netdev - Re: r8169: Performance regression and latency instability

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Message-ID: <c4f8fd11-39e4-8ea1-f405-80453a4b001e@intra2net.com>
Date:   Fri, 6 Sep 2019 13:25:59 +0200
From:   Juliana Rodrigueiro <juliana.rodrigueiro@...ra2net.com>
To:     Heiner Kallweit <hkallweit1@...il.com>,
        Holger Hoffstätte <holger@...lied-asynchrony.com>,
        Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org
Cc:     Thomas Jarosch <thomas.jarosch@...ra2net.com>,
        "Gerd v. Egidy" <gerd.von.egidy@...ra2net.com>
Subject: Re: r8169: Performance regression and latency instability

Hi all!

I would like to kindly ask your help to understand this subject, since my
familiarity with the network stack is limited. I'm still
trying to find a solution for the latency problems with Realtek cards I
reported earlier.

Why does the GSO have to be forced on the socket level
if drivers for high performance chips will have it enabled by default?

A consequence of forcing GSO is that TSO is also forced [1] in
sk_setup_caps() via the NETIF_F_GSO_SOFTWARE flag (list of features with
software fallbacks). I'm sorry for my ignorance, but I wonder if TSO
will really be done in software in case the card does have NETIF_F_TSO
capability but "might not work properly" [2]. (Plus tx-checksum and SG
are all off)

Effectively for me, the following patch shows the same good performance
as the 3.14 kernel (or kernel 4.19 with reverted "tcp: switch to GSO 
being always on").

diff --git a/net/core/sock.c b/net/core/sock.c
index 9c32e8eb64da..d792d12e0f66 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -1770,7 +1770,7 @@ void sk_setup_caps(struct sock *sk, struct 
dst_entry *dst)
         sk_dst_set(sk, dst);
         sk->sk_route_caps = dst->dev->features | sk->sk_route_forced_caps;
         if (sk->sk_route_caps & NETIF_F_GSO)
-               sk->sk_route_caps |= NETIF_F_GSO_SOFTWARE;
+               sk->sk_route_caps |= (NETIF_F_GSO_SOFTWARE & 
~NETIF_F_ALL_TSO);
         sk->sk_route_caps &= ~sk->sk_route_nocaps;
         if (sk_can_gso(sk)) {
                 if (dst->header_len && !xfrm_dst_offload_ok(dst)) {

Any help is highly appreciated.

Best regards,
Juliana.

[1] 
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/net/core/sock.c?h=linux-4.19.y#n1773
[2] 
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/drivers/net/ethernet/realtek/r8169.c?h=linux-4.19.y#n7590