[PATCH v4 00/10] steal tasks to improve CPU utilization

From: Steve Sistare
Date: Thu Dec 06 2018 - 16:39:18 EST

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When a CPU has no more CFS tasks to run, and idle_balance() fails to
find a task, then attempt to steal a task from an overloaded CPU in the
same LLC. Maintain and use a bitmap of overloaded CPUs to efficiently
identify candidates. To minimize search time, steal the first migratable
task that is found when the bitmap is traversed. For fairness, search
for migratable tasks on an overloaded CPU in order of next to run.

This simple stealing yields a higher CPU utilization than idle_balance()
alone, because the search is cheap, so it may be called every time the CPU
is about to go idle. idle_balance() does more work because it searches
widely for the busiest queue, so to limit its CPU consumption, it declines
to search if the system is too busy. Simple stealing does not offload the
globally busiest queue, but it is much better than running nothing at all.

The bitmap of overloaded CPUs is a new type of sparse bitmap, designed to
reduce cache contention vs the usual bitmap when many threads concurrently
set, clear, and visit elements.

Patch 1 defines the sparsemask type and its operations.

Patches 2, 3, and 4 implement the bitmap of overloaded CPUs.

Patches 5 and 6 refactor existing code for a cleaner merge of later
patches.

Patches 7 and 8 implement task stealing using the overloaded CPUs bitmap.

Patch 9 disables stealing on systems with more than 2 NUMA nodes for the
time being because of performance regressions that are not due to stealing
per-se. See the patch description for details.

Patch 10 adds schedstats for comparing the new behavior to the old, and
provided as a convenience for developers only, not for integration.

The patch series is based on kernel 4.20.0-rc1. It compiles, boots, and
runs with/without each of CONFIG_SCHED_SMT, CONFIG_SMP, CONFIG_SCHED_DEBUG,
and CONFIG_PREEMPT. It runs without error with CONFIG_DEBUG_PREEMPT +
CONFIG_SLUB_DEBUG + CONFIG_DEBUG_PAGEALLOC + CONFIG_DEBUG_MUTEXES +
CONFIG_DEBUG_SPINLOCK + CONFIG_DEBUG_ATOMIC_SLEEP. CPU hot plug and CPU
bandwidth control were tested.

Stealing improves utilization with only a modest CPU overhead in scheduler
code. In the following experiment, hackbench is run with varying numbers
of groups (40 tasks per group), and the delta in /proc/schedstat is shown
for each run, averaged per CPU, augmented with these non-standard stats:

%find - percent of time spent in old and new functions that search for
idle CPUs and tasks to steal and set the overloaded CPUs bitmap.

steal - number of times a task is stolen from another CPU.

X6-2: 1 socket * 10 cores * 2 hyperthreads = 20 CPUs
Intel(R) Xeon(R) CPU E5-2630 v4 @ 2.20GHz
hackbench <grps> process 100000
sched_wakeup_granularity_ns=15000000

baseline
grps time %busy slice sched idle wake %find steal
1 8.084 75.02 0.10 105476 46291 59183 0.31 0
2 13.892 85.33 0.10 190225 70958 119264 0.45 0
3 19.668 89.04 0.10 263896 87047 176850 0.49 0
4 25.279 91.28 0.10 322171 94691 227474 0.51 0
8 47.832 94.86 0.09 630636 144141 486322 0.56 0

new
grps time %busy slice sched idle wake %find steal %speedup
1 5.938 96.80 0.24 31255 7190 24061 0.63 7433 36.1
2 11.491 99.23 0.16 74097 4578 69512 0.84 19463 20.9
3 16.987 99.66 0.15 115824 1985 113826 0.77 24707 15.8
4 22.504 99.80 0.14 167188 2385 164786 0.75 29353 12.3
8 44.441 99.86 0.11 389153 1616 387401 0.67 38190 7.6

Elapsed time improves by 8 to 36%, and CPU busy utilization is up
by 5 to 22% hitting 99% for 2 or more groups (80 or more tasks).
The cost is at most 0.4% more find time.

Additional performance results follow. A negative "speedup" is a
regression. Note: for all hackbench runs, sched_wakeup_granularity_ns
is set to 15 msec. Otherwise, preemptions increase at higher loads and
distort the comparison between baseline and new.

