sched_ext: Fix queued bypassed tasks stalling when the host loses SCX_CAP_BASE

A task already queued on a bypass DSQ stalls when the host loses
SCX_CAP_BASE on that cpu, as the cpu no longer consumes the DSQ. Sending a
bypassed task to rescue at enqueue time covers only a task enqueued after
the loss, and the balancer no longer donates to a capless cpu.

Have the bypass load balancer drain such a DSQ: a cpu where the host lacks
BASE is a donor with a target of zero, and the donees take its tasks under
their ordinary fill.

A drained task no donee can take is reenqueued at its home: to rescue when
the host lacks BASE there and rescue is enabled, and to the home's bypass
DSQ otherwise.

A balancing donor leaves a task that can't move to its donee where it is.
Reenqueueing it at home would put a task homed on the donor cpu at the tail
of its own DSQ, and a blocked task that proxy execution migrated there,
which every remote cpu refuses, would go to the tail on every pass.

Without rescue, a task homed on the drained cpu goes back onto the drained
DSQ, once per pass, and waits for the host to regain BASE, the bypass to
end, or the stall watchdog to eject the host. Its reenqueues are not counted
against the reenqueue limit, as its scheduler is bypassing.

A BASE loss kicks the balancer so that the drain doesn't wait for the next
interval. With a zero interval the kick is the only pass, and a task that
lands on the drained DSQ after it waits the same way.

v2: The description drops an in-flight affinity change from the causes of a
refused balancer move.

v3:
- The description says that a stuck task's reenqueues are not counted
  against the reenqueue limit (sashiko).
- The description names the stall watchdog as the way out for a task stuck
  without rescue, and the tasks a zero interval leaves on a drained DSQ.
- bypass_lb_reenq_home()'s kerneldoc and the description give the placement
  with rescue disabled and for a task that must run at home, and the other
  two comments on that placement point to the kerneldoc.

v4:
- Skip a task the donee would refuse on a balancing donor instead of
  reenqueueing it at home (Andrea).
- bypass_lb_reenq_home()'s kerneldoc is corrected on a migration-disabled
  task homed on a donee and on the local DSQ admission.
- The description is condensed.

Fixes: 147d1885f390 ("sched_ext: Add the SCX_CAP_ENQ_IMMED cap")
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>

0 files changed, 0 insertions, 0 deletions