[PATCH RFC] nfsd: report length of the largest hash chain in reply cache stats

[Jeff Layton <jlayton@redhat.com>]

So we can get a feel for how effective the hashing function is.

As Chuck Lever pointed out to me, it's generally acceptable to do
"expensive" stuff when reading the stats since that's a relatively
rare activity.

Cc: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
---
 fs/nfsd/nfscache.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/fs/nfsd/nfscache.c b/fs/nfsd/nfscache.c
index a5ac9ab..172e211 100644
--- a/fs/nfsd/nfscache.c
+++ b/fs/nfsd/nfscache.c
@@ -551,6 +551,25 @@ nfsd_cache_append(struct svc_rqst *rqstp, struct kvec *data)
 	return 1;
 }
 
+/* Get stats on the hashtable itself */
+static unsigned int
+nfsd_repcache_max_chain_len(void)
+{
+	int i;
+	struct hlist_node *pos;
+	unsigned int max = 0;
+
+	for (i = 0; i < HASHSIZE; ++i) {
+		unsigned int cur = 0;
+
+		hlist_for_each(pos, &cache_hash[i])
+			++cur;
+		max = max(cur, max);
+	}
+
+	return max;
+}
+
 /*
  * Note that fields may be added, removed or reordered in the future. Programs
  * scraping this file for info should test the labels to ensure they're
@@ -566,6 +585,7 @@ static int nfsd_reply_cache_stats_show(struct seq_file *m, void *v)
 	seq_printf(m, "cache misses:    %u\n", nfsdstats.rcmisses);
 	seq_printf(m, "not cached:      %u\n", nfsdstats.rcnocache);
 	seq_printf(m, "checksum misses: %u\n", csum_misses);
+	seq_printf(m, "max chain len:   %u\n", nfsd_repcache_max_chain_len());
 	spin_unlock(&cache_lock);
 	return 0;
 }
-- 
1.7.11.7