percpu_ida: add an API to return free tags

Add an API to return free tags, blk-mq-tag will use it.

Note, this just returns a snapshot of free tags number. blk-mq-tag has
two usages of it. One is for info output for diagnosis. The other is to
quickly check if there are free tags for request dispatch checking.
Neither requires very precise.

Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Shaohua Li <shli@fusionio.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/percpu_ida.h

@@ -77,4 +77,5 @@ typedef int (*percpu_ida_cb)(unsigned, void *);
 int percpu_ida_for_each_free(struct percpu_ida *pool, percpu_ida_cb fn,
 	void *data);
 
+unsigned percpu_ida_free_tags(struct percpu_ida *pool, int cpu);
 #endif /* __PERCPU_IDA_H__ */

lib/percpu_ida.c

@@ -371,3 +371,20 @@ int percpu_ida_for_each_free(struct percpu_ida *pool, percpu_ida_cb fn,
 	return err;
 }
 EXPORT_SYMBOL_GPL(percpu_ida_for_each_free);
+
+/**
+ * percpu_ida_free_tags - return free tags number of a specific cpu or global pool
+ * @pool: pool related
+ * @cpu: specific cpu or global pool if @cpu == nr_cpu_ids
+ *
+ * Note: this just returns a snapshot of free tags number.
+ */
+unsigned percpu_ida_free_tags(struct percpu_ida *pool, int cpu)
+{
+	struct percpu_ida_cpu *remote;
+	if (cpu == nr_cpu_ids)
+		return pool->nr_free;
+	remote = per_cpu_ptr(pool->tag_cpu, cpu);
+	return remote->nr_free;
+}
+EXPORT_SYMBOL_GPL(percpu_ida_free_tags);