[PATCHSET sched_ext/for-7.3-fixes] sched_ext: Fix bypass leaving the running task protected and bypassed tasks stranded

[Tejun Heo <tj@kernel.org>]

Hello,

scx_bypass() handles the running task inside its walk over the runnable
list, which never contains the running task, so the protection strip at
bypass entry never runs. Separately, a bypassing sub-scheduler's tasks are
run by its nearest ancestor that isn't bypassing, but whether such a task
may stay on its cpu is still checked against the caps of its own scheduler.
A bypassed task that the bypass load balancer moved to a cpu where its
scheduler holds no caps can't stay there once its slice runs out, and the
cpu can go idle with the task queued on the ancestor's bypass DSQ.

Patch 1 handles the running task outside the walk. Patch 2 checks a bypassed
task against the caps of the scheduler running it and, in the block patch 1
adds, evicts the running task when bypass lifts if its own scheduler lacks
the caps to keep it.

Verified on bare metal by attaching and detaching a sub-scheduler repeatedly
under load. A debug knob that marks the running task protected at each
bypass entry finds every such task stripped with patch 1. Without it, they
all keep their protection and the bypass tick can't cut their slices. In a
VM, running tasks whose scheduler holds no caps on their cpu leave it within
about 50us of the bypass lift with patch 2, instead of running out their
bypass slice. The sched_ext selftests pass.

Based on sched_ext/for-7.3-fixes (43e1ef14a472).

This patchset contains the following 2 patches.

 0001 sched_ext: Fix the running task keeping its protection into bypass
 0002 sched_ext: Fix a bypassed task stalling on a cpu its scheduler has no caps on

The patchset is also available in the following git branch:

 git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext.git bypass-stall-fixes

diffstat follows. Thanks.

 kernel/sched/ext/ext.c      | 32 +++++++++++++++++++-------------
 kernel/sched/ext/internal.h | 17 +++++++++++++++++
 kernel/sched/ext/sub.c      |  7 +++++--
 kernel/sched/ext/sub.h      |  6 +++++-
 4 files changed, 46 insertions(+), 16 deletions(-)

--
tejun