zvol: Enable zvol threading functionality on FreeBSD

Make zvol I/O requests processing asynchronous on FreeBSD side in some
cases. Clone zvol threading logic and required module parameters from
Linux side. Make zvol threadpool creation/destruction logic shared for
both Linux and FreeBSD.
The IO requests are processed asynchronously in next cases:
- volmode=geom: if IO request thread is geom thread or cannot sleep.
- volmode=cdev: if IO request passed thru struct cdevsw .d_strategy
routine, mean is AIO request.
In all other cases the IO requests are processed synchronously. The
volthreading zvol property is ignored on FreeBSD side.

Sponsored-by: vStack, Inc.
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: @ImAwsumm
Signed-off-by: Fedor Uporov <fuporov.vstack@gmail.com>
Closes #17169

include/sys/zvol_impl.h

@@ -60,6 +60,32 @@ typedef struct zvol_state {
 	boolean_t		zv_threading;	/* volthreading property */
 } zvol_state_t;
 
+/*
+ * zvol taskqs
+ */
+typedef struct zv_taskq {
+	uint_t tqs_cnt;
+	taskq_t **tqs_taskq;
+} zv_taskq_t;
+
+typedef struct zv_request_stack {
+	zvol_state_t	*zv;
+	struct bio	*bio;
+#ifdef __linux__
+	struct request	*rq;
+#endif
+} zv_request_t;
+
+typedef struct zv_request_task {
+	zv_request_t	zvr;
+	taskq_ent_t	ent;
+} zv_request_task_t;
+
+/*
+ * Switch taskq at multiple of 512 MB offset. This can be set to a lower value
+ * to utilize more threads for small files but may affect prefetch hits.
+ */
+#define	ZVOL_TASKQ_OFFSET_SHIFT 29
 
 extern krwlock_t zvol_state_lock;
 #define	ZVOL_HT_SIZE	1024
@@ -69,6 +95,10 @@ extern zil_replay_func_t *const zvol_replay_vector[TX_MAX_TYPE];
 
 extern unsigned int zvol_volmode;
 extern unsigned int zvol_inhibit_dev;
+extern unsigned int zvol_threads;
+extern unsigned int zvol_num_taskqs;
+extern unsigned int zvol_request_sync;
+extern zv_taskq_t zvol_taskqs;
 
 /*
  * platform independent functions exported to platform code
@@ -94,6 +124,8 @@ int zvol_clone_range(zvol_state_handle_t *, uint64_t,
 void zvol_log_clone_range(zilog_t *zilog, dmu_tx_t *tx, int txtype,
     uint64_t off, uint64_t len, uint64_t blksz, const blkptr_t *bps,
     size_t nbps);
+zv_request_task_t *zv_request_task_create(zv_request_t zvr);
+void zv_request_task_free(zv_request_task_t *task);
 
 /*
  * platform dependent functions exported to platform independent code