Re: [PATCH v4 00/10] steal tasks to improve CPU utilization

[Vincent Guittot]

On Mon, 10 Dec 2018 at 17:33, Vincent Guittot
<vincent.guittot@xxxxxxxxxx> wrote:
>
> On Mon, 10 Dec 2018 at 17:29, Steven Sistare <steven.sistare@xxxxxxxxxx> wrote:
> >
> > On 12/10/2018 11:10 AM, Vincent Guittot wrote:
> > > Hi Steven,
> > >
> > > On Thu, 6 Dec 2018 at 22:38, 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 4.20.0-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?
>
> it's 4000000
>
> > Try 15000000.  Your 8-core system is heavily overloaded with 40 * groups tasks,
> > and I suspect preemptions are killing performance.
>
> ok. I'm going to run the tests with the proposed value

Results look better after changing /proc/sys/kernel/sched_wakeup_granularity_ns

With STEAL
grps  time
1      0.869
2      1.646
3      2.395
4      3.163
8      6.199

after echo NO_STEAL > /sys/kernel/debug/sched_features
grps  time
1      0.928
2      1.770
3      2.597
4      3.407
8      6.431

There is a 7% improvement with steal and the larger value for
/proc/sys/kernel/sched_wakeup_granularity_ns for all groups
Should we set the STEAL feature disabled by default as this provides
benefit only when changing sched_wakeup_granularity_ns value from
default value?

>
> >
> > 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.
>
> It was mainly in case you wanted the figures of these statistics
>
> Vincent
>
> >
> > - Steve