[PATCH v9 0/2] sched/fair: Scan cluster before scanning LLC in wake-up path

[Yicong Yang]

From: Yicong Yang <yangyicong at hisilicon.com>

This is the follow-up work to support cluster scheduler. Previously
we have added cluster level in the scheduler for both ARM64[1] and
X86[2] to support load balance between clusters to bring more memory
bandwidth and decrease cache contention. This patchset, on the other
hand, takes care of wake-up path by giving CPUs within the same cluster
a try before scanning the whole LLC to benefit those tasks communicating
with each other.

[1] 778c558f49a2 ("sched: Add cluster scheduler level in core and related Kconfig for ARM64")
[2] 66558b730f25 ("sched: Add cluster scheduler level for x86")

Since we're using sd->groups->flags to determine a cluster, core should ensure
the flags set correctly on domain generation. This is done by [*].

[*] https://lore.kernel.org/all/20230713013133.2314153-1-yu.c.chen@intel.com/

Change since v8:
- Peter find cpus_share_lowest_cache() is weired so fallback to cpus_share_resources()
  suggested in v4
- Use sd->groups->flags to find the cluster when scanning, save one per-cpu pointer
- Fix sched_cluster_active enabled incorrectly on domain degeneration
- Use sched_cluster_active to avoid repeated check on non-cluster machines, per Gautham
Link: https://lore.kernel.org/all/20230530070253.33306-1-yangyicong@huawei.com/

Change since v7:
- Optimize by choosing prev_cpu/recent_used_cpu when possible after failed to
  scanning for an idle CPU in cluster/LLC. Thanks Chen Yu for testing on Jacobsville
Link: https://lore.kernel.org/all/20220915073423.25535-1-yangyicong@huawei.com/

Change for RESEND:
- Collect tag from Chen Yu and rebase on the latest tip/sched/core. Thanks.
Link: https://lore.kernel.org/lkml/20220822073610.27205-1-yangyicong@huawei.com/

Change since v6:
- rebase on 6.0-rc1
Link: https://lore.kernel.org/lkml/20220726074758.46686-1-yangyicong@huawei.com/

Change since v5:
- Improve patch 2 according to Peter's suggestion:
  - use sched_cluster_active to indicate whether cluster is active
  - consider SMT case and use wrap iteration when scanning cluster
- Add Vincent's tag
Thanks.
Link: https://lore.kernel.org/lkml/20220720081150.22167-1-yangyicong@hisilicon.com/

Change since v4:
- rename cpus_share_resources to cpus_share_lowest_cache to be more informative, per Tim
- return -1 when nr==0 in scan_cluster(), per Abel
Thanks!
Link: https://lore.kernel.org/lkml/20220609120622.47724-1-yangyicong@hisilicon.com/

Change since v3:
- fix compile error when !CONFIG_SCHED_CLUSTER, reported by lkp test.
Link: https://lore.kernel.org/lkml/20220608095758.60504-1-yangyicong@hisilicon.com/

Change since v2:
- leverage SIS_PROP to suspend redundant scanning when LLC is overloaded
- remove the ping-pong suppression
- address the comment from Tim, thanks.
Link: https://lore.kernel.org/lkml/20220126080947.4529-1-yangyicong@hisilicon.com/

Change since v1:
- regain the performance data based on v5.17-rc1
- rename cpus_share_cluster to cpus_share_resources per Vincent and Gautham, thanks!
Link: https://lore.kernel.org/lkml/20211215041149.73171-1-yangyicong@hisilicon.com/

Barry Song (2):
  sched: Add cpus_share_resources API
  sched/fair: Scan cluster before scanning LLC in wake-up path

 include/linux/sched/sd_flags.h |  7 ++++
 include/linux/sched/topology.h |  8 ++++-
 kernel/sched/core.c            | 12 +++++++
 kernel/sched/fair.c            | 59 +++++++++++++++++++++++++++++++---
 kernel/sched/sched.h           |  2 ++
 kernel/sched/topology.c        | 25 ++++++++++++++
 6 files changed, 107 insertions(+), 6 deletions(-)

-- 
2.24.0