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charset="utf-8" Writing runtime or period via the per-CPU dl_server debugfs files (/sys/kernel/debug/sched/{fair,ext}_server/cpu*/{runtime,period}) on an offline CPU can trigger two distinct kernel issues: 1) Divide-by-zero in dl_server_apply_params(): Oops: divide error: 0000 [#1] SMP NOPTI RIP: 0010:dl_server_apply_params+0x239/0x3a0 Call Trace: sched_server_write_common.isra.0+0x21a/0x3c0 full_proxy_write+0x78/0xd0 vfs_write+0xe7/0x6e0 Both __dl_sub() and __dl_add() divide by cpus internally, which can be 0 once the CPU has been removed from any active root-domain span (this has been latent since the debugfs interface was introduced). 2) WARN_ON_ONCE in dl_server_start(): WARNING: kernel/sched/deadline.c:1805 at dl_server_start+0x232/0x270 Commit ee6e44dfe6e5 ("sched/deadline: Stop dl_server before CPU goes offline") added this check to catch enqueueing the server on an offline rq. There's no meaningful semantics for re-configuring the per-CPU dl_server bandwidth while the CPU is offline, so simply reject the write with -EBUSY so userspace gets a clear error. Reported-by: Sashiko Closes: https://lore.kernel.org/all/20260526092228.3B6891F00A3A@smtp.kernel= .org/ Fixes: d741f297bcea ("sched/fair: Fair server interface") Signed-off-by: Andrea Righi Reviewed-by: Juri Lelli --- kernel/sched/debug.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c index ed3a0d65da0ca..e57ad8c78a60e 100644 --- a/kernel/sched/debug.c +++ b/kernel/sched/debug.c @@ -415,6 +415,9 @@ static ssize_t sched_server_write_common(struct file *f= ilp, const char __user *u return -EINVAL; } =20 + if (!cpu_online(cpu_of(rq))) + return -EBUSY; + update_rq_clock(rq); dl_server_stop(dl_se); retval =3D dl_server_apply_params(dl_se, runtime, period, 0); base-commit: 7b197f597bc895b01204d8389a4cf3b00780bd21 --=20 2.54.0