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SubjectRe: [PATCH 00/10] steal tasks to improve CPU utilization
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Thanks very much to everyone who has commented on my patch series.
Here are the issues to be addressed in V2 of the series, and the person
that suggested it, or raised the issue that led to it.

Changes for V2:
* Remove stray patch 10 hunk from patch 5 (Valentin)
* Fix "warning: label out defined but not used" for !CONFIG_SCHED_SMT
(Valentin)
* Set SCHED_STEAL_NODE_LIMIT_DEFAULT to 2 (Steve)
* Call try_steal iff avg_idle exceeds some small threshold (Steve, Valentin)

Possible future work:
* Use sparsemask and stealing for RT (Steve, Peter)
* Remove the core and socket levels from idle_balance() and let stealing
handle those levels (Steve, Peter)
* Delete idle_balance() and use stealing exclusively for handling new idle
(Steve, Peter)
* Test specjbb multi-warehouse on 8-node systems when stealing for
large NUMA systems is revisited (Peter)
* Enhance stealing to handle misfits (Valentin)
* Lower time threshold for task_hot within LLC (Valentin)

Dropped:
* Skip try_steal() if we bail out of idle_balance() because !this_rq->rd->overload
(Valentin)
I tried it and saw no difference. Dropped for simplicity.

Does anyone else plan to review the code? Please tell me now, even if your
review will be delayed. If yes, I will wait for all comments before producing
V2. The code changes so far are small.

- Steve

On 10/22/2018 10:59 AM, 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 4.19.0-rc7. 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 imprroves 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
>
> --------------------------------------------------------------
>
> 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 +
> include/linux/sparsemask.h | 260 +++++++++++++++++++++++++++++++
> kernel/sched/core.c | 30 +++-
> kernel/sched/fair.c | 338 +++++++++++++++++++++++++++++++++++++----
> kernel/sched/features.h | 6 +
> kernel/sched/sched.h | 13 +-
> kernel/sched/stats.c | 11 +-
> kernel/sched/stats.h | 13 ++
> kernel/sched/topology.c | 117 +++++++++++++-
> lib/Makefile | 2 +-
> lib/sparsemask.c | 142 +++++++++++++++++
> 11 files changed, 898 insertions(+), 35 deletions(-)
> create mode 100644 include/linux/sparsemask.h
> create mode 100644 lib/sparsemask.c
>

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