[PATCH 05/11] NFSv4/flexfiles: drop the redundant atomic from the busy timer

[Mike Snitzer <snitzer@kernel.org>]

From: Mike Snitzer <snitzer@hammerspace.com>

nfs4_ff_busy_timer::n_ops is an atomic_t, but every access to it is
already serialised by the stripe lock:

  nfs4_ff_start_busy_timer() has one caller, nfs4_ff_layoutstat_start_io(),
  reached only from ff_layout_{read,write}_record_layoutstats_start()
  with dss[dss_id].lock held.

  nfs4_ff_end_busy_timer() has one caller,
  nfs4_ff_layout_stat_io_update_completed(), reached only from
  ff_layout_{read,write}_record_layoutstats_done() with the same lock
  held.

The two reporting paths, ff_layout_mirror_prepare_stats() and
ff_layout_encode_ff_layoutupdate(), take the stripe lock too and do not
touch n_ops at all.  There is no lockless reader.

So the atomic_inc_return()/atomic_dec_return() are locked read-modify-
writes on a line the CPU already owns exclusively -- pure serialisation
overhead, paid twice per DS RPC on the hottest path in this file, at the
275k-450k RPC/s the per-stripe lock work was measured at.

Make n_ops a plain int and document the locking on the struct.  Its size
is unchanged, so the stripe layout is unaffected.  The underflow check is
preserved.

Signed-off-by: Mike Snitzer <snitzer@hammerspace.com>
---
 fs/nfs/flexfilelayout/flexfilelayout.c | 6 ++----
 fs/nfs/flexfilelayout/flexfilelayout.h | 6 +++++-
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/fs/nfs/flexfilelayout/flexfilelayout.c b/fs/nfs/flexfilelayout/flexfilelayout.c
index 7c328f2ec900d..6a474fa7b0446 100644
--- a/fs/nfs/flexfilelayout/flexfilelayout.c
+++ b/fs/nfs/flexfilelayout/flexfilelayout.c
@@ -711,9 +711,8 @@ static void
 nfs4_ff_start_busy_timer(struct nfs4_ff_busy_timer *timer, ktime_t now)
 {
 	/* first IO request? */
-	if (atomic_inc_return(&timer->n_ops) == 1) {
+	if (++timer->n_ops == 1)
 		timer->start_time = now;
-	}
 }
 
 static ktime_t
@@ -721,8 +720,7 @@ nfs4_ff_end_busy_timer(struct nfs4_ff_busy_timer *timer, ktime_t now)
 {
 	ktime_t start;
 
-	if (atomic_dec_return(&timer->n_ops) < 0)
-		WARN_ON_ONCE(1);
+	WARN_ON_ONCE(--timer->n_ops < 0);
 
 	start = timer->start_time;
 	timer->start_time = now;
diff --git a/fs/nfs/flexfilelayout/flexfilelayout.h b/fs/nfs/flexfilelayout/flexfilelayout.h
index 658940979883a..d97001abc0d22 100644
--- a/fs/nfs/flexfilelayout/flexfilelayout.h
+++ b/fs/nfs/flexfilelayout/flexfilelayout.h
@@ -64,9 +64,13 @@ struct nfs4_ff_io_stat {
 	ktime_t				aggregate_completion_time;
 };
 
+/*
+ * Both members are serialised by the enclosing stripe's
+ * nfs4_ff_layout_ds_stripe::lock; see nfs4_ff_{start,end}_busy_timer().
+ */
 struct nfs4_ff_busy_timer {
 	ktime_t start_time;
-	atomic_t n_ops;
+	int n_ops;
 };
 
 struct nfs4_ff_layoutstat {
-- 
2.52.0