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Íñigo Huguet f28100cb9c sfc: fix lack of XDP TX queues - error XDP TX failed (-22) 
Fixes: e26ca4b535 sfc: reduce the number of requested xdp ev queues

The buggy commit intended to allocate less channels for XDP in order to
be more unlikely to reach the limit of 32 channels of the driver.

The idea was to use each IRQ/eventqeue for more XDP TX queues than
before, calculating which is the maximum number of TX queues that one
event queue can handle. For example, in EF10 each event queue could
handle up to 8 queues, better than the 4 they were handling before the
change. This way, it would have to allocate half of channels than before
for XDP TX.

The problem is that the TX queues are also contained inside the channel
structs, and there are only 4 queues per channel. Reducing the number of
channels means also reducing the number of queues, resulting in not
having the desired number of 1 queue per CPU.

This leads to getting errors on XDP_TX and XDP_REDIRECT if they're
executed from a high numbered CPU, because there only exist queues for
the low half of CPUs, actually. If XDP_TX/REDIRECT is executed in a low
numbered CPU, the error doesn't happen. This is the error in the logs
(repeated many times, even rate limited):
sfc 0000:5e:00.0 ens3f0np0: XDP TX failed (-22)

This errors happens in function efx_xdp_tx_buffers, where it expects to
have a dedicated XDP TX queue per CPU.

Reverting the change makes again more likely to reach the limit of 32
channels in machines with many CPUs. If this happen, no XDP_TX/REDIRECT
will be possible at all, and we will have this log error messages:

At interface probe:
sfc 0000:5e:00.0: Insufficient resources for 12 XDP event queues (24 other channels, max 32)

At every subsequent XDP_TX/REDIRECT failure, rate limited:
sfc 0000:5e:00.0 ens3f0np0: XDP TX failed (-22)

However, without reverting the change, it makes the user to think that
everything is OK at probe time, but later it fails in an unpredictable
way, depending on the CPU that handles the packet.

It is better to restore the predictable behaviour. If the user sees the
error message at probe time, he/she can try to configure the best way it
fits his/her needs. At least, he/she will have 2 options:
- Accept that XDP_TX/REDIRECT is not available (he/she may not need it)
- Load sfc module with modparam 'rss_cpus' with a lower number, thus
  creating less normal RX queues/channels, letting more free resources
  for XDP, with some performance penalty.

Anyway, let the calculation of maximum TX queues that can be handled by
a single event queue, and use it only if it's less than the number of TX
queues per channel. This doesn't happen in practice, but could happen if
some constant values are tweaked in the future, such us
EFX_MAX_TXQ_PER_CHANNEL, EFX_MAX_EVQ_SIZE or EFX_MAX_DMAQ_SIZE.

Related mailing list thread:
https://lore.kernel.org/bpf/20201215104327.2be76156@carbon/

Signed-off-by: Íñigo Huguet <ihuguet@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
2021-07-13 10:02:41 -07:00
arch Merge git://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf 2021-07-09 15:22:45 -07:00
block for-5.14/drivers-2021-06-29 2021-06-30 12:21:16 -07:00
certs Kbuild updates for v5.13 (2nd) 2021-05-08 10:00:11 -07:00
crypto Merge branch 'linus' of git://git.kernel.org/pub/scm/linux/kernel/git/herbert/crypto-2.6 2021-06-28 16:04:56 -07:00
Documentation netfilter: conntrack: add new sysctl to disable RST check 2021-07-06 14:15:12 +02:00
drivers sfc: fix lack of XDP TX queues - error XDP TX failed (-22) 2021-07-13 10:02:41 -07:00
fs Networking changes for 5.14. 2021-06-30 15:51:09 -07:00
include net: phy: marvell10g: fix differentiation of 88X3310 from 88X3340 2021-07-11 10:02:33 -07:00
init Clang feature updates for v5.14-rc1 2021-06-30 14:33:25 -07:00
ipc Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace 2021-06-28 20:39:26 -07:00
kernel bpf: Track subprog poke descriptors correctly and fix use-after-free 2021-07-09 12:08:27 +02:00
lib Merge branch 'akpm' (patches from Andrew) 2021-06-29 17:29:11 -07:00
LICENSES LICENSES: Add the CC-BY-4.0 license 2020-12-08 10:33:27 -07:00
mm for-5.14/block-2021-06-29 2021-06-30 12:12:56 -07:00
net net: Use nlmsg_unicast() instead of netlink_unicast() 2021-07-13 09:28:29 -07:00
samples bpf, samples: Fix xdpsock with '-M' parameter missing unload process 2021-07-05 23:34:18 +02:00
scripts Merge branch 'akpm' (patches from Andrew) 2021-06-29 17:29:11 -07:00
security One very minor code cleanup change 2021-06-30 15:30:47 -07:00
sound spi: Updates for v5.14 2021-06-28 11:10:20 -07:00
tools selftests: mptcp: fix case multiple subflows limited by server 2021-07-09 18:38:53 -07:00
usr .gitignore: prefix local generated files with a slash 2021-05-02 00:43:35 +09:00
virt Merge branch 'akpm' (patches from Andrew) 2021-06-29 17:29:11 -07:00
.clang-format clang-format: Update with the latest for_each macro list 2021-05-12 23:32:39 +02:00
.cocciconfig
.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore .gitignore: ignore only top-level modules.builtin 2021-05-02 00:43:35 +09:00
.mailmap m68k updates for v5.14 2021-06-28 14:01:03 -07:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: move Murali Karicheri to credits 2021-04-29 15:47:30 -07:00
Kbuild kbuild: rename hostprogs-y/always to hostprogs/always-y 2020-02-04 01:53:07 +09:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS MAINTAINERS: add entry for PTP virtual clock driver 2021-07-01 13:08:19 -07:00
Makefile arm64 updates for 5.14 2021-06-28 14:04:24 -07:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.