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SubjectRe: [PATCH v4 00/10] steal tasks to improve CPU utilization
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On 12/10/2018 11:10 AM, Vincent Guittot wrote:
> 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

Thanks Vincent. What is the value of /proc/sys/kernel/sched_wakeup_granularity_ns?
Try 15000000. Your 8-core system is heavily overloaded with 40 * groups tasks,
and I suspect preemptions are killing performance.

I have a python script to post-process schedstat files, but it does many things
and is large and I am not ready to share it. I can write a short bash script if
that would help.

- Steve

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