[RFC 1/1] sched: defer completion task to online CPU

[Usama Arif <usamaarif642@gmail.com>]

The following warning is being encountered at boot time:

           WARNING: CPU: 94 PID: 588 at kernel/time/hrtimer.c:1086 hrtimer_start_range_ns+0x289/0x2d0
           Modules linked in:
           CPU: 94 UID: 0 PID: 588 Comm: migration/94 Not tainted
           Stopper: multi_cpu_stop+0x0/0x120 <- stop_machine_cpuslocked+0x66/0xc0
           RIP: 0010:hrtimer_start_range_ns+0x289/0x2d0
           Code: 41 5c 41 5d 41 5e 41 5f 5d e9 63 94 ea 00 0f 0b 48 83 c4 10 5b 41 5c 41 5d 41 5e 41 5f 5d e9 39 fc 15 01 0f 0b e9 c1 fd ff ff <0f> 0b 48 8b 45 00 e9 59 ff ff ff f3 0f 1e fa 65 8b 05 1d ec e8 7e
           RSP: 0018:ffffc900019cbcc8 EFLAGS: 00010046
           RAX: ffff88bf449a4c40 RBX: 0000000000000082 RCX: 0000000000000001
           RDX: 0000000000000001 RSI: ffff88bf43224c80 RDI: ffff88bf449a4c40
           RBP: ffff88bf449a4c80 R08: ffff888280970090 R09: 0000000000000000
           R10: ffff88bf432252e0 R11: ffffffff811abf70 R12: ffff88bf449a4c40
           R13: ffff88bf43234b28 R14: ffff88bf43224c80 R15: 0000000000000000
           FS:  0000000000000000(0000) GS:ffff88bf44980000(0000) knlGS:0000000000000000
           CR2: 0000000000000000 CR3: 000000404b230001 CR4: 0000000000770ef0
           PKRU: 55555554
           Call Trace:
            <TASK>
            ? __warn+0xcf/0x1b0
            ? hrtimer_start_range_ns+0x289/0x2d0
            ? report_bug+0x120/0x1a0
            ? handle_bug+0x60/0x90
            ? exc_invalid_op+0x1a/0x50
            ? asm_exc_invalid_op+0x1a/0x20
            ? register_refined_jiffies+0xb0/0xb0
            ? hrtimer_start_range_ns+0x289/0x2d0
            ? hrtimer_start_range_ns+0x186/0x2d0
            start_dl_timer+0xfc/0x150
            enqueue_dl_entity+0x367/0x640
            dl_server_start+0x53/0xa0
            enqueue_task_fair+0x363/0x460
            enqueue_task+0x3c/0x200
            ttwu_do_activate+0x94/0x240
            try_to_wake_up+0x315/0x600
            complete+0x4b/0x80
            ? stop_two_cpus+0x2f0/0x2f0
            cpu_stopper_thread+0xb1/0x120
            ? smpboot_unregister_percpu_thread+0xc0/0xc0
            smpboot_thread_fn+0xf7/0x150
            kthread+0x121/0x130
            ? kthread_blkcg+0x40/0x40
            ret_from_fork+0x39/0x50
            ? kthread_blkcg+0x40/0x40
            ret_from_fork_asm+0x11/0x20
            </TASK>

It looks like completion that requires an hrtimer is being scheduled on a
CPU that is not yet completely online. There have been other issues with
hrtimer that have been fixed recently [1]. A possible fix would be to
defer the completion to be done by a CPU already online.
This bug might have been introduced in [2].

[1] https://lore.kernel.org/all/20240913214205.12359-2-frederic@kernel.org/
[2] https://lore.kernel.org/all/169972295552.3135.1094880886431606890.tip-bot2@tip-bot2/
Reported-by: Vlad Poenaru <vlad.wing@gmail.com>
Signed-off-by: Usama Arif <usamaarif642@gmail.com>
---
 kernel/sched/completion.c | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/completion.c b/kernel/sched/completion.c
index 3561ab533dd4..b79d0868f997 100644
--- a/kernel/sched/completion.c
+++ b/kernel/sched/completion.c
@@ -30,6 +30,13 @@ void complete_on_current_cpu(struct completion *x)
 	return complete_with_flags(x, WF_CURRENT_CPU);
 }
 
+static void complete_ipi(void *arg)
+{
+	struct completion *x = arg;
+
+	complete_with_flags(x, 0);
+}
+
 /**
  * complete: - signals a single thread waiting on this completion
  * @x:  holds the state of this particular completion
@@ -44,7 +51,23 @@ void complete_on_current_cpu(struct completion *x)
  */
 void complete(struct completion *x)
 {
-	complete_with_flags(x, 0);
+	int cpu = get_cpu();
+
+	/* The scheduler might queue an ignored hrtimer. Defer the wake up
+	 * to an online CPU instead.
+	 */
+	if (unlikely(cpu_is_offline(cpu))) {
+		int target;
+
+		target = cpumask_any_and(housekeeping_cpumask(HK_TYPE_RCU),
+					 cpu_online_mask);
+
+		smp_call_function_single(target, complete_ipi, x, 1);
+		put_cpu();
+	} else {
+		put_cpu();
+		complete_with_flags(x, 0);
+	}
 }
 EXPORT_SYMBOL(complete);
 
-- 
2.43.5