------------------ 1 Socket Results ------------------

X6-2: 1 socket * 10 cores * 2 hyperthreads = 20 CPUs
Intel(R) Xeon(R) CPU E5-2630 v4 @ 2.20GHz
Average of 10 runs of: hackbench <groups> process 100000

--- base -- --- new ---
groups time %stdev time %stdev %speedup
1 8.008 0.1 5.905 0.2 35.6
2 13.814 0.2 11.438 0.1 20.7
3 19.488 0.2 16.919 0.1 15.1
4 25.059 0.1 22.409 0.1 11.8
8 47.478 0.1 44.221 0.1 7.3

X6-2: 1 socket * 22 cores * 2 hyperthreads = 44 CPUs
Intel(R) Xeon(R) CPU E5-2699 v4 @ 2.20GHz
Average of 10 runs of: hackbench <groups> process 100000

--- base -- --- new ---
groups time %stdev time %stdev %speedup
1 4.586 0.8 4.596 0.6 -0.3
2 7.693 0.2 5.775 1.3 33.2
3 10.442 0.3 8.288 0.3 25.9
4 13.087 0.2 11.057 0.1 18.3
8 24.145 0.2 22.076 0.3 9.3
16 43.779 0.1 41.741 0.2 4.8

KVM 4-cpu
Intel(R) Xeon(R) CPU E5-2699 v3 @ 2.30GHz
tbench, average of 11 runs.

clients %speedup
1 16.2
2 11.7
4 9.9
8 12.8
16 13.7

KVM 2-cpu
Intel(R) Xeon(R) CPU E5-2699 v3 @ 2.30GHz

Benchmark %speedup
specjbb2015_critical_jops 5.7
mysql_sysb1.0.14_mutex_2 40.6
mysql_sysb1.0.14_oltp_2 3.9

------------------ 2 Socket Results ------------------

X6-2: 2 sockets * 10 cores * 2 hyperthreads = 40 CPUs
Intel(R) Xeon(R) CPU E5-2630 v4 @ 2.20GHz
Average of 10 runs of: hackbench <groups> process 100000

--- base -- --- new ---
groups time %stdev time %stdev %speedup
1 7.945 0.2 7.219 8.7 10.0
2 8.444 0.4 6.689 1.5 26.2
3 12.100 1.1 9.962 2.0 21.4
4 15.001 0.4 13.109 1.1 14.4
8 27.960 0.2 26.127 0.3 7.0

X6-2: 2 sockets * 22 cores * 2 hyperthreads = 88 CPUs
Intel(R) Xeon(R) CPU E5-2699 v4 @ 2.20GHz
Average of 10 runs of: hackbench <groups> process 100000

--- base -- --- new ---
groups time %stdev time %stdev %speedup
1 5.826 5.4 5.840 5.0 -0.3
2 5.041 5.3 6.171 23.4 -18.4
3 6.839 2.1 6.324 3.8 8.1
4 8.177 0.6 7.318 3.6 11.7
8 14.429 0.7 13.966 1.3 3.3
16 26.401 0.3 25.149 1.5 4.9

X6-2: 2 sockets * 22 cores * 2 hyperthreads = 88 CPUs
Intel(R) Xeon(R) CPU E5-2699 v4 @ 2.20GHz
Oracle database OLTP, logging disabled, NVRAM storage

Customers Users %speedup
1200000 40 -1.2
2400000 80 2.7
3600000 120 8.9
4800000 160 4.4
6000000 200 3.0

X6-2: 2 sockets * 14 cores * 2 hyperthreads = 56 CPUs
Intel(R) Xeon(R) CPU E5-2690 v4 @ 2.60GHz
Results from the Oracle "Performance PIT".

