RFC Core scheduling v5

Fifth iteration of the Core-Scheduling feature.

Core scheduling is a feature that only allows trusted tasks to run concurrently on cpus sharing compute resources(eg: hyperthreads on a core). The goal is to mitigate the core-level side-channel attacks without requiring to disable SMT (which has a significant impact on performance in some situations). So far, the feature mitigates user-space to user-space attacks but not user-space to kernel attack, when one of the hardware thread enters the kernel (syscall, interrupt etc).

By default, the feature doesn't change any of the current scheduler behavior. The user decides which tasks can run simultaneously on the same core (for now by having them in the same tagged cgroup). When a tag is enabled in a cgroup and a task from that cgroup is running on a hardware thread, the scheduler ensures that only idle or trusted tasks run on the other sibling(s). Besides security concerns, this feature can also be beneficial for RT and performance applications where we want to control how tasks make use of SMT dynamically.

This version was focusing on performance and stability. Couple of crashes related to task tagging and cpu hotplug path were fixed. This version also improves the performance considerably by making task migration and load balancing coresched aware.

In terms of performance, the major difference since the last iteration is that now even IO-heavy and mixed-resources workloads are less impacted by core-scheduling than by disabling SMT. Both host-level and VM-level benchmarks were performed. Details in:
https://lkml.org/lkml/2020/2/12/1194
https://lkml.org/lkml/2019/11/1/269

v5 is rebased on top of 5.5.5(449718782a46)
https://github.com/digitalocean/linux-coresched/tree/coresched/v5-v5.5.y

Changes in v5
-------------
- Fixes for cgroup/process tagging during corner cases like cgroup destroy, task moving across cgroups etc
- Tim Chen
- Coresched aware task migrations
- Aubrey Li
- Other minor stability fixes.

Changes in v4
-------------
- Implement a core wide min_vruntime for vruntime comparison of tasks across cpus in a core.
- Aaron Lu
- Fixes a typo bug in setting the forced_idle cpu.
- Aaron Lu

Changes in v3
-------------
- Fixes the issue of sibling picking up an incompatible task
- Aaron Lu
- Vineeth Pillai
- Julien Desfossez
- Fixes the issue of starving threads due to forced idle
- Peter Zijlstra
- Fixes the refcounting issue when deleting a cgroup with tag
- Julien Desfossez
- Fixes a crash during cpu offline/online with coresched enabled
- Vineeth Pillai
- Fixes a comparison logic issue in sched_core_find
- Aaron Lu

Changes in v2
-------------
- Fixes for couple of NULL pointer dereference crashes
- Subhra Mazumdar
- Tim Chen
- Improves priority comparison logic for process in different cpus
- Peter Zijlstra
- Aaron Lu
- Fixes a hard lockup in rq locking
- Vineeth Pillai
- Julien Desfossez
- Fixes a performance issue seen on IO heavy workloads
- Vineeth Pillai
- Julien Desfossez
- Fix for 32bit build
- Aubrey Li

ISSUES
------
- Aaron(Intel) found an issue with load balancing when the tasks have different weights(nice or cgroup shares). Task weight is not considered in coresched aware load balancing and causes those higher weights task to starve.
- Joel(ChromeOS) found an issue where RT task may be preempted by a lower class task.
- Joel(ChromeOS) found a deadlock and crash on PREEMPT kernel in the coreshed idle balance logic

TODO
----
- Work on merging patches that are ready to be merged
- Decide on the API for exposing the feature to userland
- Experiment with adding synchronization points in VMEXIT to mitigate the VM-to-host-kernel leaking
- Investigate the source of the overhead even when no tasks are tagged:
https://lkml.org/lkml/2019/10/29/242

---

Aaron Lu (2):
sched/fair: wrapper for cfs_rq->min_vruntime
sched/fair: core wide vruntime comparison

Aubrey Li (1):
sched: migration changes for core scheduling

Peter Zijlstra (9):
sched: Wrap rq::lock access
sched: Introduce sched_class::pick_task()
sched: Core-wide rq->lock
sched/fair: Add a few assertions
sched: Basic tracking of matching tasks
sched: Add core wide task selection and scheduling.
sched: Trivial forced-newidle balancer
sched: cgroup tagging interface for core scheduling
sched: Debug bits...

Tim Chen (1):
sched: Update core scheduler queue when taking cpu online/offline

 include/linux/sched.h    |    9 +-
 kernel/Kconfig.preempt   |    6 +
 kernel/sched/core.c      | 1037 +++++++++++++++++++++++++++++++++++++-
 kernel/sched/cpuacct.c   |   12 +-
 kernel/sched/deadline.c  |   69 ++-
 kernel/sched/debug.c     |    4 +-
 kernel/sched/fair.c      |  387 +++++++++++---
 kernel/sched/idle.c      |   11 +-
 kernel/sched/pelt.h      |    2 +-
 kernel/sched/rt.c        |   65 ++-
 kernel/sched/sched.h     |  248 +++++++--
 kernel/sched/stop_task.c |   13 +-
 kernel/sched/topology.c  |    4 +-
 13 files changed, 1672 insertions(+), 195 deletions(-)

2.17.1