Documentation, x86: Documentation for Intel resource allocation user interface

User Interface for Resource Allocation in Intel Resource Director Technology

Copyright (C) 2016 Intel Corporation

Fenghua Yu <fenghua.yu@intel.com>

Tony Luck <tony.luck@intel.com>

This feature is enabled by the CONFIG_INTEL_RDT_A Kconfig and the

X86 /proc/cpuinfo flag bits "rdt", "cat_l3" and "cdp_l3".

To use the feature mount the file system:

 # mount -t resctrl resctrl [-o cdp] /sys/fs/resctrl

mount options are:

"cdp": Enable code/data prioritization in L3 cache allocations.

Resource groups

---------------

Resource groups are represented as directories in the resctrl file

system. The default group is the root directory. Other groups may be

created as desired by the system administrator using the "mkdir(1)"

command, and removed using "rmdir(1)".

There are three files associated with each group:

"tasks": A list of tasks that belongs to this group. Tasks can be

	added to a group by writing the task ID to the "tasks" file

	(which will automatically remove them from the previous

	group to which they belonged). New tasks created by fork(2)

	and clone(2) are added to the same group as their parent.

	If a pid is not in any sub partition, it is in root partition

	(i.e. default partition).

"cpus": A bitmask of logical CPUs assigned to this group. Writing

	a new mask can add/remove CPUs from this group. Added CPUs

	are removed from their previous group. Removed ones are

	given to the default (root) group. You cannot remove CPUs

	from the default group.

"schemata": A list of all the resources available to this group.

	Each resource has its own line and format - see below for

	details.

When a task is running the following rules define which resources

are available to it:

1) If the task is a member of a non-default group, then the schemata

for that group is used.

2) Else if the task belongs to the default group, but is running on a

CPU that is assigned to some specific group, then the schemata for

the CPU's group is used.

3) Otherwise the schemata for the default group is used.

Schemata files - general concepts

---------------------------------

Each line in the file describes one resource. The line starts with

the name of the resource, followed by specific values to be applied

in each of the instances of that resource on the system.

Cache IDs

---------

On current generation systems there is one L3 cache per socket and L2

caches are generally just shared by the hyperthreads on a core, but this

isn't an architectural requirement. We could have multiple separate L3

caches on a socket, multiple cores could share an L2 cache. So instead

of using "socket" or "core" to define the set of logical cpus sharing

a resource we use a "Cache ID". At a given cache level this will be a

unique number across the whole system (but it isn't guaranteed to be a

contiguous sequence, there may be gaps).  To find the ID for each logical

CPU look in /sys/devices/system/cpu/cpu*/cache/index*/id

Cache Bit Masks (CBM)

---------------------

For cache resources we describe the portion of the cache that is available

for allocation using a bitmask. The maximum value of the mask is defined

by each cpu model (and may be different for different cache levels). It

is found using CPUID, but is also provided in the "info" directory of

the resctrl file system in "info/{resource}/cbm_mask". X86 hardware

requires that these masks have all the '1' bits in a contiguous block. So

0x3, 0x6 and 0xC are legal 4-bit masks with two bits set, but 0x5, 0x9

and 0xA are not.  On a system with a 20-bit mask each bit represents 5%

of the capacity of the cache. You could partition the cache into four

equal parts with masks: 0x1f, 0x3e0, 0x7c00, 0xf8000.

L3 details (code and data prioritization disabled)

--------------------------------------------------

With CDP disabled the L3 schemata format is:

	L3:<cache_id0>=<cbm>;<cache_id1>=<cbm>;...

L3 details (CDP enabled via mount option to resctrl)

----------------------------------------------------

When CDP is enabled L3 control is split into two separate resources

so you can specify independent masks for code and data like this:

	L3data:<cache_id0>=<cbm>;<cache_id1>=<cbm>;...

	L3code:<cache_id0>=<cbm>;<cache_id1>=<cbm>;...

L2 details

----------

L2 cache does not support code and data prioritization, so the

schemata format is always:

	L2:<cache_id0>=<cbm>;<cache_id1>=<cbm>;...

Example 1

---------

On a two socket machine (one L3 cache per socket) with just four bits

for cache bit masks

# mount -t resctrl resctrl /sys/fs/resctrl

# cd /sys/fs/resctrl

# mkdir p0 p1

# echo "L3:0=3;1=c" > /sys/fs/resctrl/p0/schemata

# echo "L3:0=3;1=3" > /sys/fs/resctrl/p1/schemata

The default resource group is unmodified, so we have access to all parts

of all caches (its schemata file reads "L3:0=f;1=f").

Tasks that are under the control of group "p0" may only allocate from the

"lower" 50% on cache ID 0, and the "upper" 50% of cache ID 1.

Tasks in group "p1" use the "lower" 50% of cache on both sockets.

Example 2

---------

Again two sockets, but this time with a more realistic 20-bit mask.

Two real time tasks pid=1234 running on processor 0 and pid=5678 running on

processor 1 on socket 0 on a 2-socket and dual core machine. To avoid noisy

neighbors, each of the two real-time tasks exclusively occupies one quarter

of L3 cache on socket 0.

# mount -t resctrl resctrl /sys/fs/resctrl

# cd /sys/fs/resctrl

First we reset the schemata for the default group so that the "upper"

50% of the L3 cache on socket 0 cannot be used by ordinary tasks:

# echo "L3:0=3ff;1=fffff" > schemata

Next we make a resource group for our first real time task and give

it access to the "top" 25% of the cache on socket 0.

# mkdir p0

# echo "L3:0=f8000;1=fffff" > p0/schemata

Finally we move our first real time task into this resource group. We

also use taskset(1) to ensure the task always runs on a dedicated CPU

on socket 0. Most uses of resource groups will also constrain which

processors tasks run on.

# echo 1234 > p0/tasks

# taskset -cp 1 1234

Ditto for the second real time task (with the remaining 25% of cache):

# mkdir p1

# echo "L3:0=7c00;1=fffff" > p1/schemata

# echo 5678 > p1/tasks

# taskset -cp 2 5678

Example 3

---------

A single socket system which has real-time tasks running on core 4-7 and

non real-time workload assigned to core 0-3. The real-time tasks share text

and data, so a per task association is not required and due to interaction

with the kernel it's desired that the kernel on these cores shares L3 with

the tasks.

# mount -t resctrl resctrl /sys/fs/resctrl

# cd /sys/fs/resctrl

First we reset the schemata for the default group so that the "upper"

50% of the L3 cache on socket 0 cannot be used by ordinary tasks:

# echo "L3:0=3ff" > schemata

Next we make a resource group for our real time cores and give

it access to the "top" 50% of the cache on socket 0.

# mkdir p0

# echo "L3:0=ffc00;" > p0/schemata

Finally we move core 4-7 over to the new group and make sure that the

kernel and the tasks running there get 50% of the cache.

# echo C0 > p0/cpus