[PATCH net-next v2] net/smc: Use percpu ref for wr tx reference

[Kai <KaiShen@linux.alibaba.com>]

The refcount wr_tx_refcnt may cause cache thrashing problems among
cores and we can use percpu ref to mitigate this issue here. We
gain some performance improvement with percpu ref here on our
customized smc-r verion. Applying cache alignment may also mitigate
this problem but it seem more reasonable to use percpu ref here.

redis-benchmark on smc-r with atomic wr_tx_refcnt:
SET: 525817.62 requests per second, p50=0.087 msec
GET: 570841.44 requests per second, p50=0.087 msec

redis-benchmark on the percpu_ref version:
SET: 539956.81 requests per second, p50=0.087 msec
GET: 587613.12 requests per second, p50=0.079 msec

Signed-off-by: Kai <KaiShen@linux.alibaba.com>
---
 net/smc/smc_core.h |  5 ++++-
 net/smc/smc_wr.c   | 18 ++++++++++++++++--
 net/smc/smc_wr.h   |  5 ++---
 3 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/net/smc/smc_core.h b/net/smc/smc_core.h
index 08b457c2d294..0705e33e2d68 100644
--- a/net/smc/smc_core.h
+++ b/net/smc/smc_core.h
@@ -106,7 +106,10 @@ struct smc_link {
 	unsigned long		*wr_tx_mask;	/* bit mask of used indexes */
 	u32			wr_tx_cnt;	/* number of WR send buffers */
 	wait_queue_head_t	wr_tx_wait;	/* wait for free WR send buf */
-	atomic_t		wr_tx_refcnt;	/* tx refs to link */
+	struct {
+		struct percpu_ref	wr_tx_refs;
+	} ____cacheline_aligned_in_smp;
+	struct completion	ref_comp;
 
 	struct smc_wr_buf	*wr_rx_bufs;	/* WR recv payload buffers */
 	struct ib_recv_wr	*wr_rx_ibs;	/* WR recv meta data */
diff --git a/net/smc/smc_wr.c b/net/smc/smc_wr.c
index b0678a417e09..dd923e76139f 100644
--- a/net/smc/smc_wr.c
+++ b/net/smc/smc_wr.c
@@ -648,7 +648,8 @@ void smc_wr_free_link(struct smc_link *lnk)
 
 	smc_wr_tx_wait_no_pending_sends(lnk);
 	wait_event(lnk->wr_reg_wait, (!atomic_read(&lnk->wr_reg_refcnt)));
-	wait_event(lnk->wr_tx_wait, (!atomic_read(&lnk->wr_tx_refcnt)));
+	percpu_ref_kill(&lnk->wr_tx_refs);
+	wait_for_completion(&lnk->ref_comp);
 
 	if (lnk->wr_rx_dma_addr) {
 		ib_dma_unmap_single(ibdev, lnk->wr_rx_dma_addr,
@@ -847,6 +848,13 @@ void smc_wr_add_dev(struct smc_ib_device *smcibdev)
 	tasklet_setup(&smcibdev->send_tasklet, smc_wr_tx_tasklet_fn);
 }
 
+static void smcr_wr_tx_refs_free(struct percpu_ref *ref)
+{
+	struct smc_link *lnk = container_of(ref, struct smc_link, wr_tx_refs);
+
+	complete(&lnk->ref_comp);
+}
+
 int smc_wr_create_link(struct smc_link *lnk)
 {
 	struct ib_device *ibdev = lnk->smcibdev->ibdev;
@@ -890,7 +898,13 @@ int smc_wr_create_link(struct smc_link *lnk)
 	smc_wr_init_sge(lnk);
 	bitmap_zero(lnk->wr_tx_mask, SMC_WR_BUF_CNT);
 	init_waitqueue_head(&lnk->wr_tx_wait);
-	atomic_set(&lnk->wr_tx_refcnt, 0);
+
+	rc = percpu_ref_init(&lnk->wr_tx_refs, smcr_wr_tx_refs_free,
+			     PERCPU_REF_ALLOW_REINIT, GFP_KERNEL);
+	if (rc)
+		goto dma_unmap;
+	init_completion(&lnk->ref_comp);
+
 	init_waitqueue_head(&lnk->wr_reg_wait);
 	atomic_set(&lnk->wr_reg_refcnt, 0);
 	init_waitqueue_head(&lnk->wr_rx_empty_wait);
diff --git a/net/smc/smc_wr.h b/net/smc/smc_wr.h
index 45e9b894d3f8..f3008dda222a 100644
--- a/net/smc/smc_wr.h
+++ b/net/smc/smc_wr.h
@@ -63,14 +63,13 @@ static inline bool smc_wr_tx_link_hold(struct smc_link *link)
 {
 	if (!smc_link_sendable(link))
 		return false;
-	atomic_inc(&link->wr_tx_refcnt);
+	percpu_ref_get(&link->wr_tx_refs);
 	return true;
 }
 
 static inline void smc_wr_tx_link_put(struct smc_link *link)
 {
-	if (atomic_dec_and_test(&link->wr_tx_refcnt))
-		wake_up_all(&link->wr_tx_wait);
+	percpu_ref_put(&link->wr_tx_refs);
 }
 
 static inline void smc_wr_drain_cq(struct smc_link *lnk)
-- 
2.31.1