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, 233 insertions, 0 deletions

diff --git a/arch/alpha/lib/fpreg.c b/arch/alpha/lib/fpreg.c
new file mode 100644
index 000000000..3d3216504
--- /dev/null
+++ b/arch/alpha/lib/fpreg.c
@@ -0,0 +1,233 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * arch/alpha/lib/fpreg.c
+ *
+ * (C) Copyright 1998 Linus Torvalds
+ */
+
+#include <linux/compiler.h>
+#include <linux/export.h>
+#include <linux/preempt.h>
+#include <asm/fpu.h>
+#include <asm/thread_info.h>
+
+#if defined(CONFIG_ALPHA_EV6) || defined(CONFIG_ALPHA_EV67)
+#define STT(reg,val)  asm volatile ("ftoit $f"#reg",%0" : "=r"(val));
+#else
+#define STT(reg,val)  asm volatile ("stt $f"#reg",%0" : "=m"(val));
+#endif
+
+unsigned long
+alpha_read_fp_reg (unsigned long reg)
+{
+	unsigned long val;
+
+	if (unlikely(reg >= 32))
+		return 0;
+	preempt_disable();
+	if (current_thread_info()->status & TS_SAVED_FP)
+		val = current_thread_info()->fp[reg];
+	else switch (reg) {
+	      case  0: STT( 0, val); break;
+	      case  1: STT( 1, val); break;
+	      case  2: STT( 2, val); break;
+	      case  3: STT( 3, val); break;
+	      case  4: STT( 4, val); break;
+	      case  5: STT( 5, val); break;
+	      case  6: STT( 6, val); break;
+	      case  7: STT( 7, val); break;
+	      case  8: STT( 8, val); break;
+	      case  9: STT( 9, val); break;
+	      case 10: STT(10, val); break;
+	      case 11: STT(11, val); break;
+	      case 12: STT(12, val); break;
+	      case 13: STT(13, val); break;
+	      case 14: STT(14, val); break;
+	      case 15: STT(15, val); break;
+	      case 16: STT(16, val); break;
+	      case 17: STT(17, val); break;
+	      case 18: STT(18, val); break;
+	      case 19: STT(19, val); break;
+	      case 20: STT(20, val); break;
+	      case 21: STT(21, val); break;
+	      case 22: STT(22, val); break;
+	      case 23: STT(23, val); break;
+	      case 24: STT(24, val); break;
+	      case 25: STT(25, val); break;
+	      case 26: STT(26, val); break;
+	      case 27: STT(27, val); break;
+	      case 28: STT(28, val); break;
+	      case 29: STT(29, val); break;
+	      case 30: STT(30, val); break;
+	      case 31: STT(31, val); break;
+	}
+	preempt_enable();
+	return val;
+}
+EXPORT_SYMBOL(alpha_read_fp_reg);
+
+#if defined(CONFIG_ALPHA_EV6) || defined(CONFIG_ALPHA_EV67)
+#define LDT(reg,val)  asm volatile ("itoft %0,$f"#reg : : "r"(val));
+#else
+#define LDT(reg,val)  asm volatile ("ldt $f"#reg",%0" : : "m"(val));
+#endif
+
+void
+alpha_write_fp_reg (unsigned long reg, unsigned long val)
+{
+	if (unlikely(reg >= 32))
+		return;
+
+	preempt_disable();
+	if (current_thread_info()->status & TS_SAVED_FP) {
+		current_thread_info()->status |= TS_RESTORE_FP;
+		current_thread_info()->fp[reg] = val;
+	} else switch (reg) {
+	      case  0: LDT( 0, val); break;
+	      case  1: LDT( 1, val); break;
+	      case  2: LDT( 2, val); break;
+	      case  3: LDT( 3, val); break;
+	      case  4: LDT( 4, val); break;
+	      case  5: LDT( 5, val); break;
+	      case  6: LDT( 6, val); break;
+	      case  7: LDT( 7, val); break;
+	      case  8: LDT( 8, val); break;
+	      case  9: LDT( 9, val); break;
+	      case 10: LDT(10, val); break;
+	      case 11: LDT(11, val); break;
+	      case 12: LDT(12, val); break;
+	      case 13: LDT(13, val); break;
+	      case 14: LDT(14, val); break;
+	      case 15: LDT(15, val); break;
+	      case 16: LDT(16, val); break;
+	      case 17: LDT(17, val); break;
+	      case 18: LDT(18, val); break;
+	      case 19: LDT(19, val); break;
+	      case 20: LDT(20, val); break;
+	      case 21: LDT(21, val); break;
+	      case 22: LDT(22, val); break;
+	      case 23: LDT(23, val); break;
+	      case 24: LDT(24, val); break;
+	      case 25: LDT(25, val); break;
+	      case 26: LDT(26, val); break;
+	      case 27: LDT(27, val); break;
