Re: [scx_nest] bpf_timer_cancel() unusable from select_cpu()

On Fri, 21 Aug 2026 02:22:00 +0000, Zhan Xusheng wrote:
> helpers.c:1515. .select_cpu() qualifies through irqs_disabled():
> try_to_wake_up() holds p->pi_lock across the wakeup via
> scoped_guard(raw_spinlock_irqsave) at kernel/sched/core.c:4302 and calls
> select_task_rq() inside it at 4403, which dispatches through sched_class->select_task_rq (core.c:3629) into select_task_rq_scx() and SCX_CALL_OP_TASK_RET(sch, select_cpu, ...) at kernel/sched/ext/ext.c:3530 and 3558. So a sched_ext callback that runs with IRQs enabled is unaffected, and this is not specific to CPU-pinned timers.

Unfortunately I encountered -EOPNOTSUPP with bpf_timer_cancel(), which
means it ran with IRQs disabled or within hard IRQs.

On Fri, 21 Aug 2026 02:22:00 +0000, Zhan Xusheng wrote:
> If it was -ECANCELED then the sync/async split is the whole story, and the gate is neither sleepability nor sched_ext:

Thanks for your clarification, it turns out that the
bpf_timer_cancel_async() kfunc returned -ECANCELED as you pointed out. I
erroneously use "< 0" to come to a bad conclusion without further
investigation. The gate is not about sleepability, is whether
bpf_timer_cancel can happen on .select_cpu, which might be triggered by
hard IRQs. I have changed my comment on the diff to not mislead other
people.

But I think our current workaround might be more compatible with kernels
have bpf_timer_cancel() only. I think this might not be a good solution:

	#define __COMPAT_scx_bpf_timer_cancel(timer)				\
		(bpf_ksym_exists(scx_bpf_timer_cancel_async)?			\
		bpf_timer_cancel_async((timer)) : bpf_timer_cancel((timer))

	/*
	 * bpf_timer_cancel_async returns -ECANCELED for async cancel.
	 */
	if ((ret = __COMPAT_scx_bpf_timer_cancel(&pcpu_ctx->timer)) < 0 &&
	ret != -ECANCELED)
		scx_bpf_error("Failed to cancel pcpu timer");
	/*
	 * bpf_timer_cancel_async does not need to set callback again.
	 */
	if (ret != -ECANCELED && bpf_timer_set_callback(&pcpu_ctx->timer,
	compact_primary_core))
		scx_bpf_error("Failed to re-arm pcpu timer");

This runs into a problem: On old kernels the macro gives us a synchronous
cancel. On 7.x it gives you an asynchronous one. The scheduler will have
different behavior on different kernels and thus have different
performance.

So I will keep the flag-based one as my workaround.

Thanks,
Yao YiQi
---
diff --git a/scheds/c/scx_nest.bpf.c b/scheds/c/scx_nest.bpf.c
index 2992f90b..b53a0864 100644
--- a/scheds/c/scx_nest.bpf.c
+++ b/scheds/c/scx_nest.bpf.c
@@ -195,16 +195,26 @@ static int compact_primary_core(void *map, int *key, struct bpf_timer *timer)
 	struct pcpu_ctx *pcpu_ctx;
 
 	stat_inc(NEST_STAT(CALLBACK_COMPACTED));
-	/*
-	 * If we made it to this callback, it means that the timer callback was
-	 * never cancelled, and so the core needs to be demoted from the
-	 * primary nest.
-	 */
 	pcpu_ctx = bpf_map_lookup_elem(&pcpu_ctxs, &cpu);
 	if (!pcpu_ctx) {
 		scx_bpf_error("Couldn't lookup pcpu ctx");
 		return 0;
 	}
+
+	/*
+	 * The core may have been re-promoted to the primary nest while this
+	 * timer was pending (see migrate_primary in nest_select_cpu()). We no
+	 * longer cancel the timer from there: select_cpu() runs in the task
+	 * wakeup path, which can be entered with local IRQs disabled or from
+	 * hardirq contexts (e.g. wakeups originating in interrupt handlers),
+	 * where the timer-cancel synchronization isn't available. Instead the
+	 * pending callback detects that scheduled_compaction was cleared and
+	 * bails out. Only demote the core if a compaction is still actually
+	 * scheduled.
+	 */
+	if (!pcpu_ctx->scheduled_compaction)
+		return 0;
+
 	bpf_rcu_read_lock();
 	primary = primary_cpumask;
 	reserve = reserve_cpumask;
@@ -356,11 +366,16 @@ migrate_primary:
 		tctx->prev_misses = 0;
 	pcpu_ctx = bpf_map_lookup_elem(&pcpu_ctxs, &cpu);
 	if (pcpu_ctx) {
+		/*
+		 * A compaction may have been scheduled for this core. Instead of
+		 * cancelling the timer (select_cpu() is entered from the wakeup
+		 * path, which can run with local IRQs disabled or in hardirq
+		 * contexts - not merely "non-sleepable" - so the timer cancel
+		 * isn't usable here), clear scheduled_compaction so that the
+		 * timer callback, if it fires after we land a task here, detects
+		 * it as stale and bails out.
+		 */
 		if (pcpu_ctx->scheduled_compaction) {
-			if (bpf_timer_cancel(&pcpu_ctx->timer) < 0)
-				scx_bpf_error("Failed to cancel pcpu timer");
-			if (bpf_timer_set_callback(&pcpu_ctx->timer, compact_primary_core))
-				scx_bpf_error("Failed to re-arm pcpu timer");
 			pcpu_ctx->scheduled_compaction = false;
 			stat_inc(NEST_STAT(CANCELLED_COMPACTION));
 		}