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

Tony Lu
2022-08-16T10:23:37Z

On Thu, Aug 11, 2022 at 01:47:38AM +0800, D. Wythe wrote:
> 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;

Consider do_slow?

Reverse Christmas tree.

>
> 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