Illumos #3642, #3643

3642 dsl_scan_active() should not issue I/O to determine if async
     destroying is active
3643 txg_delay should not hold the tc_lock
Reviewed by: Matthew Ahrens <[email protected]>
Reviewed by: Adam Leventhal <[email protected]>
Approved by: Gordon Ross <[email protected]>

References:
  https://www.illumos.org/issues/3642
  https://www.illumos.org/issues/3643
  illumos/illumos-gate@4a92375985c37d61406d66cd2b10ee642eb1f5e7

Ported-by: Richard Yao <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Issue #1775

Porting Notes:

1. The alignment assumptions for the tx_cpu structure assume that
   a kmutex_t is 8 bytes.  This isn't true under Linux but tc_pad[]
   was adjusted anyway for consistency since this structure was
   never carefully aligned in ZoL.  If careful alignment does impact
   performance significantly this should be reworked to be portable.

1 files changed, 47 insertions, 2 deletions

diff --git a/include/sys/txg_impl.h b/include/sys/txg_impl.h
index 7b356eac1..5a6d0e19f 100644
--- a/include/sys/txg_impl.h
+++ b/include/sys/txg_impl.h
@@ -23,6 +23,10 @@
  * Use is subject to license terms.
  */
 
+/*
+ * Copyright (c) 2013 by Delphix. All rights reserved.
+ */
+
 #ifndef _SYS_TXG_IMPL_H
 #define	_SYS_TXG_IMPL_H
 
@@ -33,14 +37,55 @@
 extern "C" {
 #endif
 
+/*
+ * The tx_cpu structure is a per-cpu structure that is used to track
+ * the number of active transaction holds (tc_count). As transactions
+ * are assigned into a transaction group the appropriate tc_count is
+ * incremented to indicate that there are pending changes that have yet
+ * to quiesce. Consumers evenutally call txg_rele_to_sync() to decrement
+ * the tc_count. A transaction group is not considered quiesced until all
+ * tx_cpu structures have reached a tc_count of zero.
+ *
+ * This structure is a per-cpu structure by design. Updates to this structure
+ * are frequent and concurrent. Having a single structure would result in
+ * heavy lock contention so a per-cpu design was implemented. With the fanned
+ * out mutex design, consumers only need to lock the mutex associated with
+ * thread's cpu.
+ *
+ * The tx_cpu contains two locks, the tc_lock and tc_open_lock.
+ * The tc_lock is used to protect all members of the tx_cpu structure with
+ * the exception of the tc_open_lock. This lock should only be held for a
+ * short period of time, typically when updating the value of tc_count.
+ *
+ * The tc_open_lock protects the tx_open_txg member of the tx_state structure.
+ * This lock is used to ensure that transactions are only assigned into
+ * the current open transaction group. In order to move the current open
+ * transaction group to the quiesce phase, the txg_quiesce thread must
+ * grab all tc_open_locks, increment the tx_open_txg, and drop the locks.
+ * The tc_open_lock is held until the transaction is assigned into the
+ * transaction group. Typically, this is a short operation but if throttling
+ * is occuring it may be held for longer periods of time.
+ */
 struct tx_cpu {
-	kmutex_t	tc_lock;
+	kmutex_t	tc_open_lock;	/* protects tx_open_txg */
+	kmutex_t	tc_lock;	/* protects the rest of this struct */
 	kcondvar_t	tc_cv[TXG_SIZE];
 	uint64_t	tc_count[TXG_SIZE];
 	list_t		tc_callbacks[TXG_SIZE]; /* commit cb list */
-	char		tc_pad[16];
+	char		tc_pad[8];		/* pad to fill 3 cache lines */
 };
 
+/*
+ * The tx_state structure maintains the state information about the different
+ * stages of the pool's transcation groups. A per pool tx_state structure
+ * is used to track this information. The tx_state structure also points to
+ * an array of tx_cpu structures (described above). Although the tx_sync_lock
+ * is used to protect the members of this structure, it is not used to
+ * protect the tx_open_txg. Instead a special lock in the tx_cpu structure
+ * is used. Readers of tx_open_txg must grab the per-cpu tc_open_lock.
+ * Any thread wishing to update tx_open_txg must grab the tc_open_lock on
+ * every cpu (see txg_quiesce()).
+ */
 typedef struct tx_state {
 	tx_cpu_t	*tx_cpu;	/* protects right to enter txg	*/
 	kmutex_t	tx_sync_lock;	/* protects tx_state_t */