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SubjectRe: [PATCH v4 00/10] steal tasks to improve CPU utilization
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>
> On 12/6/2018 4:28 PM, Steve Sistare wrote:
>> 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
>>

On 2019-01-14 8:55 a.m., Steven Sistare wrote:> Hi Peter and Ingo,
> I am waiting for one of you to review, ack, or reject this series. I
> have addressed all known issues. I have a reviewed-by from Valentin
> and a
> tested-by from Vincent which I will add to v5 if you approve the
> patch.
>

Hi Thomas, Peter, Ingo,

These patches have been around for a bit and have been reviewed by
others. Could one of you please take a look at them, and let us know if
we are heading in the right direction?

Dhaval


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