sched_ext: Maintain an online cid mask in the scheduler arenagrafted

Schedulers on the default cid mapping treat [0, nr_online_cids) as the
online set and restart on hotplug. Schedulers that install their own mapping
with scx_bpf_cid_override() have no way to learn which cids are online: the
count no longer identifies members and the CPU-form cpumask is unusable from
cid programs. This is an obvious hole in the cid API.

Add scx_bpf_online_cmask(), a kernel-maintained cmask in the scheduler's
arena, allocated alongside the per-CPU scratch masks and populated after the
cid mapping is finalized and before ops.init(), for child schedulers too.
The pointer stays valid through ops.exit() with no reference to take. It is
the arena offset as a void pointer, the same form struct_ops arena arguments
arrive in. The verifier types the void return as a scalar for the program's
arena cast.

The mask follows the SCX hotplug notifications: seeded from cpu_active_mask
and updated before ops.cid_online/offline() runs, so it lags cpu_online_mask
only inside a hotplug transition. Updates walk the scheduler list under the
lock that also serializes unlinking. Reads are live, not atomic snapshots.
Root initialization excludes hotplug.

v2: Reworded the getter kerneldoc (Andrea Righi).

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

1 files changed, 132 insertions, 0 deletions

diff --git a/lib/nmi_backtrace.c b/lib/nmi_backtrace.c
new file mode 100644
index 000000000..a3bfa9360
--- /dev/null
+++ b/lib/nmi_backtrace.c
@@ -0,0 +1,132 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ *  NMI backtrace support
+ *
+ * Gratuitously copied from arch/x86/kernel/apic/hw_nmi.c by Russell King,
+ * with the following header:
+ *
+ *  HW NMI watchdog support
+ *
+ *  started by Don Zickus, Copyright (C) 2010 Red Hat, Inc.
+ *
+ *  Arch specific calls to support NMI watchdog
+ *
+ *  Bits copied from original nmi.c file
+ */
+#include <linux/cpumask.h>
+#include <linux/delay.h>
+#include <linux/kprobes.h>
+#include <linux/stringify.h>
+#include <linux/nmi.h>
+#include <linux/cpu.h>
+#include <linux/sched/debug.h>
+
+#ifdef arch_trigger_cpumask_backtrace
+/* For reliability, we're prepared to waste bits here. */
+static DECLARE_BITMAP(backtrace_mask, NR_CPUS) __read_mostly;
+
+/* "in progress" flag of arch_trigger_cpumask_backtrace */
+static unsigned long backtrace_flag;
+
+#define NMI_BT_TIMEOUT_SEC	10
+
+/*
+ * When raise() is called it will be passed a pointer to the
+ * backtrace_mask. Architectures that call nmi_cpu_backtrace()
+ * directly from their raise() functions may rely on the mask
+ * they are passed being updated as a side effect of this call.
+ */
+void nmi_trigger_cpumask_backtrace(const cpumask_t *mask,
+				   int exclude_cpu,
+				   void (*raise)(cpumask_t *mask))
+{
+	int i, this_cpu = get_cpu();
+
+	if (test_and_set_bit(0, &backtrace_flag)) {
+		/*
+		 * If there is already a trigger_all_cpu_backtrace() in progress
+		 * (backtrace_flag == 1), don't output double cpu dump infos.
+		 */
+		put_cpu();
+		return;
+	}
+
+	cpumask_copy(to_cpumask(backtrace_mask), mask);
+	if (exclude_cpu != -1)
+		cpumask_clear_cpu(exclude_cpu, to_cpumask(backtrace_mask));
+
+	/*
+	 * Don't try to send an NMI to this cpu; it may work on some
+	 * architectures, but on others it may not, and we'll get
+	 * information at least as useful just by doing a dump_stack() here.
+	 * Note that nmi_cpu_backtrace(NULL) will clear the cpu bit.
+	 */
+	if (cpumask_test_cpu(this_cpu, to_cpumask(backtrace_mask)))
+		nmi_cpu_backtrace(NULL);
+
+	if (!cpumask_empty(to_cpumask(backtrace_mask))) {
+		pr_info("Sending NMI from CPU %d to CPUs %*pbl:\n",
+			this_cpu, nr_cpumask_bits, to_cpumask(backtrace_mask));
+		nmi_backtrace_stall_snap(to_cpumask(backtrace_mask));
+		raise(to_cpumask(backtrace_mask));
+	}
+
+	/* Wait for up to NMI_BT_TIMEOUT_SEC seconds for all CPUs to do the backtrace */
+	for (i = 0; i < NMI_BT_TIMEOUT_SEC * 1000; i++) {
+		if (cpumask_empty(to_cpumask(backtrace_mask)))
+			break;
+		mdelay(1);
+		touch_softlockup_watchdog();
+	}
+
+	if (!cpumask_empty(to_cpumask(backtrace_mask))) {
+		pr_warn("After " __stringify(NMI_BT_TIMEOUT_SEC) " seconds, these CPUS still haven't responded to the NMI: %*pbl\n",
+			cpumask_pr_args(to_cpumask(backtrace_mask)));
+
+		nmi_backtrace_stall_check(to_cpumask(backtrace_mask));
+	}
+
+	/*
+	 * Force flush any remote buffers that might be stuck in IRQ context
+	 * and therefore could not run their irq_work.
+	 */
+	printk_trigger_flush();
+
+	clear_bit_unlock(0, &backtrace_flag);
+	put_cpu();
+}
+
+// Dump stacks even for idle CPUs.
+static bool backtrace_idle;
+module_param(backtrace_idle, bool, 0644);
+
+bool nmi_cpu_backtrace(struct pt_regs *regs)
+{
+	int cpu = smp_processor_id();
+	unsigned long flags;
+
+	if (cpumask_test_cpu(cpu, to_cpumask(backtrace_mask))) {
+		/*
+		 * Allow nested NMI backtraces while serializing
+		 * against other CPUs.
+		 */
+		printk_cpu_sync_get_irqsave(flags);
+		if (!READ_ONCE(backtrace_idle) && regs && cpu_in_idle(instruction_pointer(regs))) {
+			pr_warn("NMI backtrace for cpu %d skipped: idling at %pS\n",
+				cpu, (void *)instruction_pointer(regs));
+		} else {
+			pr_warn("NMI backtrace for cpu %d\n", cpu);
+			if (regs)
+				show_regs(regs);
+			else
+				dump_stack();
+		}
+		printk_cpu_sync_put_irqrestore(flags);
+		cpumask_clear_cpu(cpu, to_cpumask(backtrace_mask));
+		return true;
+	}
+
+	return false;
+}
+NOKPROBE_SYMBOL(nmi_cpu_backtrace);
+#endif