sched_ext: Fix a cpu idling with a runnable rescuee

A scheduler preempts a running rescuee like any other task by zeroing its
slice. The rescue grant survives the preemption. put_prev_task_scx()
restores the unserved remainder through scx_rescue_keep() and queues the
rescuee at the tail of the local DSQ, behind whatever the scheduler wanted
to run first.

That breaks down when the scheduler zeroes the slice but dispatches nothing.
dispatch_one() keeps the previous task running only with slice left or, when
nothing else is runnable, with its scheduler holding SCX_CAP_BASE on the cpu
and not taking the last task through SCX_OPS_ENQ_LAST. A rescuee fails both
once its slice is zeroed if its scheduler lacks BASE there or takes the last
task:

 1. ops.tick() or scx_bpf_task_set_slice() zeroes the rescuee's slice
 2. the pick finds nothing else to run and settles on idle
 3. put_prev_task_scx() restores the remainder and queues the rescuee on the
    local DSQ
 4. nothing reschedules the cpu, as the pick has settled and __schedule()
    clears need_resched right after it

The cpu idles with the rescuee queued until an unrelated wakeup or the
rescue timer's escalation.

Restore the remainder from dispatch_one() too. When nothing else is
runnable, its last keep now keeps the rescuee running on the remainder, so
the pick never settles on idle with a rescuee runnable. put_prev_task_scx()
keeps handling a displaced rescuee, where another task won the pick.

A rescuee that passes the existing keep-last test is kept there instead and
continues as an ordinary task, its rescue ended with the zeroed slice. That
is fine as its scheduler holds SCX_CAP_BASE on the cpu.

The new keep also covers bypassed tasks run under rescue in the following
patches.

v2: The dispatch_one() comment and description now say that a rescuee whose
scheduler holds SCX_CAP_BASE on the cpu passes the keep-last test and
continues as an ordinary task (sashiko).

Fixes: 5fd501744b10 ("sched_ext: Add bandwidth-limited rescue execution for stranded tasks")
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>

0 files changed, 0 insertions, 0 deletions