Re: [PATCH 00/10] steal tasks to improve CPU utilization

[Vincent Guittot]

Hi Steve,

On Mon, 22 Oct 2018 at 17:10, Steve Sistare <steven.sistare@xxxxxxxxxx> 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.

>