From Parth Shah <> Subject [RFC v4 0/8] TurboSched: A scheduler for sustaining Turbo Frequencies for longer durations Date Thu, 25 Jul 2019 12:38:49 +0530 This is the 4th version of the patchset to sustain Turbo frequencies for

longer durations.



The previous versions can be found here:

v3: https://lkml.org/lkml/2019/6/25/25

v2: https://lkml.org/lkml/2019/5/15/1258

v1: https://lwn.net/Articles/783959/



The changes in this versions are:

v[3] -> v[4]:

- Based on Patrick Bellasi's comments, removed the use of UCLAMP based

mechanism to classify tasks as jitter

- Added support to sched_setattr to mark the task as jitter by adding a new

flag to the existing task_struct->flags attribute. This is decided to not

have any new variable inside task_struct and thus get rid of size

bloating.

- No functional changes



v[2] -> v[3]:

- Added a new attribute in task_struct to allow per task jitter

classification so that scheduler can use this as request to change wakeup

path for task packing

- Use syscall for jitter classification, removed cgroup based task

classification

- Use mutex over spinlock to get rid of task sleeping problem

- Changed _Bool->int everywhere

- Split few patches to have arch specific code separate from core scheduler

code

ToDo:

- Recompute core capacity only during CPU-Hotplug operation

- Remove smt capacity



v[1] -> v[2]:

- No CPU bound tasks' classification, only jitter tasks are classified from

the cpu cgroup controller

- Use of Spinlock rather than mutex to count number of jitters in the

system classified from cgroup

- Architecture specific implementation of Core capacity multiplication

factor changes dynamically based on the number of active threads in the

core

- Selection of non idle core in the system is bounded by DIE domain

- Use of UCLAMP mechanism to classify jitter tasks

- Removed "highutil_cpu_mask", and rather uses sd for DIE domain to find

better fit







Abstract

========



The modern servers allows multiple cores to run at range of frequencies

higher than rated range of frequencies. But the power budget of the system

inhibits sustaining these higher frequencies for longer durations.



However when certain cores are put to idle states, the power can be

effectively channelled to other busy cores, allowing them to sustain the

higher frequency.



One way to achieve this is to pack tasks onto fewer cores keeping others

idle, but it may lead to performance penalty for such tasks and sustaining

higher frequencies proves to be of no benefit. But if one can identify

unimportant low utilization tasks which can be packed on the already active

cores then waking up of new cores can be avoided. Such tasks are short

and/or bursty "jitter tasks" and waking up new core is expensive for such

case.



Current CFS algorithm in kernel scheduler is performance oriented and hence

tries to assign any idle CPU first for the waking up of new tasks. This

policy is perfect for major categories of the workload, but for jitter

tasks, one can save energy by packing them onto the active cores and allow

those cores to run at higher frequencies.



These patch-set tunes the task wake up logic in scheduler to pack

exclusively classified jitter tasks onto busy cores. The work involves the

jitter tasks classifications by using syscall based mechanisms.



In brief, if we can pack jitter tasks on busy cores then we can save power

by keeping other cores idle and allow busier cores to run at turbo

frequencies, patch-set tries to meet this solution in simplest manner.

Though, there are some challenges in implementing it(like smt_capacity,

un-needed arch hooks, etc) and I'm trying to work around that, it would be

great to have a discussion around this patches.





Implementation

==============



These patches uses syscall based mechanism to classify the tasks as jitter.

The task wakeup logic uses this information to pack such tasks onto cores

which are already running busy with CPU intensive tasks. The task packing

is done at `select_task_rq_fair` only so that in case of wrong decision

load balancer may pull the classified jitter tasks for maximizing

performance.



We define a core to be non-idle if it is over 12.5% utilized of its

capacity; the jitters are packed over these cores using First-fit approach.



To demonstrate/benchmark, one can use a synthetic workload generator

`turbo_bench.c`[1] available at

https://github.com/parthsl/tools/blob/master/benchmarks/turbo_bench.c



Following snippet demonstrates the use of TurboSched feature:

```

i=8; ./turbo_bench -t 30 -h $i -n $((i*2)) -j

```

This spawns 2*i total threads: of which i-CPU bound and i-jitter threads.



