[PATCHSET v2 sched_ext/for-6.11] sched_ext: Allow SCX_DSQ_LOCAL_ON for direct dispatches

[Tejun Heo <tj@kernel.org>]

This is v2. Changes from v1:
(http://lkml.kernel.org/r/20240709212137.1199269-1-tj@kernel.org)

- 0003-sched_ext-Make-rf-optional-for-dispatch_to_local_dsq.patch is
  replaced with
  0003-sched_ext-Unpin-and-repin-rq-lock-from-balance_scx.patch.
  balance_scx() now unpins the rq lock and the inner functions don't handle
  rq pinning. This simplifies code quite a bit.

- schedule_deferred() was missing a return after balance_work scheduling.
  Fixed.

- Other misc changes including UP build warning fix.

In ops.dispatch(), SCX_DSQ_LOCAL_ON can be used to dispatch the task to the
local DSQ of any CPU. However, during direct dispatch from ops.select_cpu()
and ops.enqueue(), this isn't allowed. This is because dispatching to the
local DSQ of a remote CPU requires locking both the task's current and new
rq's and such double locking can't be done directly from ops.enqueue(). This
gap in API forces schedulers into work-arounds which are not straightforward
or optimal such as skipping direct dispatches in such cases.

This patchset implements SCX_DSQ_LOCAL_ON support for direct dispatches.

This patchset contains the following patches:

 0001-sched-Move-struct-balance_callback-definition-upward.patch
 0002-sched_ext-Open-code-task_linked_on_dsq.patch
 0003-sched_ext-Unpin-and-repin-rq-lock-from-balance_scx.patch
 0004-sched_ext-s-SCX_RQ_BALANCING-SCX_RQ_IN_BALANCE-and-a.patch
 0005-sched_ext-Allow-SCX_DSQ_LOCAL_ON-for-direct-dispatch.patch
 0006-sched_ext-scx_qmap-Pick-idle-CPU-for-direct-dispatch.patch

While 0001 is a scheduler core change, all it does is moving the definition
of struct balance_callback so that it's visible for struct scx_rq
definition. If there are no objections, it'd make sense to route it through
the sched_ext tree with the rest of the changes.

and is also available in the following git branch:

 git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext.git scx-ddsp-local-on-v2

diffstat follows. Thanks.

 kernel/sched/ext.c             |  283 +++++++++++++++++++++++++++++++++++++++++++--------------------
 kernel/sched/sched.h           |   19 ++--
 tools/sched_ext/scx_qmap.bpf.c |   39 ++++++--
 tools/sched_ext/scx_qmap.c     |    5 -
 4 files changed, 240 insertions(+), 106 deletions(-)

--
tejun