[PATCH 00/16] Speculative page faults
Laurent Dufour
ldufour at linux.vnet.ibm.com
Wed Aug 9 00:35:33 AEST 2017
This is a port on kernel 4.13 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, so the VMA list is now managed using
SRCU allowing lockless walking. The only impact would be the deferred file
derefencing in the case of a file mapping, since the file pointer is
released once the SRCU cleaning is done. This patch relies on the change
done recently by Paul McKenney in SRCU which now runs a callback per CPU
instead of per SRCU structure [1].
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 locks 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.
Compared to the Peter's initial work, this series introduces a spin_trylock
when dealing with speculative page fault. This is required to avoid dead
lock when handling a page fault while a TLB invalidate is requested by an
other CPU holding the PTE. Another change due to a lock dependency issue
with mapping->i_mmap_rwsem.
This series builds on top of v4.13-rc4 and is functional on x86 and
PowerPC.
Tests have been made using a large commercial in-memory database on a
PowerPC system with 752 CPUs. The results are very encouraging since the
loading of the 2TB database was faster by 14% with the speculative page
fault.
Using ebizzy test [3], which spreads a lot of threads, the result are good
when running on both a large or a small system. When using kernbench, the
result are quite similar which expected as not so much multithreaded
processes are involved. But there is no performance degradation neither
which is good.
------------------
Benchmarks results
Note these test have been made on top of 4.13-rc3 with the following patch
from Paul McKenney applied:
"srcu: Provide ordering for CPU not involved in grace period" [5]
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, mean
deviation and max.
- 16 CPUs x86 VM
Records/s 4.13-rc3 4.13-rc3-spf
Average 11455.92 45803.64
Mean deviation 509.34 848.19
Max 13997 49824
- 80 CPUs Power 8 node:
Records/s 4.13-rc3 4.13-rc3-spf
Average 33848.76 63427.62
Mean deviation 684.48 1618.84
Max 36235 70401
Kernbench:
----------
This test is building a 4.12 kernel using platform default config. The
build has been run 5 times each time.
- 16 CPUs x86 VM
Average Half load -j 7 Run (std deviation)
4.13.0-rc3 4.13.0-rc3-spf
Elapsed Time 166.668 (0.462299) 167.55 (0.432724)
User Time 1083.11 (2.89018) 1083.76 (2.17015)
System Time 202.982 (0.984058) 210.364 (0.890382)
Percent CPU 771.2 (0.83666) 771.8 (1.09545)
Context Switches 46789 (519.558) 67602.4 (365.929)
Sleeps 83870.8 (836.392) 84269.4 (457.962)
Average Optimal load -j 16 Run (std deviation)
4.13.0-rc3 4.13.0-rc3-spf
Elapsed Time 85.002 (0.298111) 85.406 (0.506784)
User Time 1033.25 (52.6037) 1034.63 (51.8167)
System Time 185.46 (18.4826) 191.75 (19.6379)
Percent CPU 1062.6 (307.181) 1063.9 (307.948)
Context Switches 67423.3 (21762.7) 91316.1 (25004.4)
Sleeps 89393.6 (5860.2) 89489.9 (5563.54)
The elapsed time is in the same order, a bit larger in the case of the spf
release, but that seems to be in the error margin.
- 80 CPUs Power 8 node:
Average Half load -j 40 Run (std deviation)
4.13.0-rc3 4.13.0-rc3-spf
Elapsed Time 116.422 (0.604707) 116.898 (1.00981)
User Time 4410.13 (23.4272) 4393.49 (22.6739)
System Time 130.128 (0.567468) 132.16 (0.840238)
Percent CPU 3899.2 (13.9535) 3871 (17.6777)
Context Switches 72699.8 (585.077) 73281.4 (516.003)
Sleeps 160396 (1248.34) 161801 (522.71)
Average Optimal load -j 80 Run (std deviation)
4.13.0-rc3 4.13.0-rc3-spf
Elapsed Time 111.216 (0.826698) 110.442 (0.846505)
User Time 5911.85 (1583.04) 5932.14 (1622.02)
System Time 164.799 (36.5712) 168.29 (38.0891)
Percent CPU 5371.9 (1552.74) 5410.2 (1623.17)
Context Switches 117770 (47512.1) 130131 (59927.8)
Sleeps 161619 (2210.47) 163442 (2349.71)
Here the elapsed time is a bit shorter using the spf release, but again we
stay in the error margin. It has to be noted that this system is not
correctly balanced on the NUMA point of view as all the available memory is
attached to one core.
------------------------
Changes since RFC V5 [6]
- Port to 4.13 kernel
- Merging patch fixing lock dependency into the original patch
- Replace the 2 parameters of vma_has_changed() with the vmf pointer
- In patch 7, don't call __do_fault() in the speculative path as it may
want to unlock the mmap_sem.
- In patch 11-12, don't check for vma boundaries when
page_add_new_anon_rmap() is called during the spf path and protect against
anon_vma pointer's update.
- In patch 13-16, add performance events to report number of successful
and failed speculative events.
[1] http://linux-kernel.2935.n7.nabble.com/RFC-PATCH-0-6-Another-go-at-speculative-page-faults-tt965642.html#none
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=da915ad5cf25b5f5d358dd3670c3378d8ae8c03e
[3] http://ebizzy.sourceforge.net/
[4] http://ck.kolivas.org/apps/kernbench/kernbench-0.50/
[5] https://lkml.org/lkml/2017/7/24/829
[6] https://lwn.net/Articles/725607/
Laurent Dufour (10):
mm: Introduce pte_spinlock for FAULT_FLAG_SPECULATIVE
mm: Protect VMA modifications using VMA sequence count
mm: Try spin lock in speculative path
powerpc/mm: Add speculative page fault
mm: Introduce __page_add_new_anon_rmap()
mm: Protect SPF handler against anon_vma changes
perf: Add a speculative page fault sw events
x86/mm: Add support for SPF events
powerpc/mm: Add support for SPF events
perf tools: Add support for SPF events
Peter Zijlstra (6):
mm: Dont assume page-table invariance during faults
mm: Prepare for FAULT_FLAG_SPECULATIVE
mm: VMA sequence count
mm: RCU free VMAs
mm: Provide speculative fault infrastructure
x86/mm: Add speculative pagefault handling
arch/powerpc/mm/fault.c | 30 +++-
arch/x86/mm/fault.c | 18 ++
fs/proc/task_mmu.c | 2 +
include/linux/mm.h | 4 +
include/linux/mm_types.h | 3 +
include/linux/rmap.h | 12 +-
include/uapi/linux/perf_event.h | 2 +
kernel/fork.c | 1 +
mm/init-mm.c | 1 +
mm/internal.h | 19 +++
mm/khugepaged.c | 3 +
mm/madvise.c | 4 +
mm/memory.c | 302 ++++++++++++++++++++++++++++------
mm/mempolicy.c | 10 +-
mm/mlock.c | 9 +-
mm/mmap.c | 123 ++++++++++----
mm/mprotect.c | 2 +
mm/mremap.c | 7 +
mm/rmap.c | 5 +-
tools/include/uapi/linux/perf_event.h | 2 +
tools/perf/util/evsel.c | 2 +
tools/perf/util/parse-events.c | 8 +
tools/perf/util/parse-events.l | 2 +
tools/perf/util/python.c | 2 +
24 files changed, 484 insertions(+), 89 deletions(-)
--
2.7.4
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