GFS2: glock statistics gathering

The stats are divided into two sets: those relating to the
super block and those relating to an individual glock. The
super block stats are done on a per cpu basis in order to
try and reduce the overhead of gathering them. They are also
further divided by glock type.

In the case of both the super block and glock statistics,
the same information is gathered in each case. The super
block statistics are used to provide default values for
most of the glock statistics, so that newly created glocks
should have, as far as possible, a sensible starting point.

The statistics are divided into three pairs of mean and
variance, plus two counters. The mean/variance pairs are
smoothed exponential estimates and the algorithm used is
one which will be very familiar to those used to calculation
of round trip times in network code.

The three pairs of mean/variance measure the following
things:

 1. DLM lock time (non-blocking requests)
 2. DLM lock time (blocking requests)
 3. Inter-request time (again to the DLM)

A non-blocking request is one which will complete right
away, whatever the state of the DLM lock in question. That
currently means any requests when (a) the current state of
the lock is exclusive (b) the requested state is either null
or unlocked or (c) the "try lock" flag is set. A blocking
request covers all the other lock requests.

There are two counters. The first is there primarily to show
how many lock requests have been made, and thus how much data
has gone into the mean/variance calculations. The other counter
is counting queueing of holders at the top layer of the glock
code. Hopefully that number will be a lot larger than the number
of dlm lock requests issued.

So why gather these statistics? There are several reasons
we'd like to get a better idea of these timings:

1. To be able to better set the glock "min hold time"
2. To spot performance issues more easily
3. To improve the algorithm for selecting resource groups for
allocation (to base it on lock wait time, rather than blindly
using a "try lock")
Due to the smoothing action of the updates, a step change in
some input quantity being sampled will only fully be taken
into account after 8 samples (or 4 for the variance) and this
needs to be carefully considered when interpreting the
results.

Knowing both the time it takes a lock request to complete and
the average time between lock requests for a glock means we
can compute the total percentage of the time for which the
node is able to use a glock vs. time that the rest of the
cluster has its share. That will be very useful when setting
the lock min hold time.

The other point to remember is that all times are in
nanoseconds. Great care has been taken to ensure that we
measure exactly the quantities that we want, as accurately
as possible. There are always inaccuracies in any
measuring system, but I hope this is as accurate as we
can reasonably make it.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

1 files changed, 44 insertions, 5 deletions

diff --git a/fs/gfs2/incore.h b/fs/gfs2/incore.h
index 97742a7..4d546df 100644
--- a/fs/gfs2/incore.h
+++ b/fs/gfs2/incore.h
@@ -19,6 +19,8 @@
 #include <linux/rculist_bl.h>
 #include <linux/completion.h>
 #include <linux/rbtree.h>
+#include <linux/ktime.h>
+#include <linux/percpu.h>
 
 #define DIO_WAIT	0x00000010
 #define DIO_METADATA	0x00000020
@@ -205,6 +207,22 @@ struct gfs2_glock_operations {
 };
 
 enum {
+	GFS2_LKS_SRTT = 0,	/* Non blocking smoothed round trip time */
+	GFS2_LKS_SRTTVAR = 1,	/* Non blocking smoothed variance */
+	GFS2_LKS_SRTTB = 2,	/* Blocking smoothed round trip time */
+	GFS2_LKS_SRTTVARB = 3,	/* Blocking smoothed variance */
+	GFS2_LKS_SIRT = 4,	/* Smoothed Inter-request time */
+	GFS2_LKS_SIRTVAR = 5,	/* Smoothed Inter-request variance */
+	GFS2_LKS_DCOUNT = 6,	/* Count of dlm requests */
+	GFS2_LKS_QCOUNT = 7,	/* Count of gfs2_holder queues */
+	GFS2_NR_LKSTATS
+};
+
+struct gfs2_lkstats {
+	s64 stats[GFS2_NR_LKSTATS];
+};
+
+enum {
 	/* States */
 	HIF_HOLDER		= 6,  /* Set for gh that "holds" the glock */
 	HIF_FIRST		= 7,
@@ -238,10 +256,12 @@ enum {
 	GLF_QUEUED			= 12,
 	GLF_LRU				= 13,
 	GLF_OBJECT			= 14, /* Used only for tracing */
+	GLF_BLOCKING			= 15,
 };
 
 struct gfs2_glock {
 	struct hlist_bl_node gl_list;
+	struct gfs2_sbd *gl_sbd;
 	unsigned long gl_flags;		/* GLF_... */
 	struct lm_lockname gl_name;
 	atomic_t gl_ref;
@@ -261,16 +281,14 @@ struct gfs2_glock {
 	struct list_head gl_holders;
 
 	const struct gfs2_glock_operations *gl_ops;
-	char gl_strname[GDLM_STRNAME_BYTES];
+	ktime_t gl_dstamp;
+	struct gfs2_lkstats gl_stats;
 	struct dlm_lksb gl_lksb;
 	char gl_lvb[32];
 	unsigned long gl_tchange;
 	void *gl_object;
 
 	struct list_head gl_lru;
-
-	struct gfs2_sbd *gl_sbd;
-
 	struct list_head gl_ail_list;
 	atomic_t gl_ail_count;
 	atomic_t gl_revokes;
@@ -560,8 +578,14 @@ struct lm_lockstruct {
 	uint32_t *ls_recover_result; /* result of last jid recovery */
 };
 
+struct gfs2_pcpu_lkstats {
+	/* One struct for each glock type */
+	struct gfs2_lkstats lkstats[10];
+};
+
 struct gfs2_sbd {
 	struct super_block *sd_vfs;
+	struct gfs2_pcpu_lkstats __percpu *sd_lkstats;
 	struct kobject sd_kobj;
 	unsigned long sd_flags;	/* SDF_... */
 	struct gfs2_sb_host sd_sb;
@@ -725,8 +749,23 @@ struct gfs2_sbd {
 
 	unsigned long sd_last_warning;
 	struct dentry *debugfs_dir;    /* debugfs directory */
-	struct dentry *debugfs_dentry_glocks; /* for debugfs */
+	struct dentry *debugfs_dentry_glocks;
+	struct dentry *debugfs_dentry_glstats;
+	struct dentry *debugfs_dentry_sbstats;
 };
 
+static inline void gfs2_glstats_inc(struct gfs2_glock *gl, int which)
+{
+	gl->gl_stats.stats[which]++;
+}
+
+static inline void gfs2_sbstats_inc(const struct gfs2_glock *gl, int which)
+{
+	const struct gfs2_sbd *sdp = gl->gl_sbd;
+	preempt_disable();
+	this_cpu_ptr(sdp->sd_lkstats)->lkstats[gl->gl_name.ln_type].stats[which]++;
+	preempt_enable();
+}
+
 #endif /* __INCORE_DOT_H__ */