[Bug 2129005] Re: tc class show dev provides incorrect value for cburst

[Frode Nordahl]

The offending iproute2 commit appears to be:

commit d947f365602b30657d1b797e7464000d0ab88d5a
Author: Jonathan Lennox <jonathan.lennox42 at gmail.com>
Date:   Wed Feb 26 18:53:21 2025 +0000

    tc: Fix rounding in tc_calc_xmittime and tc_calc_xmitsize.
    
    Currently, tc_calc_xmittime and tc_calc_xmitsize round from double to
    int three times — once when they call tc_core_time2tick /
    tc_core_tick2time (whose argument is int), once when those functions
    return (their return value is int), and then finally when the tc_calc_*
    functions return.  This leads to extremely granular and inaccurate
    conversions.
    
    As a result, for example, on my test system (where tick_in_usec=15.625,
    clock_factor=1, and hz=1000000000) for a bitrate of 1Gbps, all tc htb
    burst values between 0 and 999 bytes get encoded as 0 ticks; all values
    between 1000 and 1999 bytes get encoded as 15 ticks (equivalent to 960
    bytes); all values between 2000 and 2999 bytes as 31 ticks (1984 bytes);
    etc.
    
    The patch changes the code so these calculations are done internally in
    floating-point, and only rounded to integer values when the value is
    returned. It also changes tc_calc_xmittime to round its calculated value
    up, rather than down, to ensure that the calculated time is actually
    sufficient for the requested size.
    
    Signed-off-by: Jonathan Lennox <jonathan.lennox at 8x8.com>
    Signed-off-by: David Ahern <dsahern at kernel.org>

It looks like a deliberate change of behavior.

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https://bugs.launchpad.net/bugs/2129005

Title:
  tc class show dev provides incorrect value for cburst

Status in iproute2 package in Ubuntu:
  New
Status in ovn package in Ubuntu:
  New

Bug description:
  As detected by the ovn system testsuite the following test fails when
  upgrading iprotue2 from 6.14 to more recent versions:

      egress qos

  The command being executed by the testsuite is:

      tc class show dev ovs-public

  And the printed value becomes:

      class htb 1:2 parent 1:fffe prio 0 rate 12Kbit ceil 34359Mbit
  burst 375000b cburst 374933b

  While the expected output is:

      class htb 1:2 parent 1:fffe prio 0 rate 12Kbit ceil 34359Mbit
  burst 375000b cburst 373662b

  The issue appears to be present in iproute2 versions 6.17.0-1,
  6.16.0-1ubuntu2

  Downgrading to 6.14.0-1ubuntu1 makes the problem go away.

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