/a 97.989

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

Ganesh Mahendran had backported the series on top of a 4.9 kernel and gave it a try on an android device. He reported that the application launch time was improved in average by 6%, and for large applications (~100 threads) by 20%.

Here are the launch time Ganesh mesured on Android 8.0 on top of a Qcom MSM845 (8 cores) with 6GB (the less is better):

Application 4.9 4.9+spf delta
com.tencent.mm 416 389 -7%
com.eg.android.AlipayGphone 1135 986 -13%
com.tencent.mtt 455 454 0%
com.qqgame.hlddz 1497 1409 -6%
com.autonavi.minimap 711 701 -1%
com.tencent.tmgp.sgame 788 748 -5%
com.immomo.momo 501 487 -3%
com.tencent.peng 2145 2112 -2%
com.smile.gifmaker 491 461 -6%
com.baidu.BaiduMap 479 366 -23%
com.taobao.taobao 1341 1198 -11%
com.baidu.searchbox 333 314 -6%
com.tencent.mobileqq 394 384 -3%
com.sina.weibo 907 906 0%
com.youku.phone 816 731 -11%
com.happyelements.AndroidAnimal.qq 763 717 -6%
com.UCMobile 415 411 -1%
com.tencent.tmgp.ak 1464 1431 -2%
com.tencent.qqmusic 336 329 -2%
com.sankuai.meituan 1661 1302 -22%
com.netease.cloudmusic 1193 1200 1%
air.tv.douyu.android 4257 4152 -2%

------------------
Benchmarks results

Base kernel is v4.17.0-rc4-mm1
SPF is BASE + this series

Kernbench:
----------
Here are the results on a 16 CPUs X86 guest using kernbench on a 4.15 kernel (kernel is build 5 times):

Average Half load -j 8

		 Run	(std deviation)
		 BASE			SPF

Elapsed Time 1448.65 (5.72312) 1455.84 (4.84951) 0.50%
User Time 10135.4 (30.3699) 10148.8 (31.1252) 0.13%
System Time 900.47 (2.81131) 923.28 (7.52779) 2.53%
Percent CPU 761.4 (1.14018) 760.2 (0.447214) -0.16%
Context Switches 85380 (3419.52) 84748 (1904.44) -0.74%
Sleeps 105064 (1240.96) 105074 (337.612) 0.01%

Average Optimal load -j 16

		 Run	(std deviation)
		 BASE			SPF

Elapsed Time 920.528 (10.1212) 927.404 (8.91789) 0.75%
User Time 11064.8 (981.142) 11085 (990.897) 0.18%
System Time 979.904 (84.0615) 1001.14 (82.5523) 2.17%
Percent CPU 1089.5 (345.894) 1086.1 (343.545) -0.31%
Context Switches 159488 (78156.4) 158223 (77472.1) -0.79%
Sleeps 110566 (5877.49) 110388 (5617.75) -0.16%

During a run on the SPF, perf events were captured:
Performance counter stats for '../kernbench -M':

         526743764      faults                   
               210      spf                      
                 3      pagefault:spf_vma_changed
                 0      pagefault:spf_vma_noanon 
              2278      pagefault:spf_vma_notsup 
                 0      pagefault:spf_vma_access 
                 0      pagefault:spf_pmd_changed

Very few speculative page faults were recorded as most of the processes involved are monothreaded (sounds that on this architecture some threads were created during the kernel build processing).

Here are the kerbench results on a 80 CPUs Power8 system:

Average Half load -j 40

		 Run	(std deviation)
		 BASE			SPF

Elapsed Time 117.152 (0.774642) 117.166 (0.476057) 0.01%
User Time 4478.52 (24.7688) 4479.76 (9.08555) 0.03%
System Time 131.104 (0.720056) 134.04 (0.708414) 2.24%
Percent CPU 3934 (19.7104) 3937.2 (19.0184) 0.08%
Context Switches 92125.4 (576.787) 92581.6 (198.622) 0.50%
Sleeps 317923 (652.499) 318469 (1255.59) 0.17%

Average Optimal load -j 80

		 Run	(std deviation)
		 BASE			SPF

Elapsed Time 107.73 (0.632416) 107.31 (0.584936) -0.39%
User Time 5869.86 (1466.72) 5871.71 (1467.27) 0.03%
System Time 153.728 (23.8573) 157.153 (24.3704) 2.23%
Percent CPU 5418.6 (1565.17) 5436.7 (1580.91) 0.33%
Context Switches 223861 (138865) 225032 (139632) 0.52%
Sleeps 330529 (13495.1) 332001 (14746.2) 0.45%

During a run on the SPF, perf events were captured:
Performance counter stats for '../kernbench -M':

         116730856      faults
                 0      spf
                 3      pagefault:spf_vma_changed
                 0      pagefault:spf_vma_noanon
               476      pagefault:spf_vma_notsup
                 0      pagefault:spf_vma_access
                 0      pagefault:spf_pmd_changed

Most of the processes involved are monothreaded so SPF is not activated but there is no impact on the performance.

