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SubjectRe: [PATCH v12 00/31] Speculative page faults
Hi Laurent,

Regression test for v12 patch serials have been run on Intel 2s skylake platform,
some regressions were found by LKP-tools (linux kernel performance). Only tested the
cases that have been run and found regressions on v11 patch serials.

Get the patch serials from https://github.com/ldu4/linux/tree/spf-v12.
Kernel commit:
base: a297558ad4479e0c9c5c14f3f69fe43113f72d1c (v5.1-rc4-mmotm-2019-04-09-17-51)
head: 02c5a1f984a8061d075cfd74986ac8aa01d81064 (spf-v12)

Benchmark: will-it-scale
Download link: https://github.com/antonblanchard/will-it-scale/tree/master
Metrics: will-it-scale.per_thread_ops=threads/nr_cpu
test box: lkp-skl-2sp8(nr_cpu=72,memory=192G)
THP: enable / disable
nr_task: 100%

The following is benchmark results, tested 4 times for every case.

a). Enable THP
base %stddev change head %stddev
will-it-scale.page_fault3.per_thread_ops 63216 ±3% -16.9% 52537 ±4%
will-it-scale.page_fault2.per_thread_ops 36862 -9.8% 33256

b). Disable THP
base %stddev change head %stddev
will-it-scale.page_fault3.per_thread_ops 65111 -18.6% 53023 ±2%
will-it-scale.page_fault2.per_thread_ops 38164 -12.0% 33565

