[PATCH net-next 07/10] net/smc: reduce unnecessary blocking in smcr_lgr_reg_rmbs()

D. Wythe
2022-08-10T17:48:04Z

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

Unlike smc_buf_create() and smcr_buf_unuse(), smcr_lgr_reg_rmbs() is
exclusive when assigned rmb_desc was not registered, although it can be
executed in parallel when assigned rmb_desc was registered already
and only performs read semtamics on it. Hence, we can not simply replace
it with read semaphore.

The idea here is that if the assigned rmb_desc was registered already,
use read semaphore to protect the critical section, once the assigned
rmb_desc was not registered, keep using keep write semaphore still
to keep its exclusivity.

Thanks to the reusable features of rmb_desc, which allows us to execute
in parallel in most cases.

Signed-off-by: D. Wythe <alibuda@linux.alibaba.com>
---
net/smc/af_smc.c | 19 +++++++++++++++++--
1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/net/smc/af_smc.c b/net/smc/af_smc.c
index 51b90e2..39dbf39 100644
--- a/net/smc/af_smc.c
+++ b/net/smc/af_smc.c
@@ -516,10 +516,25 @@ static int smcr_lgr_reg_rmbs(struct smc_link *link,
{
struct smc_link_group *lgr = link->lgr;
int i, rc = 0;
+ bool slow = false;

rc = smc_llc_flow_initiate(lgr, SMC_LLC_FLOW_RKEY);
if (rc)
return rc;
+
+ down_read(&lgr->llc_conf_mutex);
+ for (i = 0; i < SMC_LINKS_PER_LGR_MAX; i++) {
+ if (!smc_link_active(&lgr->lnk[i]))
+ continue;
+ if (!rmb_desc->is_reg_mr[link->link_idx]) {
+ up_read(&lgr->llc_conf_mutex);
+ goto slow_path;
+ }
+ }
+ /* mr register already */
+ goto fast_path;
+slow_path:
+ slow = true;
/* protect against parallel smc_llc_cli_rkey_exchange() and
* parallel smcr_link_reg_buf()
*/
@@ -531,7 +546,7 @@ static int smcr_lgr_reg_rmbs(struct smc_link *link,
if (rc)
goto out;
}
-
+fast_path:
/* exchange confirm_rkey msg with peer */
rc = smc_llc_do_confirm_rkey(link, rmb_desc);
if (rc) {
@@ -540,7 +555,7 @@ static int smcr_lgr_reg_rmbs(struct smc_link *link,
}
rmb_desc->is_conf_rkey = true;
out:
- up_write(&lgr->llc_conf_mutex);
+ slow ? up_write(&lgr->llc_conf_mutex) : up_read(&lgr->llc_conf_mutex);
smc_llc_flow_stop(lgr, &lgr->llc_flow_lcl);
return rc;
}
--
1.8.3.1