Re: [PATCH 00/10] steal tasks to improve CPU utilization
From: Vincent Guittot
Date: Thu Oct 25 2018 - 08:44:04 EST
On Thu, 25 Oct 2018 at 13:29, Steven Sistare <steven.sistare@xxxxxxxxxx> wrote:
> On 10/25/2018 3:50 AM, Vincent Guittot wrote:
> > 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
> I changed it for the test, and I explain why a few paragraphs later.
> The value matches the one set by tuned.service, for those running it.
ok. I haven't noticed that later explanation.
You said " 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."
What do you mean exactly by distort ?
> - Steve
> >> 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.