[PATCH v3 0/2] sched/fair: nohz.next_balance vs newly-idle CPUs

[Valentin Schneider <valentin.schneider@arm.com>]

Hi folks,

This was caught up by our testing on an arm64 RB5 board - that's an 8 CPUs
DynamIQ SoC with 4 littles, 3 mediums and 1 big. It seems to rely more on NOHZ
balancing than our other boards being tested, which highlighted that not
including a newly-idle CPU into nohz.next_balance can cause issues (especially
when the other CPUs have had their balance_interval inflated by pinned tasks).

As suggested by Vincent, the approach here is to mimic what was done for
nohz.has_blocked, which gives us sane(ish) ordering guarantees.

Revisions
=========

v2 -> v3
++++++++

o Rebased against latest tip/sched/core: 234b8ab6476c ("sched: Introduce
  dl_task_check_affinity() to check proposed affinity")
  
o Kept NOHZ_NEXT_KICK in NOHZ_KICK_MASK, but changed nohz_balancer_kick() to
  issue kicks with NOHZ_STATS_KICK | NOHZ_BALANCE_KICK instead (Dietmar)
o Added missing NOHZ_STATS_KICK gate for nohz.next_blocked update (Vincent)

v1 -> v2
++++++++

o Ditched the extra cpumasks and went with a sibling of nohz.has_blocked
  (Vincent) 

Cheers,
Valentin

Valentin Schneider (2):
  sched/fair: Add NOHZ balancer flag for nohz.next_balance updates
  sched/fair: Trigger nohz.next_balance updates when a CPU goes
    NOHZ-idle

 kernel/sched/fair.c  | 39 +++++++++++++++++++++++++++------------
 kernel/sched/sched.h |  8 +++++++-
 2 files changed, 34 insertions(+), 13 deletions(-)

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2.25.1