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SubjectRe: [PATCH v4 00/10] steal tasks to improve CPU utilization
Hi Steven,

On Thu, 6 Dec 2018 at 22:38, Steve Sistare <steven.sistare@oracle.com> 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.

I have run some hackbench tests on my hikey arm64 octo cores with your
patchset. My original intent was to send a tested-by but I have some
performances regressions.
This hikey is the smp one and not the asymetric hikey960 that Valentin
used for his tests
The sched domain topology is
domain-0: span=0-3 level=MC and domain-0: span=4-7 level=MC
domain-1: span=0-7 level=DIE

I have run 12 times hackbench -g $j -P -l 2000 with j equals to 1 2 3 4 8

grps time
1 1.396
2 2.699
3 3.617
4 4.498
8 7.721

Then after disabling STEAL in sched_feature with echo NO_STEAL >
/sys/kernel/debug/sched_features , the results become:
grps time
1 1.217
2 1.973
3 2.855
4 3.932
8 7.674

I haven't looked in details about some possible reasons of such
difference yet and haven't collected the stats that you added with
patch 10.
Have you got a script to collect and post process them ?

Regards,
Vincent

>
> 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
>

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