netdev - Re: Throughput regression with `tcp: refine TSO autosizing`

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Date:	Wed, 04 Feb 2015 13:11:43 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Michal Kazior <michal.kazior@...to.com>
Cc:	Neal Cardwell <ncardwell@...gle.com>,
	linux-wireless <linux-wireless@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	eyalpe@....mellanox.co.il
Subject: Re: Throughput regression with `tcp: refine TSO autosizing`

I do not see how a TSO patch could hurt a flow not using TSO/GSO.

This makes no sense.

ath10k tx completions being batched/deferred to a tasklet might increase
probability to hit this condition in tcp_wfree() :

        /* If this softirq is serviced by ksoftirqd, we are likely under stress.
         * Wait until our queues (qdisc + devices) are drained.
         * This gives :
         * - less callbacks to tcp_write_xmit(), reducing stress (batches)
         * - chance for incoming ACK (processed by another cpu maybe)
         *   to migrate this flow (skb->ooo_okay will be eventually set)
         */
        if (wmem >= SKB_TRUESIZE(1) && this_cpu_ksoftirqd() == current)
                goto out;

Meaning tcp stack waits all skbs left qdisc/NIC queues before queuing
additional packets.

I would try to call skb_orphan() in ath10k if you really want to keep
these batches.

I have hard time to understand why tx completed packets go through
ath10k_htc_rx_completion_handler().. anyway...

Most conservative patch would be :

diff --git a/drivers/net/wireless/ath/ath10k/htt_rx.c b/drivers/net/wireless/ath/ath10k/htt_rx.c
index 9c782a42665e1aaf43bfbca441631ee58da50c09..6a36317d6bb0447202dee15528130bd5e21248c4 100644
--- a/drivers/net/wireless/ath/ath10k/htt_rx.c
+++ b/drivers/net/wireless/ath/ath10k/htt_rx.c
@@ -1642,6 +1642,7 @@ void ath10k_htt_t2h_msg_handler(struct ath10k *ar, struct sk_buff *skb)
 		break;
 	}
 	case HTT_T2H_MSG_TYPE_TX_COMPL_IND:
+		skb_orphan(skb);
 		spin_lock_bh(&htt->tx_lock);
 		__skb_queue_tail(&htt->tx_compl_q, skb);
 		spin_unlock_bh(&htt->tx_lock);


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