Best regards,
Haiyan Song

On Tue, Apr 16, 2019 at 03:44:51PM +0200, Laurent Dufour wrote:
> This is a port on kernel 5.1 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 of page fault is named speculative
> page fault. If the speculative page fault fails because a concurrency has
> been detected or because underlying PMD or PTE tables are not yet
> allocating, it is failing its processing and a regular 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 protecting the MM RB
> tree with RCU and by using a reference counter on each VMA. When fetching a
> VMA under the RCU protection, the VMA's reference counter is incremented to
> ensure that the VMA will not freed in our back during the SPF
> processing. Once that processing is done the VMA's reference counter is
> decremented. To ensure that a VMA is still present when walking the RB tree
> locklessly, the VMA's reference counter is incremented when that VMA is
> linked in the RB tree. When the VMA is unlinked from the RB tree, its
> reference counter will be decremented at the end of the RCU grace period,
> ensuring it will be available during this time. This means that the VMA
> freeing could be delayed and could delay the file closing for file
> mapping. Since the SPF handler is not able to manage file mapping, file is
> closed synchronously and not during the RCU cleaning. This is safe since
> the page fault handler is aborting if a file pointer is associated to the
> VMA.
>
> Using RCU fixes the overhead seen by Haiyan Song using the will-it-scale
> benchmark [2].
>
> 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 = find_vma_rcu()
> 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
> ...
> put_vma(vma)
> }
>
> 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
>
> It is possible to deactivate the speculative page fault handler by echoing
> 0 in /proc/sys/vm/speculative_page_fault.
>
> This series builds on top of v5.1-rc4-mmotm-2019-04-09-17-51 and is
> functional on x86, PowerPC. I cross built it on arm64 but I was not able to
> test it.
>
> This series is also available on github [4].
>
> ---------------------
> Real Workload results
>
> Test using a "popular in memory multithreaded database product" on 128cores
> SMT8 Power system are in progress and I will come back with performance
> mesurement as soon as possible. With the previous series we seen up to 30%
> improvements in the number of transaction processed per second, and we hope
> this will be the case with this series too.
>
> ------------------
> Benchmarks results
>
> Base kernel is v5.1-rc4-mmotm-2019-04-09-17-51
> SPF is BASE + this series
>
> Kernbench:
> ----------
> Here are the results on a 48 CPUs X86 system using kernbench on a 5.0
> kernel (kernel is build 5 times):
>
> Average Half load -j 24
> Run (std deviation)
> BASE SPF
> Elapsed Time 56.52 (1.39185) 56.256 (1.15106) 0.47%
> User Time 980.018 (2.94734) 984.958 (1.98518) -0.50%
> System Time 130.744 (1.19148) 133.616 (0.873573) -2.20%
> Percent CPU 1965.6 (49.682) 1988.4 (40.035) -1.16%
> Context Switches 29926.6 (272.789) 30472.4 (109.569) -1.82%
> Sleeps 124793 (415.87) 125003 (591.008) -0.17%
>
> Average Optimal load -j 48
> Run (std deviation)
> BASE SPF
> Elapsed Time 46.354 (0.917949) 45.968 (1.42786) 0.83%
> User Time 1193.42 (224.96) 1196.78 (223.28) -0.28%
> System Time 143.306 (13.2726) 146.177 (13.2659) -2.00%
> Percent CPU 2668.6 (743.157) 2699.9 (753.767) -1.17%
> Context Switches 62268.3 (34097.1) 62721.7 (33999.1) -0.73%
> Sleeps 132556 (8222.99) 132607 (8077.6) -0.04%
>
> During a run on the SPF, perf events were captured:
> Performance counter stats for '../kernbench -M':
> 525,873,132 faults
> 242 spf
> 0 pagefault:spf_vma_changed
> 0 pagefault:spf_vma_noanon
> 441 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 1024 CPUs Power8 VM:
>
> 5.1.0-rc4-mm1+ 5.1.0-rc4-mm1-spf-rcu+
> Average Half load -j 512 Run (std deviation):
> Elapsed Time 52.52 (0.906697) 52.778 (0.510069) -0.49%
> User Time 3855.43 (76.378) 3890.44 (73.0466) -0.91%
> System Time 1977.24 (182.316) 1974.56 (166.097) 0.14%
> Percent CPU 11111.6 (540.461) 11115.2 (458.907) -0.03%
> Context Switches 83245.6 (3061.44) 83651.8 (1202.31) -0.49%
> Sleeps 613459 (23091.8) 628378 (27485.2) -2.43%
>
> Average Optimal load -j 1024 Run (std deviation):
> Elapsed Time 52.964 (0.572346) 53.132 (0.825694) -0.32%
> User Time 4058.22 (222.034) 4070.2 (201.646) -0.30%
> System Time 2672.81 (759.207) 2712.13 (797.292) -1.47%
> Percent CPU 12756.7 (1786.35) 12806.5 (1858.89) -0.39%
> Context Switches 88818.5 (6772) 87890.6 (5567.72) 1.04%
> Sleeps 618658 (20842.2) 636297 (25044) -2.85%
>
> During a run on the SPF, perf events were captured:
> Performance counter stats for '../kernbench -M':
> 149 375 832 faults
> 1 spf
> 0 pagefault:spf_vma_changed
> 0 pagefault:spf_vma_noanon
> 561 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
> 24 CPUs x86 5492.69 9383.07 70.83%
> 1024 CPUS P8 VM 8476.74 17144.38 102%
>
> Here are the performance counter read during a run on a 48 CPUs x86 node:
> Performance counter stats for './ebizzy -mTt 48':
> 11,846,569 faults
> 10,886,706 spf
> 957,702 pagefault:spf_vma_changed
> 0 pagefault:spf_vma_noanon
> 815 pagefault:spf_vma_notsup
> 0 pagefault:spf_vma_access
> 0 pagefault:spf_pmd_changed
>
> And the ones captured during a run on a 1024 CPUs Power VM:
> Performance counter stats for './ebizzy -mTt 1024':
> 1 359 789 faults
> 1 284 910 spf
> 72 085 pagefault:spf_vma_changed
> 0 pagefault:spf_vma_noanon
> 2 669 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 v11 [3]
> - Check vm_ops.fault instead of vm_ops since now all the VMA as a vm_ops.
> - Abort speculative page fault when doing swap readhead because VMA's
> boundaries are not protected at this time. Doing this the first swap in
> is doing a readhead, the next fault should be handled in a speculative
> way as the page is present in the swap read page.
> - Handle a race between copy_pte_range() and the wp_page_copy called by
> the speculative page fault handler.
> - Ported to Kernel v5.0
> - Moved VM_FAULT_PTNOTSAME define in mm_types.h
> - Use RCU to protect the MM RB tree instead of a rwlock.
> - Add a toggle interface: /proc/sys/vm/speculative_page_fault
>
> [1] https://lore.kernel.org/linux-mm/20141020215633.717315139@infradead.org/
> [2] https://lore.kernel.org/linux-mm/9FE19350E8A7EE45B64D8D63D368C8966B847F54@SHSMSX101.ccr.corp.intel.com/
> [3] https://lore.kernel.org/linux-mm/1526555193-7242-1-git-send-email-ldufour@linux.vnet.ibm.com/
> [4] https://github.com/ldu4/linux/tree/spf-v12
>
> Laurent Dufour (25):
> 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 against PTE changes done by dup_mmap()
> mm: protect the RB tree with a sequence lock
> mm: introduce vma reference counter
> mm: Introduce find_vma_rcu()
> mm: don't do swap readahead during speculative page fault
> 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
> mm: Add a speculative page fault switch in sysctl
>
> 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 | 14 +
> fs/exec.c | 1 +
> 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 | 138 +++++-
> include/linux/mm_types.h | 16 +-
> 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 | 35 +-
> kernel/sysctl.c | 9 +
> mm/Kconfig | 22 +
> mm/huge_memory.c | 6 +-
> mm/hugetlb.c | 2 +
> mm/init-mm.c | 3 +
> mm/internal.h | 45 ++
> mm/khugepaged.c | 5 +
> mm/madvise.c | 6 +-
> mm/memory.c | 631 ++++++++++++++++++++++----
> mm/mempolicy.c | 51 ++-
> mm/migrate.c | 6 +-
> mm/mlock.c | 13 +-
> mm/mmap.c | 249 ++++++++--
> mm/mprotect.c | 4 +-
> mm/mremap.c | 13 +
> mm/nommu.c | 1 +
> mm/rmap.c | 5 +-
> mm/swap.c | 6 +-
> mm/swap_state.c | 10 +-
> 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 +
> 45 files changed, 1277 insertions(+), 196 deletions(-)
> create mode 100644 include/trace/events/pagefault.h
>
> --
> 2.21.0
>

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