sched_ext: Add scx_cmask_test() and scx_cmask_for_each_cid()

Add single-bit test and iterator over set cids in an scx_cmask.

v2: Bound scx_cmask_for_each_cid() to the active span. (sashiko AI)

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>

kernel/sched/ext_cid.h

@@ -225,4 +225,47 @@ static inline void __scx_cmask_set(u32 cid, struct scx_cmask *m)
 	*__scx_cmask_word(cid, m) |= BIT_U64(cid & 63);
 }
 
+/**
+ * scx_cmask_test - test whether @cid is set in @m
+ * @cid: cid to test
+ * @m: cmask to test
+ *
+ * Return %false if @cid is outside @m's active range. Otherwise return the
+ * bit's value. Read via READ_ONCE so callers can race set/clear writers.
+ */
+static inline bool scx_cmask_test(u32 cid, const struct scx_cmask *m)
+{
+	if (!__scx_cmask_contains(cid, m))
+		return false;
+	return READ_ONCE(*__scx_cmask_word(cid, m)) & BIT_U64(cid & 63);
+}
+
+/*
+ * Words of bits[] the active range spans, 0 if empty. Tighter than the storage
+ * SCX_CMASK_NR_WORDS() sizes for the worst-case base alignment.
+ */
+static inline u32 scx_cmask_nr_used_words(const struct scx_cmask *m)
+{
+	if (!m->nr_cids)
+		return 0;
+	return ((m->base & 63) + m->nr_cids - 1) / 64 + 1;
+}
+
+/**
+ * scx_cmask_for_each_cid - iterate set cids in @m
+ * @cid: s32 loop var that receives each set cid in turn
+ * @m: cmask to iterate
+ *
+ * Visits set bits within @m's active range in ascending order. Scans only the
+ * words the active range spans, where head and tail padding is kept zero, so
+ * no per-cid range check is needed.
+ */
+#define scx_cmask_for_each_cid(cid, m)						\
+	for (u64 __bs = (m)->base & ~63u, __wi = 0,				\
+		     __nw = scx_cmask_nr_used_words(m);				\
+	     __wi < __nw; __wi++)						\
+		for (u64 __w = READ_ONCE((m)->bits[__wi]);			\
+		     __w && ((cid) = __bs + __wi * 64 + __ffs64(__w), true);	\
+		     __w &= __w - 1)
+
 #endif /* _KERNEL_SCHED_EXT_CID_H */