sched_ext: Factor out bypass_lb_move() and insert with the donor DSQ unlocked

Prepare the bypass load balancer for moving tasks homed on other cpus. Such
a move takes the task's own rq lock through the remote consumer's handshake,
which drops the donor DSQ lock on the way, so its insert runs with the donor
DSQ unlocked and under the task's rq lock. The balancer's own move holds the
donor DSQ lock throughout, with the donee DSQ lock nested inside.

Switch the donor-homed move to the same scheme so that one body can serve
both: factor it out of bypass_lb_cpu() into bypass_lb_move(), which takes
the task off the donor DSQ and inserts with the DSQ unlocked. A task that
can't run on the donee goes back to the head of the donor DSQ instead of
being skipped in place.

The walk parks its cursor behind the task before each move and resumes from
it afterwards. It is bounded to the tasks queued when it began, by their DSQ
sequence numbers, so a task it put back is not visited again in the same
pass.

Before, the walk held both the donor rq lock and the donor DSQ lock across
every move. The donor DSQ can be very long and contended, a walk competing
with the cpu's dispatch and the enqueues for the DSQ lock would crawl, and
the walk is what relieves that contention by moving tasks off the DSQ. The
walk now cycles the DSQ lock around each insert but still holds the rq lock
throughout, and that alone keeps dispatch and enqueues away. The DSQ lock is
uncontended when retaken, so cycling it is cheap.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>

0 files changed, 0 insertions, 0 deletions