[PATCH net-next 06/10] net/smc: use read semaphores to reduce unnecessary blocking in smc_buf_create() & smcr_buf_unuse()

D. Wythe
2022-08-10T17:48:08Z

From: "D. Wythe" <alibuda@linux.alibaba.com>

Following is part of Off-CPU graph during frequent SMC-R short-lived
processing:

process_one_work (51.19%)
smc_close_passive_work (28.36%)
smcr_buf_unuse (28.34%)
rwsem_down_write_slowpath (28.22%)

smc_listen_work (22.83%)
smc_clc_wait_msg (1.84%)
smc_buf_create (20.45%)
smcr_buf_map_usable_links
rwsem_down_write_slowpath (20.43%)
smcr_lgr_reg_rmbs (0.53%)
rwsem_down_write_slowpath (0.43%)
smc_llc_do_confirm_rkey (0.08%)

We can clearly see that during the connection establishment time,
waiting time of connections is not on IO, but on llc_conf_mutex.

What is more important, the core critical area (smcr_buf_unuse() &
smc_buf_create()) only perfroms read semantics on links, we can
easily replace it with read semaphore.

Signed-off-by: D. Wythe <alibuda@linux.alibaba.com>
---
net/smc/smc_core.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/net/smc/smc_core.c b/net/smc/smc_core.c
index fc8deec..113804d 100644
--- a/net/smc/smc_core.c
+++ b/net/smc/smc_core.c
@@ -1388,10 +1388,10 @@ static void smcr_buf_unuse(struct smc_buf_desc *buf_desc, bool is_rmb,
rc = smc_llc_flow_initiate(lgr, SMC_LLC_FLOW_RKEY);
if (!rc) {
/* protect against smc_llc_cli_rkey_exchange() */
- down_write(&lgr->llc_conf_mutex);
+ down_read(&lgr->llc_conf_mutex);
smc_llc_do_delete_rkey(lgr, buf_desc);
buf_desc->is_conf_rkey = false;
- up_write(&lgr->llc_conf_mutex);
+ up_read(&lgr->llc_conf_mutex);
smc_llc_flow_stop(lgr, &lgr->llc_flow_lcl);
}
}
@@ -2581,7 +2581,7 @@ static int smcr_buf_map_usable_links(struct smc_link_group *lgr,
int i, rc = 0;

/* protect against parallel link reconfiguration */
- down_write(&lgr->llc_conf_mutex);
+ down_read(&lgr->llc_conf_mutex);
for (i = 0; i < SMC_LINKS_PER_LGR_MAX; i++) {
struct smc_link *lnk = &lgr->lnk[i];

@@ -2593,7 +2593,7 @@ static int smcr_buf_map_usable_links(struct smc_link_group *lgr,
}
}
out:
- up_write(&lgr->llc_conf_mutex);
+ up_read(&lgr->llc_conf_mutex);
return rc;
}

--
1.8.3.1