[PATCH v2 10/12] NFSv4/flexfiles: rotate LAYOUTSTATS reporting across a mirror's stripes

[Mike Snitzer <snitzer@kernel.org>]

From: Mike Snitzer <snitzer@hammerspace.com>

Both callers of ff_layout_mirror_prepare_stats() limit a report to four
ff_layoutupdate4 entries, and the scan of mirror->dss[] stops when the
limit is reached.  On a mirror with more than four busy stripes, every
report contains the same four stripes, and the others are reported only
at LAYOUTRETURN.

Record where each mirror's scan stopped, and start the next scan there.
The index moves past each stripe that gets a slot, and idle stripes do
not use a slot.  So every busy stripe is reported within
ceil(busy_stripes / 4) report intervals.  The values and the encoding
do not change.

The mirror loop gets the same dev_limit test as the stripe loop, so a
full report stops the scan.

[jonathan.flynn: read dss_report_index once per mirror, before the
 stripe loop.  Reading it again in the loop skipped stripes and could
 report one stripe twice.]
Signed-off-by: Jonathan Flynn <jonathan.flynn@hammerspace.com>
Signed-off-by: Mike Snitzer <snitzer@hammerspace.com>
---
 fs/nfs/flexfilelayout/flexfilelayout.c | 15 ++++++++++++---
 fs/nfs/flexfilelayout/flexfilelayout.h |  1 +
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/fs/nfs/flexfilelayout/flexfilelayout.c b/fs/nfs/flexfilelayout/flexfilelayout.c
index 91600086526ee..b4d4971816145 100644
--- a/fs/nfs/flexfilelayout/flexfilelayout.c
+++ b/fs/nfs/flexfilelayout/flexfilelayout.c
@@ -3136,14 +3136,21 @@ ff_layout_mirror_prepare_stats(struct pnfs_layout_hdr *lo,
 	struct nfs4_ff_layout_ds_stripe *dss_info;
 	struct nfs4_ff_layout_ds *mirror_ds;
 	__u64 read_count, read_bytes, write_count, write_bytes, ops;
-	int i = 0, dss_id;
+	u32 dss_id, scanned, base;
+	int i = 0;
 
 	rcu_read_lock();
 	list_for_each_entry(mirror, &ff_layout->mirrors, mirrors) {
-		for (dss_id = 0; dss_id < mirror->dss_count; ++dss_id) {
-			dss_info = &mirror->dss[dss_id];
+		if (i >= dev_limit)
+			break;
+		base = mirror->dss_report_index;
+		for (scanned = 0; scanned < mirror->dss_count; ++scanned) {
 			if (i >= dev_limit)
 				break;
+			dss_id = base + scanned;
+			if (dss_id >= mirror->dss_count)
+				dss_id -= mirror->dss_count;
+			dss_info = &mirror->dss[dss_id];
 			mirror_ds = rcu_dereference(dss_info->mirror_ds);
 			if (IS_ERR_OR_NULL(mirror_ds))
 				continue;
@@ -3167,6 +3174,8 @@ ff_layout_mirror_prepare_stats(struct pnfs_layout_hdr *lo,
 			if (!refcount_inc_not_zero(&mirror->ref))
 				continue;
 			dss_info->last_reported_ops = ops;
+			mirror->dss_report_index =
+				dss_id + 1 < mirror->dss_count ? dss_id + 1 : 0;
 			/* The pin holds a reference; it is exchanged out only
 			 * under i_lock.  Put in ff_layout_free_layoutstats().
 			 */
diff --git a/fs/nfs/flexfilelayout/flexfilelayout.h b/fs/nfs/flexfilelayout/flexfilelayout.h
index df91165ba72ee..33ad85d1d07a2 100644
--- a/fs/nfs/flexfilelayout/flexfilelayout.h
+++ b/fs/nfs/flexfilelayout/flexfilelayout.h
@@ -108,6 +108,7 @@ struct nfs4_ff_layout_mirror {
 	struct nfs4_ff_layout_ds_stripe *dss;
 	refcount_t			ref;
 	u32				report_interval;
+	u32				dss_report_index;
 };
 
 struct nfs4_ff_layout_segment {
-- 
2.52.0