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SubjectRE: [PATCH v11 00/26] Speculative page faults

Some regression and improvements is found by LKP-tools(linux kernel performance) on V9 patch series
tested on Intel 4s Skylake platform.

The regression result is sorted by the metric will-it-scale.per_thread_ops.
Branch: Laurent-Dufour/Speculative-page-faults/20180316-151833 (V9 patch series)
Commit id:
base commit: d55f34411b1b126429a823d06c3124c16283231f
head commit: 0355322b3577eeab7669066df42c550a56801110
Benchmark suite: will-it-scale
Download link:
test box: lkp-skl-4sp1(nr_cpu=192,memory=768G)
THP: enable / disable
nr_task: 100%

1. Regressions:
a) THP enabled:
testcase base change head metric
page_fault3/ enable THP 10092 -17.5% 8323 will-it-scale.per_thread_ops
page_fault2/ enable THP 8300 -17.2% 6869 will-it-scale.per_thread_ops
brk1/ enable THP 957.67 -7.6% 885 will-it-scale.per_thread_ops
page_fault3/ enable THP 172821 -5.3% 163692 will-it-scale.per_process_ops
signal1/ enable THP 9125 -3.2% 8834 will-it-scale.per_process_ops

b) THP disabled:
testcase base change head metric
page_fault3/ disable THP 10107 -19.1% 8180 will-it-scale.per_thread_ops
page_fault2/ disable THP 8432 -17.8% 6931 will-it-scale.per_thread_ops
context_switch1/ disable THP 215389 -6.8% 200776 will-it-scale.per_thread_ops
brk1/ disable THP 939.67 -6.6% 877.33 will-it-scale.per_thread_ops
page_fault3/ disable THP 173145 -4.7% 165064 will-it-scale.per_process_ops
signal1/ disable THP 9162 -3.9% 8802 will-it-scale.per_process_ops

2. Improvements:
a) THP enabled:
testcase base change head metric
malloc1/ enable THP 66.33 +469.8% 383.67 will-it-scale.per_thread_ops
writeseek3/ enable THP 2531 +4.5% 2646 will-it-scale.per_thread_ops
signal1/ enable THP 989.33 +2.8% 1016 will-it-scale.per_thread_ops

b) THP disabled:
testcase base change head metric
malloc1/ disable THP 90.33 +417.3% 467.33 will-it-scale.per_thread_ops
read2/ disable THP 58934 +39.2% 82060 will-it-scale.per_thread_ops
page_fault1/ disable THP 8607 +36.4% 11736 will-it-scale.per_thread_ops
read1/ disable THP 314063 +12.7% 353934 will-it-scale.per_thread_ops
writeseek3/ disable THP 2452 +12.5% 2759 will-it-scale.per_thread_ops
signal1/ disable THP 971.33 +5.5% 1024 will-it-scale.per_thread_ops

Notes: for above values in column "change", the higher value means that the related testcase result
on head commit is better than that on base commit for this benchmark.

Best regards
Haiyan Song

From: [] on behalf of Laurent Dufour []
Sent: Thursday, May 17, 2018 7:06 PM
To:;;;;;;; Matthew Wilcox;;;;;; Thomas Gleixner; Ingo Molnar;; Will Deacon; Sergey Senozhatsky;; Andrea Arcangeli; Alexei Starovoitov; Wang, Kemi; Daniel Jordan; David Rientjes; Jerome Glisse; Ganesh Mahendran; Minchan Kim; Punit Agrawal; vinayak menon; Yang Shi
Cc:;;;;;; Tim Chen;;
Subject: [PATCH v11 00/26] 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

In pseudo code, this could be seen as:
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
page = alloc_page()
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

if (speculative_page_fault(&vma))
goto done
vma = find_vma();
if retry
goto again;
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

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
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

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

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 416 389 -7% 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% 491 461 -6% 479 366 -23% 1341 1198 -11% 333 314 -6%
com.tencent.mobileqq 394 384 -3% 907 906 0% 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% 4257 4152 -2%

Benchmarks results

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

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)
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)
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)
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)
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.

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

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 (
- Accounted for all review feedbacks from Punit Agrawal, Ganesh Mahendran
and Minchan Kim, hopefully.
- Remove unneeded check on CONFIG_SPECULATIVE_PAGE_FAULT in
- Loop in pte_spinlock() and pte_map_lock() when pte try lock fails
of aborting the speculative page fault handling. Dropping the now
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.


Laurent Dufour (20):
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: add speculative page fault

Peter Zijlstra (4):
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


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