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2f530df76c
Current taprio software implementation is haunted by the shadow of the igb/igc hardware model. It iterates over child qdiscs in increasing order of TXQ index, therefore giving higher xmit priority to TXQ 0 and lower to TXQ N. According to discussions with Vinicius, that is the default (perhaps even unchangeable) prioritization scheme used for the NICs that taprio was first written for (igb, igc), and we have a case of two bugs canceling out, resulting in a functional setup on igb/igc, but a less sane one on other NICs. To the best of my understanding, taprio should prioritize based on the traffic class, so it should really dequeue starting with the highest traffic class and going down from there. We get to the TXQ using the tc_to_txq[] netdev property. TXQs within the same TC have the same (strict) priority, so we should pick from them as fairly as we can. We can achieve that by implementing something very similar to q->curband from multiq_dequeue(). Since igb/igc really do have TXQ 0 of higher hardware priority than TXQ 1 etc, we need to preserve the behavior for them as well. We really have no choice, because in txtime-assist mode, taprio is essentially a software scheduler towards offloaded child tc-etf qdiscs, so the TXQ selection really does matter (not all igb TXQs support ETF/SO_TXTIME, says Kurt Kanzenbach). To preserve the behavior, we need a capability bit so that taprio can determine if it's running on igb/igc, or on something else. Because igb doesn't offload taprio at all, we can't piggyback on the qdisc_offload_query_caps() call from taprio_enable_offload(), but instead we need a separate call which is also made for software scheduling. Introduce two static keys to minimize the performance penalty on systems which only have igb/igc NICs, and on systems which only have other NICs. For mixed systems, taprio will have to dynamically check whether to dequeue using one prioritization algorithm or using the other. Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com> 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.