NFSD: Report average age of filecache items

This is a measure of how long items stay in the filecache, to help
assess how efficient the cache is.
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/filecache.c

@@ -45,6 +45,7 @@ struct nfsd_fcache_bucket {
 static DEFINE_PER_CPU(unsigned long, nfsd_file_cache_hits);
 static DEFINE_PER_CPU(unsigned long, nfsd_file_acquisitions);
 static DEFINE_PER_CPU(unsigned long, nfsd_file_releases);
+static DEFINE_PER_CPU(unsigned long, nfsd_file_total_age);
 
 struct nfsd_fcache_disposal {
 	struct work_struct work;
@@ -178,6 +179,7 @@ nfsd_file_alloc(struct inode *inode, unsigned int may, unsigned int hashval,
 	if (nf) {
 		INIT_HLIST_NODE(&nf->nf_node);
 		INIT_LIST_HEAD(&nf->nf_lru);
+		nf->nf_birthtime = ktime_get();
 		nf->nf_file = NULL;
 		nf->nf_cred = get_current_cred();
 		nf->nf_net = net;
@@ -195,9 +197,11 @@ nfsd_file_alloc(struct inode *inode, unsigned int may, unsigned int hashval,
 static bool
 nfsd_file_free(struct nfsd_file *nf)
 {
+	s64 age = ktime_to_ms(ktime_sub(ktime_get(), nf->nf_birthtime));
 	bool flush = false;
 
 	this_cpu_inc(nfsd_file_releases);
+	this_cpu_add(nfsd_file_total_age, age);
 
 	trace_nfsd_file_put_final(nf);
 	if (nf->nf_mark)
@@ -1055,7 +1059,7 @@ static int nfsd_file_cache_stats_show(struct seq_file *m, void *v)
 {
 	unsigned long hits = 0, acquisitions = 0, releases = 0;
 	unsigned int i, count = 0, longest = 0;
-	unsigned long lru = 0;
+	unsigned long lru = 0, total_age = 0;
 
 	/*
 	 * No need for spinlocks here since we're not terribly interested in
@@ -1076,6 +1080,7 @@ static int nfsd_file_cache_stats_show(struct seq_file *m, void *v)
 		hits += per_cpu(nfsd_file_cache_hits, i);
 		acquisitions += per_cpu(nfsd_file_acquisitions, i);
 		releases += per_cpu(nfsd_file_releases, i);
+		total_age += per_cpu(nfsd_file_total_age, i);
 	}
 
 	seq_printf(m, "total entries: %u\n", count);
@@ -1084,6 +1089,10 @@ static int nfsd_file_cache_stats_show(struct seq_file *m, void *v)
 	seq_printf(m, "cache hits:    %lu\n", hits);
 	seq_printf(m, "acquisitions:  %lu\n", acquisitions);
 	seq_printf(m, "releases:      %lu\n", releases);
+	if (releases)
+		seq_printf(m, "mean age (ms): %ld\n", total_age / releases);
+	else
+		seq_printf(m, "mean age (ms): -\n");
 	return 0;
 }
 

fs/nfsd/filecache.h

@@ -44,6 +44,7 @@ struct nfsd_file {
 	refcount_t		nf_ref;
 	unsigned char		nf_may;
 	struct nfsd_file_mark	*nf_mark;
+	ktime_t			nf_birthtime;
 };
 
 int nfsd_file_cache_init(void);