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b88dd52c62
Whenever MQ is not used on a multiqueue device, we experience
serious reordering problems. Bisection found the cited
commit.
The issue can be described this way :
- A single qdisc hierarchy is shared by all transmit queues.
(eg : tc qdisc replace dev eth0 root fq_codel)
- When/if try_bulk_dequeue_skb_slow() dequeues a packet targetting
a different transmit queue than the one used to build a packet train,
we stop building the current list and save the 'bad' skb (P1) in a
special queue. (bad_txq)
- When dequeue_skb() calls qdisc_dequeue_skb_bad_txq() and finds this
skb (P1), it checks if the associated transmit queues is still in frozen
state. If the queue is still blocked (by BQL or NIC tx ring full),
we leave the skb in bad_txq and return NULL.
- dequeue_skb() calls q->dequeue() to get another packet (P2)
The other packet can target the problematic queue (that we found
in frozen state for the bad_txq packet), but another cpu just ran
TX completion and made room in the txq that is now ready to accept
new packets.
- Packet P2 is sent while P1 is still held in bad_txq, P1 might be sent
at next round. In practice P2 is the lead of a big packet train
(P2,P3,P4 ...) filling the BQL budget and delaying P1 by many packets :/
To solve this problem, we have to block the dequeue process as long
as the first packet in bad_txq can not be sent. Reordering issues
disappear and no side effects have been seen.
Fixes:
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|---|---|---|
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| Documentation | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| ipc | ||
| kernel | ||
| lib | ||
| LICENSES | ||
| mm | ||
| net | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .cocciconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| 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.