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, 220 insertions, 0 deletions

diff --git a/arch/m68k/atari/stdma.c b/arch/m68k/atari/stdma.c
new file mode 100644
index 000000000..155fefff1
--- /dev/null
+++ b/arch/m68k/atari/stdma.c
@@ -0,0 +1,220 @@
+/*
+ *  linux/arch/m68k/atari/stmda.c
+ *
+ *  Copyright (C) 1994 Roman Hodek
+ *
+ *
+ * This file is subject to the terms and conditions of the GNU General Public
+ * License.  See the file COPYING in the main directory of this archive
+ * for more details.
+ */
+
+
+/* This file contains some function for controlling the access to the  */
+/* ST-DMA chip that may be shared between devices. Currently we have:  */
+/*   TT:     Floppy and ACSI bus                                       */
+/*   Falcon: Floppy and SCSI                                           */
+/*                                                                     */
+/* The controlling functions set up a wait queue for access to the     */
+/* ST-DMA chip. Callers to stdma_lock() that cannot granted access are */
+/* put onto a queue and waked up later if the owner calls              */
+/* stdma_release(). Additionally, the caller gives his interrupt       */
+/* service routine to stdma_lock().                                    */
+/*                                                                     */
+/* On the Falcon, the IDE bus uses just the ACSI/Floppy interrupt, but */
+/* not the ST-DMA chip itself. So falhd.c needs not to lock the        */
+/* chip. The interrupt is routed to falhd.c if IDE is configured, the  */
+/* model is a Falcon and the interrupt was caused by the HD controller */
+/* (can be determined by looking at its status register).              */
+
+
+#include <linux/types.h>
+#include <linux/kdev_t.h>
+#include <linux/sched.h>
+#include <linux/init.h>
+#include <linux/interrupt.h>
+#include <linux/wait.h>
+#include <linux/module.h>
+
+#include <asm/atari_stdma.h>
+#include <asm/atariints.h>
+#include <asm/atarihw.h>
+#include <asm/io.h>
+#include <asm/irq.h>
+
+static int stdma_locked;			/* the semaphore */
+						/* int func to be called */
+static irq_handler_t stdma_isr;
+static void *stdma_isr_data;			/* data passed to isr */
+static DECLARE_WAIT_QUEUE_HEAD(stdma_wait);	/* wait queue for ST-DMA */
+
+
+
+
+/***************************** Prototypes *****************************/
+
+static irqreturn_t stdma_int (int irq, void *dummy);
+
+/************************* End of Prototypes **************************/
+
+
+/**
+ * stdma_try_lock - attempt to acquire ST DMA interrupt "lock"
+ * @handler: interrupt handler to use after acquisition
+ * @data: cookie passed to the interrupt handler function
+ *
+ * Returns !0 if lock was acquired; otherwise 0.
+ */
+
+int stdma_try_lock(irq_handler_t handler, void *data)
+{
+	unsigned long flags;
+
+	local_irq_save(flags);
+	if (stdma_locked) {
+		local_irq_restore(flags);
+		return 0;
+	}
+
+	stdma_locked   = 1;
+	stdma_isr      = handler;
+	stdma_isr_data = data;
+	local_irq_restore(flags);
+	return 1;
+}
+EXPORT_SYMBOL(stdma_try_lock);
+
+
+/*
+ * Function: void stdma_lock( isrfunc isr, void *data )
+ *
+ * Purpose: Tries to get a lock on the ST-DMA chip that is used by more
+ *   then one device driver. Waits on stdma_wait until lock is free.
+ *   stdma_lock() may not be called from an interrupt! You have to
+ *   get the lock in your main routine and release it when your
+ *   request is finished.
+ *
+ * Inputs: A interrupt function that is called until the lock is
+ *   released.
+ *
+ * Returns: nothing
+ *
+ */
+
+void stdma_lock(irq_handler_t handler, void *data)
+{
+	/* Since the DMA is used for file system purposes, we
+	 have to sleep uninterruptible (there may be locked
+	 buffers) */
+	wait_event(stdma_wait, stdma_try_lock(handler, data));
+}
+EXPORT_SYMBOL(stdma_lock);
+
+
+/*
+ * Function: void stdma_release( void )
+ *
+ * Purpose: Releases the lock on the ST-DMA chip.
+ *
+ * Inputs: none
+ *
+ * Returns: nothing
+ *
+ */
+
+void stdma_release(void)
+{
+	unsigned long flags;
+
+	local_irq_save(flags);
+
+	stdma_locked   = 0;
+	stdma_isr      = NULL;
+	stdma_isr_data = NULL;
+	wake_up(&stdma_wait);
+
+	local_irq_restore(flags);
+}
+EXPORT_SYMBOL(stdma_release);
+
+
+/**
+ * stdma_is_locked_by - allow lock holder to check whether it needs to release.
+ * @handler: interrupt handler previously used to acquire lock.
+ *
+ * Returns !0 if locked for the given handler; 0 otherwise.
+ */
+
+int stdma_is_locked_by(irq_handler_t handler)
+{
+	unsigned long flags;
+	int result;
+
+	local_irq_save(flags);
+	result = stdma_locked && (stdma_isr == handler);
+	local_irq_restore(flags);
+
+	return result;
+}
+EXPORT_SYMBOL(stdma_is_locked_by);
+
+
+/*
+ * Function: int stdma_islocked( void )
+ *
+ * Purpose: Check if the ST-DMA is currently locked.
+ * Note: Returned status is only valid if ints are disabled while calling and
+ *       as long as they remain disabled.
+ *       If called with ints enabled, status can change only from locked to
+ *       unlocked, because ints may not lock the ST-DMA.
+ *
+ * Inputs: none
+ *
+ * Returns: != 0 if locked, 0 otherwise
+ *
+ */
+
+int stdma_islocked(void)
+{
+	return stdma_locked;
+}
+EXPORT_SYMBOL(stdma_islocked);
+
+
+/*
+ * Function: void stdma_init( void )
+ *
+ * Purpose: Initialize the ST-DMA chip access controlling.
+ *   It sets up the interrupt and its service routine. The int is registered
+ *   as slow int, client devices have to live with that (no problem
+ *   currently).
+ *
+ * Inputs: none
+ *
+ * Return: nothing
+ *
+ */
+
+void __init stdma_init(void)
+{
+	stdma_isr = NULL;
+	if (request_irq(IRQ_MFP_FDC, stdma_int, IRQF_SHARED,
+			"ST-DMA floppy,ACSI,IDE,Falcon-SCSI", stdma_int))
+		pr_err("Couldn't register ST-DMA interrupt\n");
+}
+
+
+/*
+ * Function: void stdma_int()
+ *
+ * Purpose: The interrupt routine for the ST-DMA. It calls the isr
+ *   registered by stdma_lock().
+ *
+ */
+
+static irqreturn_t stdma_int(int irq, void *dummy)
+{
+  if (stdma_isr)
+      (*stdma_isr)(irq, stdma_isr_data);
+  return IRQ_HANDLED;
+}