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

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Date:	Thu, 5 Feb 2015 09:38:23 +0100
From:	Michal Kazior <michal.kazior@...to.com>
To:	Eric Dumazet <eric.dumazet@...il.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`

On 4 February 2015 at 22:11, Eric Dumazet <eric.dumazet@...il.com> wrote:
> I do not see how a TSO patch could hurt a flow not using TSO/GSO.
>
> This makes no sense.

Hmm..

@@ -2018,8 +2053,8 @@ static bool tcp_write_xmit(struct sock *sk,
unsigned int mss_now, int nonagle,
                 * of queued bytes to ensure line rate.
                 * One example is wifi aggregation (802.11 AMPDU)
                 */
-               limit = max_t(unsigned int, sysctl_tcp_limit_output_bytes,
-                             sk->sk_pacing_rate >> 10);
+               limit = max(2 * skb->truesize, sk->sk_pacing_rate >> 10);
+               limit = min_t(u32, limit, sysctl_tcp_limit_output_bytes);

                if (atomic_read(&sk->sk_wmem_alloc) > limit) {
                        set_bit(TSQ_THROTTLED, &tp->tsq_flags);

Doesn't this effectively invert how tcp_limit_output_bytes is used?
This would explain why raising the limit wasn't changing anything
anymore when you asked me do so. Only decreasing it yielded any
change.

I've added a printk to show up the new and old values. Excerpt from logs:

[  114.782740] (4608 39126 131072 = 39126) vs (131072 39126 = 131072)

(2*truesize, pacing_rate, tcp_limit = limit) vs (tcp_limit, pacing_rate = limit)

Reverting this patch hunk alone fixes my TCP problem. Not that I'm
saying the old logic was correct (it seems it wasn't, a limit should
be applied as min(value, max_value), right?).

Anyway the change doesn't seem to be TSO-only oriented so it would
explain the "makes no sense".


Michał
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