Benchmark %speedup

mysql_sysb1.0.14_fileio_56_rndrd 19.6
mysql_sysb1.0.14_fileio_56_seqrd 12.1
mysql_sysb1.0.14_fileio_56_rndwr 0.4
mysql_sysb1.0.14_fileio_56_seqrewr -0.3

pgsql_sysb1.0.14_fileio_56_rndrd 19.5
pgsql_sysb1.0.14_fileio_56_seqrd 8.6
pgsql_sysb1.0.14_fileio_56_rndwr 1.0
pgsql_sysb1.0.14_fileio_56_seqrewr 0.5

opatch_time_ASM_12.2.0.1.0_HP2M 7.5
select-1_users-warm_asmm_ASM_12.2.0.1.0_HP2M 5.1
select-1_users_asmm_ASM_12.2.0.1.0_HP2M 4.4
swingbenchv3_asmm_soebench_ASM_12.2.0.1.0_HP2M 5.8

lm3_memlat_L2 4.8
lm3_memlat_L1 0.0

ub_gcc_56CPUs-56copies_Pipe-based_Context_Switching 60.1
ub_gcc_56CPUs-56copies_Shell_Scripts_1_concurrent 5.2
ub_gcc_56CPUs-56copies_Shell_Scripts_8_concurrent -3.0
ub_gcc_56CPUs-56copies_File_Copy_1024_bufsize_2000_maxblocks 2.4

X5-2: 2 sockets * 18 cores * 2 hyperthreads = 72 CPUs
Intel(R) Xeon(R) CPU E5-2699 v3 @ 2.30GHz

NAS_OMP
bench class ncpu %improved(Mops)
dc B 72 1.3
is C 72 0.9
is D 72 0.7

sysbench mysql, average of 24 runs
--- base --- --- new ---
nthr events %stdev events %stdev %speedup
1 331.0 0.25 331.0 0.24 -0.1
2 661.3 0.22 661.8 0.22 0.0
4 1297.0 0.88 1300.5 0.82 0.2
8 2420.8 0.04 2420.5 0.04 -0.1
16 4826.3 0.07 4825.4 0.05 -0.1
32 8815.3 0.27 8830.2 0.18 0.1
64 12823.0 0.24 12823.6 0.26 0.0

--------------------------------------------------------------

Changes from v1 to v2:
- Remove stray find_time hunk from patch 5
- Fix "warning: label out defined but not used" for !CONFIG_SCHED_SMT
- Set SCHED_STEAL_NODE_LIMIT_DEFAULT to 2
- Steal iff avg_idle exceeds the cost of stealing

Changes from v2 to v3:
- Update series for kernel 4.20. Context changes only.

Changes from v3 to v4:
- Avoid 64-bit division on 32-bit processors in compute_skid()
- Replace IF_SMP with inline functions to set idle_stamp
- Push ZALLOC_MASK body into calling function
- Set rq->cfs_overload_cpus in update_top_cache_domain instead of
cpu_attach_domain
- Rewrite sparsemask iterator for complete inlining
- Cull and clean up sparsemask functions and moved all into
sched/sparsemask.h

Steve Sistare (10):
sched: Provide sparsemask, a reduced contention bitmap
sched/topology: Provide hooks to allocate data shared per LLC
sched/topology: Provide cfs_overload_cpus bitmap
sched/fair: Dynamically update cfs_overload_cpus
sched/fair: Hoist idle_stamp up from idle_balance
sched/fair: Generalize the detach_task interface
sched/fair: Provide can_migrate_task_llc
sched/fair: Steal work from an overloaded CPU when CPU goes idle
sched/fair: disable stealing if too many NUMA nodes
sched/fair: Provide idle search schedstats

include/linux/sched/topology.h | 1 +
kernel/sched/core.c | 31 +++-
kernel/sched/fair.c | 354 +++++++++++++++++++++++++++++++++++++----
kernel/sched/features.h | 6 +
kernel/sched/sched.h | 13 +-
kernel/sched/sparsemask.h | 210 ++++++++++++++++++++++++
kernel/sched/stats.c | 11 +-
kernel/sched/stats.h | 13 ++
kernel/sched/topology.c | 121 +++++++++++++-
9 files changed, 726 insertions(+), 34 deletions(-)
create mode 100644 kernel/sched/sparsemask.h

--
1.8.3.1

Next message: Steve Sistare: "[PATCH v4 04/10] sched/fair: Dynamically update cfs_overload_cpus"
Previous message: Steve Sistare: "[PATCH v4 09/10] sched/fair: disable stealing if too many NUMA nodes"
Next in thread: Steve Sistare: "[PATCH v4 06/10] sched/fair: Generalize the detach_task interface"
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