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, 190 insertions, 0 deletions

diff --git a/arch/alpha/boot/main.c b/arch/alpha/boot/main.c
new file mode 100644
index 000000000..22a1cb026
--- /dev/null
+++ b/arch/alpha/boot/main.c
@@ -0,0 +1,190 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * arch/alpha/boot/main.c
+ *
+ * Copyright (C) 1994, 1995 Linus Torvalds
+ *
+ * This file is the bootloader for the Linux/AXP kernel
+ */
+#include <linux/kernel.h>
+#include <linux/slab.h>
+#include <linux/string.h>
+#include <generated/utsrelease.h>
+#include <linux/mm.h>
+
+#include <asm/console.h>
+#include <asm/hwrpb.h>
+
+#include <linux/stdarg.h>
+
+#include "ksize.h"
+
+extern unsigned long switch_to_osf_pal(unsigned long nr,
+	struct pcb_struct * pcb_va, struct pcb_struct * pcb_pa,
+	unsigned long *vptb);
+struct hwrpb_struct *hwrpb = INIT_HWRPB;
+static struct pcb_struct pcb_va[1];
+
+/*
+ * Find a physical address of a virtual object..
+ *
+ * This is easy using the virtual page table address.
+ */
+
+static inline void *
+find_pa(unsigned long *vptb, void *ptr)
+{
+	unsigned long address = (unsigned long) ptr;
+	unsigned long result;
+
+	result = vptb[address >> 13];
+	result >>= 32;
+	result <<= 13;
+	result |= address & 0x1fff;
+	return (void *) result;
+}	
+
+/*
+ * This function moves into OSF/1 pal-code, and has a temporary
+ * PCB for that. The kernel proper should replace this PCB with
+ * the real one as soon as possible.
+ *
+ * The page table muckery in here depends on the fact that the boot
+ * code has the L1 page table identity-map itself in the second PTE
+ * in the L1 page table. Thus the L1-page is virtually addressable
+ * itself (through three levels) at virtual address 0x200802000.
+ */
+
+#define VPTB	((unsigned long *) 0x200000000)
+#define L1	((unsigned long *) 0x200802000)
+
+void
+pal_init(void)
+{
+	unsigned long i, rev;
+	struct percpu_struct * percpu;
+	struct pcb_struct * pcb_pa;
+
+	/* Create the dummy PCB.  */
+	pcb_va->ksp = 0;
+	pcb_va->usp = 0;
+	pcb_va->ptbr = L1[1] >> 32;
+	pcb_va->asn = 0;
+	pcb_va->pcc = 0;
+	pcb_va->unique = 0;
+	pcb_va->flags = 1;
+	pcb_va->res1 = 0;
+	pcb_va->res2 = 0;
+	pcb_pa = find_pa(VPTB, pcb_va);
+
+	/*
+	 * a0 = 2 (OSF)
+	 * a1 = return address, but we give the asm the vaddr of the PCB
+	 * a2 = physical addr of PCB
+	 * a3 = new virtual page table pointer
+	 * a4 = KSP (but the asm sets it)
+	 */
+	srm_printk("Switching to OSF PAL-code .. ");
+
+	i = switch_to_osf_pal(2, pcb_va, pcb_pa, VPTB);
+	if (i) {
+		srm_printk("failed, code %ld\n", i);
+		__halt();
+	}
+
+	percpu = (struct percpu_struct *)
+		(INIT_HWRPB->processor_offset + (unsigned long) INIT_HWRPB);
+	rev = percpu->pal_revision = percpu->palcode_avail[2];
+
+	srm_printk("Ok (rev %lx)\n", rev);
+
+	tbia(); /* do it directly in case we are SMP */
+}
+
+static inline long openboot(void)
+{
+	char bootdev[256];
+	long result;
+
+	result = callback_getenv(ENV_BOOTED_DEV, bootdev, 255);
+	if (result < 0)
+		return result;
+	return callback_open(bootdev, result & 255);
+}
+
+static inline long close(long dev)
+{
+	return callback_close(dev);
+}
+
+static inline long load(long dev, unsigned long addr, unsigned long count)
+{
+	char bootfile[256];
+	extern char _end;
+	long result, boot_size = &_end - (char *) BOOT_ADDR;
+
+	result = callback_getenv(ENV_BOOTED_FILE, bootfile, 255);
+	if (result < 0)
+		return result;
+	result &= 255;
+	bootfile[result] = '\0';
+	if (result)
+		srm_printk("Boot file specification (%s) not implemented\n",
+		       bootfile);
+	return callback_read(dev, count, (void *)addr, boot_size/512 + 1);
+}
+
+/*
+ * Start the kernel.
+ */
+static void runkernel(void)
+{
+	__asm__ __volatile__(
+		"bis %1,%1,$30\n\t"
+		"bis %0,%0,$26\n\t"
+		"ret ($26)"
+		: /* no outputs: it doesn't even return */
+		: "r" (START_ADDR),
+		  "r" (PAGE_SIZE + INIT_STACK));
+}
+
+void start_kernel(void)
+{
+	long i;
+	long dev;
+	int nbytes;
+	char envval[256];
+
+	srm_printk("Linux/AXP bootloader for Linux " UTS_RELEASE "\n");
+	if (INIT_HWRPB->pagesize != 8192) {
+		srm_printk("Expected 8kB pages, got %ldkB\n", INIT_HWRPB->pagesize >> 10);
+		return;
+	}
+	pal_init();
+	dev = openboot();
+	if (dev < 0) {
+		srm_printk("Unable to open boot device: %016lx\n", dev);
+		return;
+	}
+	dev &= 0xffffffff;
+	srm_printk("Loading vmlinux ...");
+	i = load(dev, START_ADDR, KERNEL_SIZE);
+	close(dev);
+	if (i != KERNEL_SIZE) {
+		srm_printk("Failed (%lx)\n", i);
+		return;
+	}
+
+	nbytes = callback_getenv(ENV_BOOTED_OSFLAGS, envval, sizeof(envval));
+	if (nbytes < 0) {
+		nbytes = 0;
+	}
+	envval[nbytes] = '\0';
+	strcpy((char*)ZERO_PGE, envval);
+
+	srm_printk(" Ok\nNow booting the kernel\n");
+	runkernel();
+	for (i = 0 ; i < 0x100000000 ; i++)
+		/* nothing */;
+	__halt();
+}