In ingress and clsact qdisc TCA_OPTIONS are ignored, since it's
parameterless. In tc, we add an empty addattr_l(... TCA_OPTIONS,
NULL, 0) to the netlink message nevertheless. This has the
side effect that when someone tries a 'tc qdisc replace' and
already an existing such qdisc is present, tc fails with
EINVAL here.
Reason is that in the kernel, this invokes qdisc_change() when
such requested qdisc is already present. When TCA_OPTIONS are
passed to modify parameters, it looks whether qdisc implements
.change() callback, and if not present (like in both cases here)
it returns with error. Rather than adding an empty stub to the
kernel that ignores TCA_OPTIONS again, just don't add TCA_OPTIONS
to the netlink message in the first place.
Before:
# tc qdisc replace dev foo clsact # first try
# tc qdisc replace dev foo clsact # second one
RTNETLINK answers: Invalid argument
After:
# tc qdisc replace dev foo clsact
# tc qdisc replace dev foo clsact
# tc qdisc replace dev foo clsact
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Kernel code and interface.
--------------------------
* Compile time switches
There is only one, but very important, compile time switch.
It is not settable by "make config", but should be selected
manually and after a bit of thinking in <include/net/pkt_sched.h>
PSCHED_CLOCK_SOURCE can take three values:
PSCHED_GETTIMEOFDAY
PSCHED_JIFFIES
PSCHED_CPU
PSCHED_GETTIMEOFDAY
Default setting is the most conservative PSCHED_GETTIMEOFDAY.
It is very slow both because of weird slowness of do_gettimeofday()
and because it forces code to use unnatural "timeval" format,
where microseconds and seconds fields are separate.
Besides that, it will misbehave, when delays exceed 2 seconds
(f.e. very slow links or classes bounded to small slice of bandwidth)
To resume: as only you will get it working, select correct clock
source and forget about PSCHED_GETTIMEOFDAY forever.
PSCHED_JIFFIES
Clock is derived from jiffies. On architectures with HZ=100
granularity of this clock is not enough to make reasonable
bindings to real time. However, taking into account Linux
architecture problems, which force us to use artificial
integrated clock in any case, this switch is not so bad
for schduling even on high speed networks, though policing
is not reliable.
PSCHED_CPU
It is available only for alpha and pentiums with correct
CPU timestamp. It is the fastest way, use it when it is available,
but remember: not all pentiums have this facility, and
a lot of them have clock, broken by APM etc. etc.
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