sched_ext: Auto-register/unregister dl_server reservations

Commit cd959a3562050d ("sched_ext: Add a DL server for sched_ext tasks")
introduced an ext_server deadline server to protect sched_ext tasks from
fair/RT starvation, mirroring the existing fair_server.

Currently, both servers reserve their 50ms/1000ms bandwidth at boot,
regardless of whether a BPF scheduler is loaded. Unused bandwidth is
still reclaimed at runtime by other classes, but the static reservation
prevents the RT class from implicitly using that headroom when one of
the two classes is guaranteed to be empty.

A sysadmin can work around this by writing
/sys/kernel/debug/sched/{fair,ext}_server/cpu*/runtime, but that
requires manual action and not all systems expose debugfs.

A better approach is to make server bandwidth reservations dynamic: only
the scheduling policy that is currently active should register its
reservation, while the inactive one should not artificially hold
capacity (keeping both reservations only when the BPF scheduler is
running in partial mode):

 +---------------------------------------------+-------------+------------+
 | BPF scheduler state                         | fair server | ext server |
 +---------------------------------------------+-------------+------------+
 | not loaded (default boot)                   | reserved    | none       |
 | loaded full mode (!SCX_OPS_SWITCH_PARTIAL)  | none        | reserved   |
 | loaded partial mode (SCX_OPS_SWITCH_PARTIAL)| reserved    | reserved   |
 +---------------------------------------------+-------------+------------+

To achieve this, introduce an "attached/detached" state for each
deadline server, so the kernel can decide whether a server's bandwidth
should be accounted in global bandwidth tracking.

At boot, the system starts with only the fair server contributing to
bandwidth accounting. When a BPF scheduler is enabled, the ext server is
attached and may replace or complement the fair server depending on
whether full or partial mode is used. When sched_ext is disabled, the
system restores the previous deadline bandwidth values and behavior.

The transition logic ensures that switching between scheduling modes is
consistent and reversible, without losing runtime configuration or
requiring manual intervention.

Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Juri Lelli <juri.lelli@redhat.com>
Link: https://patch.msgid.link/20260526164420.638711-2-arighi@nvidia.com

0 files changed, 0 insertions, 0 deletions