+	      case 28: LDT(28, val); break;
+	      case 29: LDT(29, val); break;
+	      case 30: LDT(30, val); break;
+	      case 31: LDT(31, val); break;
+	}
+	preempt_enable();
+}
+EXPORT_SYMBOL(alpha_write_fp_reg);
+
+#if defined(CONFIG_ALPHA_EV6) || defined(CONFIG_ALPHA_EV67)
+#define STS(reg,val)  asm volatile ("ftois $f"#reg",%0" : "=r"(val));
+#else
+#define STS(reg,val)  asm volatile ("sts $f"#reg",%0" : "=m"(val));
+#endif
+
+unsigned long
+alpha_read_fp_reg_s (unsigned long reg)
+{
+	unsigned long val;
+
+	if (unlikely(reg >= 32))
+		return 0;
+
+	preempt_disable();
+	if (current_thread_info()->status & TS_SAVED_FP) {
+		LDT(0, current_thread_info()->fp[reg]);
+		STS(0, val);
+	} else switch (reg) {
+	      case  0: STS( 0, val); break;
+	      case  1: STS( 1, val); break;
+	      case  2: STS( 2, val); break;
+	      case  3: STS( 3, val); break;
+	      case  4: STS( 4, val); break;
+	      case  5: STS( 5, val); break;
+	      case  6: STS( 6, val); break;
+	      case  7: STS( 7, val); break;
+	      case  8: STS( 8, val); break;
+	      case  9: STS( 9, val); break;
+	      case 10: STS(10, val); break;
+	      case 11: STS(11, val); break;
+	      case 12: STS(12, val); break;
+	      case 13: STS(13, val); break;
+	      case 14: STS(14, val); break;
+	      case 15: STS(15, val); break;
+	      case 16: STS(16, val); break;
+	      case 17: STS(17, val); break;
+	      case 18: STS(18, val); break;
+	      case 19: STS(19, val); break;
+	      case 20: STS(20, val); break;
+	      case 21: STS(21, val); break;
+	      case 22: STS(22, val); break;
+	      case 23: STS(23, val); break;
+	      case 24: STS(24, val); break;
+	      case 25: STS(25, val); break;
+	      case 26: STS(26, val); break;
+	      case 27: STS(27, val); break;
+	      case 28: STS(28, val); break;
+	      case 29: STS(29, val); break;
+	      case 30: STS(30, val); break;
+	      case 31: STS(31, val); break;
+	}
+	preempt_enable();
+	return val;
+}
+EXPORT_SYMBOL(alpha_read_fp_reg_s);
+
+#if defined(CONFIG_ALPHA_EV6) || defined(CONFIG_ALPHA_EV67)
+#define LDS(reg,val)  asm volatile ("itofs %0,$f"#reg : : "r"(val));
+#else
+#define LDS(reg,val)  asm volatile ("lds $f"#reg",%0" : : "m"(val));
+#endif
+
+void
+alpha_write_fp_reg_s (unsigned long reg, unsigned long val)
+{
+	if (unlikely(reg >= 32))
+		return;
+
+	preempt_disable();
+	if (current_thread_info()->status & TS_SAVED_FP) {
+		current_thread_info()->status |= TS_RESTORE_FP;
+		LDS(0, val);
+		STT(0, current_thread_info()->fp[reg]);
+	} else switch (reg) {
+	      case  0: LDS( 0, val); break;
+	      case  1: LDS( 1, val); break;
+	      case  2: LDS( 2, val); break;
+	      case  3: LDS( 3, val); break;
+	      case  4: LDS( 4, val); break;
+	      case  5: LDS( 5, val); break;
+	      case  6: LDS( 6, val); break;
+	      case  7: LDS( 7, val); break;
+	      case  8: LDS( 8, val); break;
+	      case  9: LDS( 9, val); break;
+	      case 10: LDS(10, val); break;
+	      case 11: LDS(11, val); break;
+	      case 12: LDS(12, val); break;
+	      case 13: LDS(13, val); break;
+	      case 14: LDS(14, val); break;
+	      case 15: LDS(15, val); break;
+	      case 16: LDS(16, val); break;
+	      case 17: LDS(17, val); break;
+	      case 18: LDS(18, val); break;
+	      case 19: LDS(19, val); break;
+	      case 20: LDS(20, val); break;
+	      case 21: LDS(21, val); break;
+	      case 22: LDS(22, val); break;
+	      case 23: LDS(23, val); break;
+	      case 24: LDS(24, val); break;
+	      case 25: LDS(25, val); break;
+	      case 26: LDS(26, val); break;
+	      case 27: LDS(27, val); break;
+	      case 28: LDS(28, val); break;
+	      case 29: LDS(29, val); break;
+	      case 30: LDS(30, val); break;
+	      case 31: LDS(31, val); break;
+	}
+	preempt_enable();
+}
+EXPORT_SYMBOL(alpha_write_fp_reg_s);