sched_ext: Pass the initial cmask to cid-form ops.enable()

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The cid-form API has an obvious hole. A task's cid mask is only visible
through ops.set_cmask(), which fires on affinity changes and class switches
but not when a task enters a scheduler through fork, sub-sched enable or
re-home, and there is no p->cpus_ptr equivalent to fall back on. Schedulers
work around it by seeding the mask in ops.init_task() from p->cpus_ptr cid
by cid, which is subtly wrong: on sub-sched enable and re-home, an affinity
change between init_task() and enable() is delivered to the sched the task
is still on, and nothing corrects the new sched's copy afterwards.

Fix it by adding struct scx_enable_args to cid-form ops.enable() carrying
the task's cmask, built in the per-cpu scratch under the rq lock as the task
enters the scheduler, and calling set_cmask() with the same mask right after
enable(), ahead of set_weight(). A scheduler can then track affinity in
set_cmask() alone, and scx_qmap drops its init_task() seed. set_cmask() no
longer fires for a cid-form task before it is enabled, and the class-switch
republish in switching_to_scx() is limited to the cpu form.

This changes the cid-form ops.enable() signature, which is fine as the
cid-form API is still considered unreleased. An args struct rather than a
bare cmask argument leaves room for more initial state without another
signature change, and the cmask travels as a plain arena address because BTF
can't type arena struct members yet.

v2: The cmask travels as a u64 arena address, cmask_arena_addr, instead of a
kernel-typed pointer, with the typing limitation and the planned typed alias
documented (Sashiko review).

v3: The initial set_cmask() is delivered before set_weight() so that the
mask is in place when weight-dependent state is derived (Andrea Righi).
Selftest added.

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

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This commit is contained in:

Tejun Heo 2026-09-18 14:28:37 -10:00

parent 9ec7ba20c9

commit 3bd46666cf

3 changed files with 110 additions and 41 deletions

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92

kernel/sched/ext/ext.c

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56

kernel/sched/ext/internal.h

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3

tools/sched_ext/scx_qmap.bpf.c

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