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Petr Machata says: ==================== RED: Introduce an ECN tail-dropping mode When the RED qdisc is currently configured to enable ECN, the RED algorithm is used to decide whether a certain SKB should be marked. If that SKB is not ECN-capable, it is early-dropped. It is also possible to keep all traffic in the queue, and just mark the ECN-capable subset of it, as appropriate under the RED algorithm. Some switches support this mode, and some installations make use of it. There is currently no way to put the RED qdiscs to this mode. Therefore this patchset adds a new RED flag, TC_RED_TAILDROP. When the qdisc is configured with this flag, non-ECT traffic is enqueued (and tail-dropped when the queue size is exhausted) instead of being early-dropped. Unfortunately, adding a new RED flag is not as simple as it sounds. RED flags are passed in tc_red_qopt.flags. However RED neglects to validate the flag field, and just copies it over wholesale to its internal structure, and later dumps it back. A broken userspace can therefore configure a RED qdisc with arbitrary unsupported flags, and later expect to see the flags on qdisc dump. The current ABI thus allows storage of 5 bits of custom data along with the qdisc instance. GRED, SFQ and CHOKE qdiscs are in the same situation. (GRED validates VQ flags, but not the flags for the main queue.) E.g. if SFQ ever needs to support TC_RED_ADAPTATIVE, it needs another way of doing it, and at the same time it needs to retain the possibility to store 6 bits of uninterpreted data. For RED, this problem is resolved in patch #2, which adds a new attribute, and a way to separate flags from userbits that can be reused by other qdiscs. The flag itself and related behavioral changes are added in patch To test the new feature, patch #1 first introduces a TDC testsuite that covers the existing RED flags. Patch #5 later extends it with taildrop coverage. Patch #6 contains a forwarding selftest for the offloaded datapath. To test the SW datapath, I took the mlxsw selftest and adapted it in mostly obvious ways. The test is stable enough to verify that RED, ECN and ECN taildrop actually work. However, I have no confidence in its portability to other people's machines or mildly different configurations. I therefore do not find it suitable for upstreaming. GRED and CHOKE can use the same method as RED if they ever need to support extra flags. SFQ uses the length of TCA_OPTIONS to dispatch on binary control structure version, and would therefore need a different approach. v2: - Patch #1 - Require nsPlugin in each RED test - Match end-of-line to catch cases of more flags reported than requested - Patch #2: - Replaced with another patch. - Patch #3: - Fix red_use_taildrop() condition in red_enqueue switch for probabilistic case. - Patch #5: - Require nsPlugin in each RED test - Match end-of-line to catch cases of more flags reported than requested - Add a test for creation of non-ECN taildrop, which should fail ==================== Signed-off-by: David S. Miller <davem@davemloft.net> |
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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.