Manual: tc-taprio

TAPRIO(8) Linux TAPRIO(8)

NAME
TAPRIO - Time Aware Priority Shaper

SYNOPSIS
tc qdisc ... dev dev parent classid [ handle major: ] taprio num_tc tcs
map P0 P1 P2 ... queues count1@offset1 count2@offset2 ...
base-time base-time clockid clockid
sched-entry
sched-entry
sched-entry
sched-entry
[ max-sdu ]
[ fp ]

DESCRIPTION
The TAPRIO qdisc implements a simplified version of the scheduling state machine defined by IEEE 802.1Q-2018 Section
8.6.9, which allows configuration of a sequence of gate states, where each gate state allows outgoing traffic for a
subset (potentially empty) of traffic classes.

How traffic is mapped to different hardware queues is similar to mqprio(8) and so the map and queues parameters have
the same meaning.

The other parameters specify the schedule, and at what point in time it should start (it can behave as the schedule
started in the past).

PARAMETERS
num_tc Number of traffic classes to use. Up to 16 classes supported.

map
The priority to traffic class map. Maps priorities 0..15 to a specified traffic class. See mqprio(8) for more
details.

queues
Provide count and offset of queue range for each traffic class. In the format, count@offset. Queue ranges for
each traffic classes cannot overlap and must be a contiguous range of queues.

base-time
Specifies the instant in nanoseconds, using the reference of clockid, defining the time when the schedule
starts. If 'base-time' is a time in the past, the schedule will start at

base-time + (N * cycle-time)

where N is the smallest integer so the resulting time is greater than "now", and "cycle-time" is the sum of
all the intervals of the entries in the schedule;

clockid
Specifies the clock to be used by qdisc's internal timer for measuring time and scheduling events. This argu‐
ment must be omitted when using the full-offload feature (flags 0x2), since in that case, the clockid is im‐
plicitly /dev/ptpN (where N is given by ethtool -T eth0 | grep 'PTP Hardware Clock' ), and therefore not nec‐
essarily synchronized with the system's CLOCK_TAI.

sched-entry
There may multiple sched-entry parameters in a single schedule. Each one has the

sched-entry

format.

support the following values:
S for SetGateStates
H for Set-And-Hold-MAC
R for Set-And-Release-MAC
These commands follow the IEEE 802.1Q-2018 definition (Table 8-7).

is a bitmask where each bit is associated with a traffic class, so bit 0 (the least significant
bit) being "on" means that traffic class 0 is "active" for that schedule entry.

is a time duration, in nanoseconds, that specifies for how long that state defined by and
should be held before moving to the next entry.

flags
This is a bit mask which specifies different modes for taprio.

0x1 Enables the txtime-assist feature. In this mode, taprio will set the transmit timestamp depending on
the interval in which the packet needs to be transmitted. It will then utililize the etf(8) qdisc to
sort and transmit the packets at the right time. The second example can be used as a reference to con‐
figure this mode.

0x2 Enables the full-offload feature. In this mode, taprio will pass the gate control list to the NIC which
will execute it cyclically in hardware. When using full-offload, there is no need to specify the
clockid argument.

The txtime-assist and full-offload features are mutually exclusive, i.e. setting flags to 0x3 is in‐
valid.

txtime-delay
This parameter is specific to the txtime offload mode. It specifies the maximum time a packet might take to
reach the network card from the taprio qdisc. The value should always be greater than the delta specified in
the etf(8) qdisc.

max-sdu
Specifies an array containing at most 16 elements, one per traffic class, which corresponds to the queueMaxSDU
table from IEEE 802.1Q-2018. Each array element represents the maximum L2 payload size that can egress that
traffic class. Elements that are not filled in default to 0. The value 0 means that the traffic class can
send packets up to the port's maximum MTU in size.

fp
Selects whether traffic classes are express or preemptible. See tc-mqprio(8) for details.

EXAMPLES
The following example shows how an traffic schedule with three traffic classes ("num_tc 3"), which are separated dif‐
ferent traffic classes, we are going to call these TC 0, TC 1 and TC 2. We could read the "map" parameter below as:
traffic with priority 3 is classified as TC 0, priority 2 is classified as TC 1 and the rest is classified as TC 2.

The schedule will start at instant 1528743495910289987 using the reference CLOCK_TAI. The schedule is composed of
three entries each of 300us duration.

# tc qdisc replace dev eth0 parent root handle 100 stab overhead 24 taprio \
num_tc 3 \
map 2 2 1 0 2 2 2 2 2 2 2 2 2 2 2 2 \
queues 1@0 1@1 2@2 \
base-time 1528743495910289987 \
sched-entry S 01 300000 \
sched-entry S 02 300000 \
sched-entry S 04 300000 \
clockid CLOCK_TAI

Following is an example to enable the txtime offload mode in taprio. See etf(8) for more information about configur‐
ing the ETF qdisc.

# tc qdisc replace dev eth0 parent root handle 100 stab overhead 24 taprio \
num_tc 3 \
map 2 2 1 0 2 2 2 2 2 2 2 2 2 2 2 2 \
queues 1@0 1@0 1@0 \
base-time 1528743495910289987 \
sched-entry S 01 300000 \
sched-entry S 02 300000 \
sched-entry S 04 400000 \
flags 0x1 \
txtime-delay 200000 \
clockid CLOCK_TAI

# tc qdisc replace dev $IFACE parent 100:1 etf skip_skb_check \
offload delta 200000 clockid CLOCK_TAI

The following is a schedule in full offload mode. The base-time is 200 ns and the cycle-time is implicitly calculated
as the sum of all sched-entry durations (i.e. 20 us + 20 us + 60 us = 100 us). Although the base-time is in the past,
the hardware will start executing the schedule at a PTP time equal to the smallest integer multiple of 100 us, plus
200 ns, that is larger than the NIC's current PTP time. In addition, the MTU for traffic class 5 is limited to 200
octets, so that the interference this creates upon traffic class 7 during the time window when their gates are both
open is bounded. The interference is determined by the transmit time of the max SDU, plus the L2 header length, plus
the L1 overhead (determined from the size table specified using tc-stab(8)).

# tc qdisc add dev eth0 parent root stab overhead 24 taprio \
num_tc 8 \
map 0 1 2 3 4 5 6 7 \
queues 1@0 1@1 1@2 1@3 1@4 1@5 1@6 1@7 \
max-sdu 0 0 0 0 0 200 0 0 \
base-time 200 \
sched-entry S 80 20000 \
sched-entry S a0 20000 \
sched-entry S 5f 60000 \
flags 0x2

Manual: tc-taprio

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