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 -mTt <nrcpus>'. 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 742.57 1490.24 100.69%
80 CPUs P8 node 13105.4 24174.23 84.46%

Here are the performance counter read during a run on a 16 CPUs x86 VM:
Performance counter stats for './ebizzy -mTt 16':

           1706379      faults
           1674599      spf
             30588      pagefault:spf_vma_changed
                 0      pagefault:spf_vma_noanon
               363      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 -mTt 80':

           1874773      faults
           1461153      spf
            413293      pagefault:spf_vma_changed
                 0      pagefault:spf_vma_noanon
               200      pagefault:spf_vma_notsup
                 0      pagefault:spf_vma_access
                 0      pagefault:spf_pmd_changed 

In ebizzy's case most of the page fault were handled in a speculative way, leading the ebizzy performance boost.

------------------
Changes since v10 (https://lkml.org/lkml/2018/4/17/572):
- Accounted for all review feedbacks from Punit Agrawal, Ganesh Mahendran and Minchan Kim, hopefully.
- Remove unneeded check on CONFIG_SPECULATIVE_PAGE_FAULT in __do_page_fault().
- Loop in pte_spinlock() and pte_map_lock() when pte try lock fails instead
of aborting the speculative page fault handling. Dropping the now
useless
trace event pagefault:spf_pte_lock.
- No more try to reuse the fetched VMA during the speculative page fault handling when retrying is needed. This adds a lot of complexity and additional tests done didn't show a significant performance improvement.
- Convert IS_ENABLED(CONFIG_NUMA) back to #ifdef due to build error.

[1] http://linux-kernel.2935.n7.nabble.com/RFC-PATCH-0-6-Another-go-at-speculative-page-faults-tt965642.html#none
[2] https://patchwork.kernel.org/patch/9999687/

Laurent Dufour (20):
mm: introduce CONFIG_SPECULATIVE_PAGE_FAULT
x86/mm: define ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT powerpc/mm: set ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT
mm: introduce pte_spinlock for FAULT_FLAG_SPECULATIVE
mm: make pte_unmap_same compatible with SPF
mm: introduce INIT_VMA()
mm: protect VMA modifications using VMA sequence count
mm: protect mremap() against SPF hanlder
mm: protect SPF handler against anon_vma changes
mm: cache some VMA fields in the vm_fault structure mm/migrate: Pass vm_fault pointer to migrate_misplaced_page()
mm: introduce __lru_cache_add_active_or_unevictable
mm: introduce __vm_normal_page()
mm: introduce __page_add_new_anon_rmap()
mm: protect mm_rb tree with a rwlock
mm: adding speculative page fault failure trace events
perf: add a speculative page fault sw event
perf tools: add support for the SPF perf event
mm: add speculative page fault vmstats
powerpc/mm: add speculative page fault

Mahendran Ganesh (2):
arm64/mm: define ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT arm64/mm: add speculative page fault

Peter Zijlstra (4):
mm: prepare for FAULT_FLAG_SPECULATIVE
mm: VMA sequence count
mm: provide speculative fault infrastructure
x86/mm: add speculative pagefault handling

 arch/arm64/Kconfig                    |   1 +
 arch/arm64/mm/fault.c                 |  12 +
 arch/powerpc/Kconfig                  |   1 +
 arch/powerpc/mm/fault.c               |  16 +
 arch/x86/Kconfig                      |   1 +
 arch/x86/mm/fault.c                   |  27 +-
 fs/exec.c                             |   2 +-
 fs/proc/task_mmu.c                    |   5 +-
 fs/userfaultfd.c                      |  17 +-
 include/linux/hugetlb_inline.h        |   2 +-
 include/linux/migrate.h               |   4 +-
 include/linux/mm.h                    | 136 +++++++-
 include/linux/mm_types.h              |   7 +
 include/linux/pagemap.h               |   4 +-
 include/linux/rmap.h                  |  12 +-
 include/linux/swap.h                  |  10 +-
 include/linux/vm_event_item.h         |   3 +
 include/trace/events/pagefault.h      |  80 +++++
 include/uapi/linux/perf_event.h       |   1 +
 kernel/fork.c                         |   5 +-
 mm/Kconfig                            |  22 ++
 mm/huge_memory.c                      |   6 +-
 mm/hugetlb.c                          |   2 +
 mm/init-mm.c                          |   3 +
 mm/internal.h                         |  20 ++
 mm/khugepaged.c                       |   5 +
 mm/madvise.c                          |   6 +-
 mm/memory.c                           | 612 +++++++++++++++++++++++++++++-----
 mm/mempolicy.c                        |  51 ++-
 mm/migrate.c                          |   6 +-
 mm/mlock.c                            |  13 +-
 mm/mmap.c                             | 229 ++++++++++---
 mm/mprotect.c                         |   4 +-
 mm/mremap.c                           |  13 +
 mm/nommu.c                            |   2 +-
 mm/rmap.c                             |   5 +-
 mm/swap.c                             |   6 +-
 mm/swap_state.c                       |   8 +-
 mm/vmstat.c                           |   5 +-
 tools/include/uapi/linux/perf_event.h |   1 +
 tools/perf/util/evsel.c               |   1 +
 tools/perf/util/parse-events.c        |   4 +
 tools/perf/util/parse-events.l        |   1 +
 tools/perf/util/python.c              |   1 +
 44 files changed, 1161 insertions(+), 211 deletions(-)
 create mode 100644 include/trace/events/pagefault.h

2.7.4