sched_ext: Allow ops.cgroup_set_bandwidth() to be sleepable

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ops.cgroup_set_bandwidth() is delivered from scx_group_set_bandwidth(),
which runs from the cpu.max cgroup interface write path (tg_set_bandwidth())
in process context. scx_group_set_bandwidth() holds
percpu_down_read(&scx_cgroup_ops_rwsem), whose read side may sleep.
The call site is therefore sleepable, like ops.cgroup_init().

bpf_scx_check_member() rejects a sleepable program on any member not on its
allow-list, so a BPF scheduler cannot allocate -- which is sleepable -- when
a cgroup gains a cpu.max limit at runtime; it must instead pre-reserve memory
for a callback that cannot allocate. Add cgroup_set_bandwidth() to the
allow-list so the callback can allocate on demand, and document that it may
block.

A scheduler must decide at load time whether to mark the callback sleepable,
but the allow-list entry is a verifier property with no symbol to probe. Add
a compatibility marker whose presence in the kernel's BTF lets userspace detect
this support: DEFINE_SCX_COMPAT_MARKER() emits an empty, callerless function,
here scx_compat_marker_cgroup_set_bandwidth_may_sleep(). It is __used
__retain so neither the compiler nor the linker (under
CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) drops it. The markers share the
scx_compat_marker_ prefix and are collected near the end of ext.c so more
can be added as further capabilities appear.

Signed-off-by: Changwoo Min <changwoo@igalia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

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This commit is contained in:

Changwoo Min 2026-08-19 01:04:29 +09:00 • committed by Tejun Heo

parent fab183d632

commit 5f01293930

2 changed files with 36 additions and 1 deletions

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14

kernel/sched/ext/ext.c

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23

kernel/sched/ext/internal.h

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