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| author | Eric Dumazet <edumazet@google.com> | 2023-03-23 09:28:42 -0700 |
|---|---|---|
| committer | Jakub Kicinski <kuba@kernel.org> | 2023-03-27 19:29:22 -0700 |
| commit | 3948b05950fdd64002a5f182c65ba5cf2d53cf71 (patch) | |
| tree | 6397f0b034631ef13231dc76df8352292dbd54b5 /net/Kconfig | |
| parent | e5b42483ccce50d5b957f474fd332afd4ef0c27b (diff) | |
| download | lwn-3948b05950fdd64002a5f182c65ba5cf2d53cf71.tar.gz lwn-3948b05950fdd64002a5f182c65ba5cf2d53cf71.zip | |
net: introduce a config option to tweak MAX_SKB_FRAGS
Currently, MAX_SKB_FRAGS value is 17.
For standard tcp sendmsg() traffic, no big deal because tcp_sendmsg()
attempts order-3 allocations, stuffing 32768 bytes per frag.
But with zero copy, we use order-0 pages.
For BIG TCP to show its full potential, we add a config option
to be able to fit up to 45 segments per skb.
This is also needed for BIG TCP rx zerocopy, as zerocopy currently
does not support skbs with frag list.
We have used MAX_SKB_FRAGS=45 value for years at Google before
we deployed 4K MTU, with no adverse effect, other than
a recent issue in mlx4, fixed in commit 26782aad00cc
("net/mlx4: MLX4_TX_BOUNCE_BUFFER_SIZE depends on MAX_SKB_FRAGS")
Back then, goal was to be able to receive full size (64KB) GRO
packets without the frag_list overhead.
Note that /proc/sys/net/core/max_skb_frags can also be used to limit
the number of fragments TCP can use in tx packets.
By default we keep the old/legacy value of 17 until we get
more coverage for the updated values.
Sizes of struct skb_shared_info on 64bit arches
MAX_SKB_FRAGS | sizeof(struct skb_shared_info):
==============================================
17 320
21 320+64 = 384
25 320+128 = 448
29 320+192 = 512
33 320+256 = 576
37 320+320 = 640
41 320+384 = 704
45 320+448 = 768
This inflation might cause problems for drivers assuming they could pack
both the incoming packet (for MTU=1500) and skb_shared_info in half a page,
using build_skb().
v3: fix build error when CONFIG_NET=n
v2: fix two build errors assuming MAX_SKB_FRAGS was "unsigned long"
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Link: https://lore.kernel.org/r/20230323162842.1935061-1-eric.dumazet@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Diffstat (limited to 'net/Kconfig')
| -rw-r--r-- | net/Kconfig | 12 |
1 files changed, 12 insertions, 0 deletions
diff --git a/net/Kconfig b/net/Kconfig index 48c33c222199..f806722bccf4 100644 --- a/net/Kconfig +++ b/net/Kconfig @@ -251,6 +251,18 @@ config PCPU_DEV_REFCNT network device refcount are using per cpu variables if this option is set. This can be forced to N to detect underflows (with a performance drop). +config MAX_SKB_FRAGS + int "Maximum number of fragments per skb_shared_info" + range 17 45 + default 17 + help + Having more fragments per skb_shared_info can help GRO efficiency. + This helps BIG TCP workloads, but might expose bugs in some + legacy drivers. + This also increases memory overhead of small packets, + and in drivers using build_skb(). + If unsure, say 17. + config RPS bool depends on SMP && SYSFS |