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

On Mon, 22 Oct 2018 at 17:10, 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 4.19.0-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

Why do you mention this sched_wakeup_granularity_ns value ?
It is something that you changed for you tests ?
The comment for this tunable says that default value is 1ms *
ilog(ncpus) = 4ms for 20CPUs

>
> 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.

>

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