Subject: Re: [PATCH net-next 6/7] net/smc: don't req_notify until all CQEs drained

From: Leon Romanovsky <[email-masked]>
Date: Tue, 1 Mar 2022 12:14:15 +0200

On Tue, Mar 01, 2022 at 05:44:01PM +0800, Dust Li wrote:
> When we are handling softirq workload, enable hardirq may
> again interrupt the current routine of softirq, and then
> try to raise softirq again. This only wastes CPU cycles
> and won't have any real gain.
> 
> Since IB_CQ_REPORT_MISSED_EVENTS already make sure if
> ib_req_notify_cq() returns 0, it is safe to wait for the
> next event, with no need to poll the CQ again in this case.
> 
> This patch disables hardirq during the processing of softirq,
> and re-arm the CQ after softirq is done. Somehow like NAPI.
> 
> Co-developed-by: Guangguan Wang <[email-masked]>
> Signed-off-by: Guangguan Wang <[email-masked]>
> Signed-off-by: Dust Li <[email-masked]>
> ---
>  net/smc/smc_wr.c | 49 +++++++++++++++++++++++++++---------------------
>  1 file changed, 28 insertions(+), 21 deletions(-)
> 
> diff --git a/net/smc/smc_wr.c b/net/smc/smc_wr.c
> index 24be1d03fef9..34d616406d51 100644
> --- a/net/smc/smc_wr.c
> +++ b/net/smc/smc_wr.c
> @@ -137,25 +137,28 @@ static void smc_wr_tx_tasklet_fn(struct tasklet_struct *t)
>  {
>  	struct smc_ib_device *dev = from_tasklet(dev, t, send_tasklet);
>  	struct ib_wc wc[SMC_WR_MAX_POLL_CQE];
> -	int i = 0, rc;
> -	int polled = 0;
> +	int i, rc;
>  
>  again:
> -	polled++;
>  	do {
>  		memset(&wc, 0, sizeof(wc));
>  		rc = ib_poll_cq(dev->roce_cq_send, SMC_WR_MAX_POLL_CQE, wc);
> -		if (polled == 1) {
> -			ib_req_notify_cq(dev->roce_cq_send,
> -					 IB_CQ_NEXT_COMP |
> -					 IB_CQ_REPORT_MISSED_EVENTS);
> -		}
> -		if (!rc)
> -			break;
>  		for (i = 0; i < rc; i++)
>  			smc_wr_tx_process_cqe(&wc[i]);
> +		if (rc < SMC_WR_MAX_POLL_CQE)
> +			/* If < SMC_WR_MAX_POLL_CQE, the CQ should have been
> +			 * drained, no need to poll again. --Guangguan Wang

1. Please remove "--Guangguan Wang".
2. We already discussed that. SMC should be changed to use RDMA CQ pool API
drivers/infiniband/core/cq.c. 
ib_poll_handler() has much better implementation (tracing, IRQ rescheduling,
proper error handling) than this SMC variant.

Thanks

> +			 */
> +			break;
>  	} while (rc > 0);
> -	if (polled == 1)
> +
> +	/* IB_CQ_REPORT_MISSED_EVENTS make sure if ib_req_notify_cq() returns
> +	 * 0, it is safe to wait for the next event.
> +	 * Else we must poll the CQ again to make sure we won't miss any event
> +	 */
> +	if (ib_req_notify_cq(dev->roce_cq_send,
> +			     IB_CQ_NEXT_COMP |
> +			     IB_CQ_REPORT_MISSED_EVENTS))
>  		goto again;
>  }
>  
> @@ -478,24 +481,28 @@ static void smc_wr_rx_tasklet_fn(struct tasklet_struct *t)
>  {
>  	struct smc_ib_device *dev = from_tasklet(dev, t, recv_tasklet);
>  	struct ib_wc wc[SMC_WR_MAX_POLL_CQE];
> -	int polled = 0;
>  	int rc;
>  
>  again:
> -	polled++;
>  	do {
>  		memset(&wc, 0, sizeof(wc));
>  		rc = ib_poll_cq(dev->roce_cq_recv, SMC_WR_MAX_POLL_CQE, wc);
> -		if (polled == 1) {
> -			ib_req_notify_cq(dev->roce_cq_recv,
> -					 IB_CQ_SOLICITED_MASK
> -					 | IB_CQ_REPORT_MISSED_EVENTS);
> -		}
> -		if (!rc)
> +		if (rc > 0)
> +			smc_wr_rx_process_cqes(&wc[0], rc);
> +		if (rc < SMC_WR_MAX_POLL_CQE)
> +			/* If < SMC_WR_MAX_POLL_CQE, the CQ should have been
> +			 * drained, no need to poll again. --Guangguan Wang
> +			 */
>  			break;
> -		smc_wr_rx_process_cqes(&wc[0], rc);
>  	} while (rc > 0);
> -	if (polled == 1)
> +
> +	/* IB_CQ_REPORT_MISSED_EVENTS make sure if ib_req_notify_cq() returns
> +	 * 0, it is safe to wait for the next event.
> +	 * Else we must poll the CQ again to make sure we won't miss any event
> +	 */
> +	if (ib_req_notify_cq(dev->roce_cq_recv,
> +			     IB_CQ_SOLICITED_MASK |
> +			     IB_CQ_REPORT_MISSED_EVENTS))
>  		goto again;
>  }
>  
> -- 
> 2.19.1.3.ge56e4f7
>