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, 237 insertions, 0 deletions

diff --git a/tools/perf/util/comm.c b/tools/perf/util/comm.c
new file mode 100644
index 000000000..9880247a2
--- /dev/null
+++ b/tools/perf/util/comm.c
@@ -0,0 +1,237 @@
+// SPDX-License-Identifier: GPL-2.0
+#include "comm.h"
+#include <errno.h>
+#include <string.h>
+#include <internal/rc_check.h>
+#include <linux/refcount.h>
+#include <linux/zalloc.h>
+#include <tools/libc_compat.h> // reallocarray
+
+#include "rwsem.h"
+
+DECLARE_RC_STRUCT(comm_str) {
+	refcount_t refcnt;
+	char str[];
+};
+
+static struct comm_strs {
+	struct rw_semaphore lock;
+	struct comm_str **strs;
+	int num_strs;
+	int capacity;
+} _comm_strs;
+
+static void comm_strs__remove_if_last(struct comm_str *cs);
+
+static void comm_strs__init(void)
+	NO_THREAD_SAFETY_ANALYSIS /* Inherently single threaded due to pthread_once. */
+{
+	init_rwsem(&_comm_strs.lock);
+	_comm_strs.capacity = 16;
+	_comm_strs.num_strs = 0;
+	_comm_strs.strs = calloc(16, sizeof(*_comm_strs.strs));
+}
+
+static struct comm_strs *comm_strs__get(void)
+{
+	static pthread_once_t comm_strs_type_once = PTHREAD_ONCE_INIT;
+
+	pthread_once(&comm_strs_type_once, comm_strs__init);
+
+	return &_comm_strs;
+}
+
+static refcount_t *comm_str__refcnt(struct comm_str *cs)
+{
+	return &RC_CHK_ACCESS(cs)->refcnt;
+}
+
+static const char *comm_str__str(const struct comm_str *cs)
+{
+	return &RC_CHK_ACCESS(cs)->str[0];
+}
+
+static struct comm_str *comm_str__get(struct comm_str *cs)
+{
+	struct comm_str *result;
+
+	if (RC_CHK_GET(result, cs))
+		refcount_inc_not_zero(comm_str__refcnt(cs));
+
+	return result;
+}
+
+static void comm_str__put(struct comm_str *cs)
+{
+	if (!cs)
+		return;
+
+	if (refcount_dec_and_test(comm_str__refcnt(cs))) {
+		RC_CHK_FREE(cs);
+	} else {
+		if (refcount_read(comm_str__refcnt(cs)) == 1)
+			comm_strs__remove_if_last(cs);
+
+		RC_CHK_PUT(cs);
+	}
+}
+
+static struct comm_str *comm_str__new(const char *str)
+{
+	struct comm_str *result = NULL;
+	RC_STRUCT(comm_str) *cs;
+
+	cs = malloc(sizeof(*cs) + strlen(str) + 1);
+	if (ADD_RC_CHK(result, cs)) {
+		refcount_set(comm_str__refcnt(result), 1);
+		strcpy(&cs->str[0], str);
+	}
+	return result;
+}
+
+static int comm_str__search(const void *_key, const void *_member)
+{
+	const char *key = _key;
+	const struct comm_str *member = *(const struct comm_str * const *)_member;
+
+	return strcmp(key, comm_str__str(member));
+}
+
+static void comm_strs__remove_if_last(struct comm_str *cs)
+{
+	struct comm_strs *comm_strs = comm_strs__get();
+
+	down_write(&comm_strs->lock);
+	/*
+	 * Are there only references from the array, if so remove the array
+	 * reference under the write lock so that we don't race with findnew.
+	 */
+	if (refcount_read(comm_str__refcnt(cs)) == 1) {
+		struct comm_str **entry;
+
+		entry = bsearch(comm_str__str(cs), comm_strs->strs, comm_strs->num_strs,
+				sizeof(struct comm_str *), comm_str__search);
+		comm_str__put(*entry);
+		for (int i = entry - comm_strs->strs; i < comm_strs->num_strs - 1; i++)
+			comm_strs->strs[i] = comm_strs->strs[i + 1];
+		comm_strs->num_strs--;
+	}
+	up_write(&comm_strs->lock);
+}
+
+static struct comm_str *__comm_strs__find(struct comm_strs *comm_strs, const char *str)
+	SHARED_LOCKS_REQUIRED(comm_strs->lock)
+{
+	struct comm_str **result;
+
+	result = bsearch(str, comm_strs->strs, comm_strs->num_strs, sizeof(struct comm_str *),
+			 comm_str__search);
+
+	if (!result)
+		return NULL;
+
+	return comm_str__get(*result);
+}
+
+static struct comm_str *comm_strs__findnew(const char *str)
+{
+	struct comm_strs *comm_strs = comm_strs__get();
+	struct comm_str *result;
+
+	if (!comm_strs)
+		return NULL;
+
+	down_read(&comm_strs->lock);
+	result = __comm_strs__find(comm_strs, str);
+	up_read(&comm_strs->lock);
+	if (result)
+		return result;
+
+	down_write(&comm_strs->lock);
+	result = __comm_strs__find(comm_strs, str);
+	if (!result) {
+		if (comm_strs->num_strs == comm_strs->capacity) {
+			struct comm_str **tmp;
+
+			tmp = reallocarray(comm_strs->strs,
+					   comm_strs->capacity + 16,
+					   sizeof(*comm_strs->strs));
+			if (!tmp) {
+				up_write(&comm_strs->lock);
+				return NULL;
+			}
+			comm_strs->strs = tmp;
+			comm_strs->capacity += 16;
+		}
+		result = comm_str__new(str);
+		if (result) {
+			int low = 0, high = comm_strs->num_strs - 1;
+			int insert = comm_strs->num_strs; /* Default to inserting at the end. */
+
+			while (low <= high) {
+				int mid = low + (high - low) / 2;
+				int cmp = strcmp(comm_str__str(comm_strs->strs[mid]), str);
+
+				if (cmp < 0) {
+					low = mid + 1;
+				} else {
+					high = mid - 1;
+					insert = mid;
+				}
+			}
+			memmove(&comm_strs->strs[insert + 1], &comm_strs->strs[insert],
+				(comm_strs->num_strs - insert) * sizeof(struct comm_str *));
+			comm_strs->num_strs++;
+			comm_strs->strs[insert] = result;
+		}
+	}
+	up_write(&comm_strs->lock);
+	return comm_str__get(result);
+}
+
+struct comm *comm__new(const char *str, u64 timestamp, bool exec)
+{
+	struct comm *comm = zalloc(sizeof(*comm));
+
+	if (!comm)
+		return NULL;
+
+	comm->start = timestamp;
+	comm->exec = exec;
+
+	comm->comm_str = comm_strs__findnew(str);
+	if (!comm->comm_str) {
+		free(comm);
+		return NULL;
+	}
+
+	return comm;
+}
+
+int comm__override(struct comm *comm, const char *str, u64 timestamp, bool exec)
+{
+	struct comm_str *new, *old = comm->comm_str;
+
+	new = comm_strs__findnew(str);
+	if (!new)
+		return -ENOMEM;
+
+	comm_str__put(old);
+	comm->comm_str = new;
+	comm->start = timestamp;
+	if (exec)
+		comm->exec = true;
+
+	return 0;
+}
+
+void comm__free(struct comm *comm)
+{
+	comm_str__put(comm->comm_str);
+	free(comm);
+}
+
+const char *comm__str(const struct comm *comm)
+{
+	return comm_str__str(comm->comm_str);
+}