[Cluster-devel] [GFS2 PATCH 3/6] GFS2: Refactor function fast_to_acquire

[Bob Peterson <rpeterso@redhat.com>]

Function fast_to_acquire determines if a rgrp ought to be fast to
acquire, but the logic was cryptic. This patch makes the logic more
straightforward:

1. If the rgrp is one of our "preferred" rgrps, consider it fast
   to acquire. This is how each node tries to confine its allocations
   to a set of rgrps unique from the other nodes, to reduce contention.
2. If the rgrp glock is unlocked, consider it slow, because it will
   take some time to lock it, either for the DLM or for the glock
   state machine.
3. If there are glock holders, consider it slow because we'd have to
   wait for another process to dequeue before we get our turn.
4. If the glock is being demoted, consider it slow.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
---
 fs/gfs2/rgrp.c | 30 +++++++++++++++++++++---------
 1 file changed, 21 insertions(+), 9 deletions(-)

diff --git a/fs/gfs2/rgrp.c b/fs/gfs2/rgrp.c
index 960fbc2d39d7..fa72e3e6ce4e 100644
--- a/fs/gfs2/rgrp.c
+++ b/fs/gfs2/rgrp.c
@@ -1938,20 +1938,32 @@ static bool gfs2_select_rgrp(struct gfs2_rgrpd **pos, const struct gfs2_rgrpd *b
 /**
  * fast_to_acquire - determine if a resource group will be fast to acquire
  *
- * If this is one of our preferred rgrps, it should be quicker to acquire,
- * because we tried to set ourselves up as dlm lock master.
+ * 1. If this is one of our preferred rgrps, it should be quicker to acquire,
+ *    because we tried to set ourselves up as dlm lock master.
+ * 2. If the glock is unlocked, consider it slow because it will take time for
+ *    the dlm and the glock state machine to transition it to a locked state.
+ * 3. If there are glock holder records queued to the glock, consider it slow
+ *    because this process will need to be queued up behind another process:
+ *    Our request can't be enqueued until the other is dequeued.
+ * 4. If the rgrp glock is being demoted, consider it slow because the glock
+ *    will need to be demoted, possibly used on another node, the promoted,
+ *    all of which will take time.
+ *
  */
-static inline int fast_to_acquire(struct gfs2_rgrpd *rgd)
+static inline bool fast_to_acquire(struct gfs2_rgrpd *rgd)
 {
 	struct gfs2_glock *gl = rgd->rd_gl;
 
-	if (gl->gl_state != LM_ST_UNLOCKED && list_empty(&gl->gl_holders) &&
-	    !test_bit(GLF_DEMOTE_IN_PROGRESS, &gl->gl_flags) &&
-	    !test_bit(GLF_DEMOTE, &gl->gl_flags))
-		return 1;
 	if (rgd->rd_flags & GFS2_RDF_PREFERRED)
-		return 1;
-	return 0;
+		return true;
+	if (gl->gl_state == LM_ST_UNLOCKED)
+		return false;
+	if (!list_empty(&gl->gl_holders))
+		return false;
+	if (test_bit(GLF_DEMOTE_IN_PROGRESS, &gl->gl_flags) ||
+	    test_bit(GLF_DEMOTE, &gl->gl_flags))
+		return false;
+	return true;
 }
 
 /**
-- 
2.14.3