Current implementation uses only jitter classified tasks to be packed on

the first busy cores, but can be further optimized by getting userspace

input of important tasks and keeping track of such tasks. This leads to

optimized searching of non idle cores and also more accurate as userspace

hints are safer than auto classified busy cores/tasks.





Result

======



The patch-set proves to be useful for the system and the workload where

frequency boost is found to be useful than packing tasks into cores. IBM

POWER 9 system shows the benefit for a workload can be up to 13%.



Performance benefit of TurboSched w.r.t. CFS

+--+--+--+--+--+--+-+--+--+--+--+--+--+--+--+--+--+--+-+--+--+--+--+--+

| + + + + + + + + + + + + + + + + + + + + + + + |

15 +-+ Performance benefit in % +-+

| ** |

| ** ** |

10 +-+ ** ** ** +-+

| ** ** ** |

| ** ** ** |

5 +-+ ** ** ** ** ** ** +-+

| ** ** ** ** ** ** ** ** |

| ** ** ** ** ** ** ** ** ** ** |

| * ** ** ** ** ** ** ** ** ** ** ** * |

0 +-+** ** ** ** ** * ** ** ** ** ** ** ** ** ** ** ** * ** ** ** ** **-+

| ** ** ** ** |

| ** |

-5 +-+ +-+

| + + + + + + + + + + + + + + + + + + + + + + + |

+--+--+--+--+--+--+-+--+--+--+--+--+--+--+--+--+--+--+-+--+--+--+--+--+

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 1920 21 22 23 24

No. of workload threads





Frequency benefit of TurboSched w.r.t. CFS

+--+--+--+--+--+--+-+--+--+--+--+--+--+--+--+--+--+--+-+--+--+--+--+--+

| + + + + + + + + + + + + + + + + + + + + + + + |

15 +-+ Frequency benefit in % +-+

| ** |

| ** |

10 +-+ ** ** +-+

| ** ** ** |

| ** ** * ** ** ** |

5 +-+ ** ** ** * ** ** ** ** +-+

| ** ** ** ** * ** ** ** ** ** |

| ** ** ** ** ** * ** ** ** ** ** ** |

| ** ** ** ** ** * ** ** ** ** ** ** ** ** ** |

0 +-+** ** ** ** ** * ** ** ** ** ** ** ** ** ** ** ** * ** ** ** ** **-+

| |

| |

-5 +-+ +-+

| + + + + + + + + + + + + + + + + + + + + + + + |

+--+--+--+--+--+--+-+--+--+--+--+--+--+--+--+--+--+--+-+--+--+--+--+--+

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 1920 21 22 23 24

No. of workload threads





These numbers are w.r.t. `turbo_bench.c` multi-threaded test benchmark

which can create two kinds of tasks: CPU bound (High Utilization) and

Jitters (Low Utilization). N in X-axis represents N-CPU bound and N-Jitter

tasks spawned.





Series organization

==============

- Patches [01-03]: Jitter tasks classification using syscall

- Patches [04-05]: Defines Core Capacity to limit task packing

- Patches [06-08]: Tune CFS task wakeup logic to pack tasks onto busy

cores



Series can be applied on the top of tip/sched/core at

commit af24bde8df20 ("sched/uclamp: Add uclamp support to energy_compute()")





Parth Shah (8):

sched/core: Add manual jitter classification using sched_setattr

syscall

sched: Introduce switch to enable TurboSched mode

sched/core: Update turbo_sched count only when required

sched/fair: Define core capacity to limit task packing

powerpc: Define Core Capacity for POWER systems

sched/fair: Tune task wake-up logic to pack jitter tasks

sched/fair: Bound non idle core search within LLC domain

powerpc: Set turbo domain to NUMA node for task packing



arch/powerpc/include/asm/topology.h | 7 ++

arch/powerpc/kernel/smp.c | 38 ++++++++

include/linux/sched.h | 1 +

include/uapi/linux/sched.h | 4 +-

kernel/sched/core.c | 39 ++++++++

kernel/sched/fair.c | 135 +++++++++++++++++++++++++++-

kernel/sched/sched.h | 9 ++

7 files changed, 231 insertions(+), 2 deletions(-)



--

2.17.1



