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sched_ext: Drop unlocked scx_rq_clock_invalidate() from scx_root_disable()

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scx_root_disable() invalidates each rq's clock before taking the rq lock.
scx_rq_clock_invalidate() is a plain read-modify-write of rq->scx.flags and
every other writer of the word runs under the rq lock, so the unlocked
update can race a concurrent flags update and lose one side's bits.

The invalidation doesn't matter in the first place. The cached clock is read
only by scx_bpf_now() from a loaded scheduler's BPF programs, nothing can
re-validate the clock while sched_ext is disabled as scx_rq_clock_update()
is gated on scx_enabled() too, and the usual rq lock cycles under the next
scheduler refresh or invalidate it before it's practically observable. Drop
the invalidation instead of fixing the locking.

v2: Description and comment updated - the invalidation is unnecessary rather
    than subsumed by the rq lock cycle below.

Fixes: 3a9910b590 ("sched_ext: Implement scx_bpf_now()")
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Changwoo Min <changwoo@igalia.com>
This commit is contained in:
Tejun Heo 2026-08-14 23:47:39 -10:00
parent e72979d326
commit d440933231
1 changed files with 3 additions and 8 deletions
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11
kernel/sched/ext/ext.c
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@ -6476,12 +6476,9 @@ static void scx_root_disable(struct scx_sched *sch)
percpu_up_write(&scx_fork_rwsem);
/*
* Invalidate all the rq clocks to prevent getting outdated
* rq clocks from a previous scx scheduler.
*
* Also re-balance the dl_server bandwidth reservations: detach
* ext_server (no more sched_ext tasks) and reinstate fair_server if it
* was previously detached because we were running in full mode.
* Re-balance the dl_server bandwidth reservations: detach ext_server
* (no more sched_ext tasks) and reinstate fair_server if it was
* previously detached because we were running in full mode.
*
* Unlike the enable path, this runs on a recovery path that cannot
* fail, so we use dl_server_swap_bw() to atomically free ext_server's
@ -6494,8 +6491,6 @@ static void scx_root_disable(struct scx_sched *sch)
for_each_possible_cpu(cpu) {
struct rq *rq = cpu_rq(cpu);
scx_rq_clock_invalidate(rq);
scoped_guard(rq_lock_irqsave, rq) {
update_rq_clock(rq);
if (was_switched_all) {