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, 219 insertions, 0 deletions

diff --git a/drivers/pps/kapi.c b/drivers/pps/kapi.c
new file mode 100644
index 000000000..1bf0335a1
--- /dev/null
+++ b/drivers/pps/kapi.c
@@ -0,0 +1,219 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * kernel API
+ *
+ * Copyright (C) 2005-2009   Rodolfo Giometti <giometti@linux.it>
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/init.h>
+#include <linux/sched.h>
+#include <linux/time.h>
+#include <linux/timex.h>
+#include <linux/spinlock.h>
+#include <linux/fs.h>
+#include <linux/pps_kernel.h>
+#include <linux/slab.h>
+
+#include "kc.h"
+
+/*
+ * Local functions
+ */
+
+static void pps_add_offset(struct pps_ktime *ts, struct pps_ktime *offset)
+{
+	ts->nsec += offset->nsec;
+	while (ts->nsec >= NSEC_PER_SEC) {
+		ts->nsec -= NSEC_PER_SEC;
+		ts->sec++;
+	}
+	while (ts->nsec < 0) {
+		ts->nsec += NSEC_PER_SEC;
+		ts->sec--;
+	}
+	ts->sec += offset->sec;
+}
+
+static void pps_echo_client_default(struct pps_device *pps, int event,
+		void *data)
+{
+	dev_info(&pps->dev, "echo %s %s\n",
+		event & PPS_CAPTUREASSERT ? "assert" : "",
+		event & PPS_CAPTURECLEAR ? "clear" : "");
+}
+
+/*
+ * Exported functions
+ */
+
+/* pps_register_source - add a PPS source in the system
+ * @info: the PPS info struct
+ * @default_params: the default PPS parameters of the new source
+ *
+ * This function is used to add a new PPS source in the system. The new
+ * source is described by info's fields and it will have, as default PPS
+ * parameters, the ones specified into default_params.
+ *
+ * The function returns, in case of success, the PPS device. Otherwise
+ * ERR_PTR(errno).
+ */
+
+struct pps_device *pps_register_source(struct pps_source_info *info,
+		int default_params)
+{
+	struct pps_device *pps;
+	int err;
+
+	/* Sanity checks */
+	if ((info->mode & default_params) != default_params) {
+		pr_err("%s: unsupported default parameters\n",
+					info->name);
+		err = -EINVAL;
+		goto pps_register_source_exit;
+	}
+	if ((info->mode & (PPS_TSFMT_TSPEC | PPS_TSFMT_NTPFP)) == 0) {
+		pr_err("%s: unspecified time format\n",
+					info->name);
+		err = -EINVAL;
+		goto pps_register_source_exit;
+	}
+
+	/* Allocate memory for the new PPS source struct */
+	pps = kzalloc_obj(struct pps_device);
+	if (pps == NULL) {
+		err = -ENOMEM;
+		goto pps_register_source_exit;
+	}
+
+	/* These initializations must be done before calling idr_alloc()
+	 * in order to avoid reces into pps_event().
+	 */
+	pps->params.api_version = PPS_API_VERS;
+	pps->params.mode = default_params;
+	pps->info = *info;
+
+	/* check for default echo function */
+	if ((pps->info.mode & (PPS_ECHOASSERT | PPS_ECHOCLEAR)) &&
+			pps->info.echo == NULL)
+		pps->info.echo = pps_echo_client_default;
+
+	init_waitqueue_head(&pps->queue);
+	spin_lock_init(&pps->lock);
+
+	/* Create the char device */
+	err = pps_register_cdev(pps);
+	if (err < 0) {
+		pr_err("%s: unable to create char device\n",
+					info->name);
+		goto pps_register_source_exit;
+	}
+
+	dev_dbg(&pps->dev, "new PPS source %s\n", info->name);
+
+	return pps;
+
+pps_register_source_exit:
+	pr_err("%s: unable to register source\n", info->name);
+
+	return ERR_PTR(err);
+}
+EXPORT_SYMBOL(pps_register_source);
+
+/* pps_unregister_source - remove a PPS source from the system
+ * @pps: the PPS source
+ *
+ * This function is used to remove a previously registered PPS source from
+ * the system.
+ */
+
+void pps_unregister_source(struct pps_device *pps)
+{
+	pps_kc_remove(pps);
+	pps_unregister_cdev(pps);
+
+	/* don't have to kfree(pps) here because it will be done on
+	 * device destruction */
+}
+EXPORT_SYMBOL(pps_unregister_source);
+
+/* pps_event - register a PPS event into the system
+ * @pps: the PPS device
+ * @ts: the event timestamp
+ * @event: the event type
+ * @data: userdef pointer
+ *
+ * This function is used by each PPS client in order to register a new
+ * PPS event into the system (it's usually called inside an IRQ handler).
+ *
+ * If an echo function is associated with the PPS device it will be called
+ * as:
+ *	pps->info.echo(pps, event, data);
+ */
+void pps_event(struct pps_device *pps, struct pps_event_time *ts, int event,
+		void *data)
+{
+	unsigned long flags;
+	int captured = 0;
+	struct pps_ktime ts_real = { .sec = 0, .nsec = 0, .flags = 0 };
+
+	/* check event type */
+	BUG_ON((event & (PPS_CAPTUREASSERT | PPS_CAPTURECLEAR)) == 0);
+
+	dev_dbg(&pps->dev, "PPS event at %ptSp\n", &ts->ts_real);
+
+	timespec_to_pps_ktime(&ts_real, ts->ts_real);
+
+	spin_lock_irqsave(&pps->lock, flags);
+
+	/* Must call the echo function? */
+	if ((pps->params.mode & (PPS_ECHOASSERT | PPS_ECHOCLEAR)))
+		pps->info.echo(pps, event, data);
+
+	/* Check the event */
+	pps->current_mode = pps->params.mode;
+	if (event & pps->params.mode & PPS_CAPTUREASSERT) {
+		/* We have to add an offset? */
+		if (pps->params.mode & PPS_OFFSETASSERT)
+			pps_add_offset(&ts_real,
+					&pps->params.assert_off_tu);
+
+		/* Save the time stamp */
+		pps->assert_tu = ts_real;
+		pps->assert_sequence++;
+		dev_dbg(&pps->dev, "capture assert seq #%u\n",
+			pps->assert_sequence);
+
+		captured = ~0;
+	}
+	if (event & pps->params.mode & PPS_CAPTURECLEAR) {
+		/* We have to add an offset? */
+		if (pps->params.mode & PPS_OFFSETCLEAR)
+			pps_add_offset(&ts_real,
+					&pps->params.clear_off_tu);
+
+		/* Save the time stamp */
+		pps->clear_tu = ts_real;
+		pps->clear_sequence++;
+		dev_dbg(&pps->dev, "capture clear seq #%u\n",
+			pps->clear_sequence);
+
+		captured = ~0;
+	}
+
+	pps_kc_event(pps, ts, event);
+
+	/* Wake up if captured something */
+	if (captured) {
+		pps->last_ev++;
+		wake_up_interruptible_all(&pps->queue);
+
+		kill_fasync(&pps->async_queue, SIGIO, POLL_IN);
+	}
+
+	spin_unlock_irqrestore(&pps->lock, flags);
+}
+EXPORT_SYMBOL(pps_event);