Documentation/x86/intel_rdt_ui.txt

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>
Vikas Shivappa <vikas.shivappa@intel.com>

This feature is enabled by the CONFIG_INTEL_RDT Kconfig and the
X86 /proc/cpuinfo flag bits:
RDT (Resource Director Technology) Allocation - "rdt_a"
CAT (Cache Allocation Technology) - "cat_l3", "cat_l2"
CDP (Code and Data Prioritization ) - "cdp_l3", "cdp_l2"
CQM (Cache QoS Monitoring) - "cqm_llc", "cqm_occup_llc"
MBM (Memory Bandwidth Monitoring) - "cqm_mbm_total", "cqm_mbm_local"
MBA (Memory Bandwidth Allocation) - "mba"

To use the feature mount the file system:

 # mount -t resctrl resctrl [-o cdp[,cdpl2][,mba_MBps]] /sys/fs/resctrl

mount options are:

"cdp": Enable code/data prioritization in L3 cache allocations.
"cdpl2": Enable code/data prioritization in L2 cache allocations.
"mba_MBps": Enable the MBA Software Controller(mba_sc) to specify MBA
 bandwidth in MBps

L2 and L3 CDP are controlled seperately.

RDT features are orthogonal. A particular system may support only
monitoring, only control, or both monitoring and control.

The mount succeeds if either of allocation or monitoring is present, but
only those files and directories supported by the system will be created.
For more details on the behavior of the interface during monitoring
and allocation, see the "Resource alloc and monitor groups" section.

Info directory
--------------

The 'info' directory contains information about the enabled
resources. Each resource has its own subdirectory. The subdirectory
names reflect the resource names.

Each subdirectory contains the following files with respect to
allocation:

Cache resource(L3/L2)  subdirectory contains the following files
related to allocation:

"num_closids":  	The number of CLOSIDs which are valid for this
			resource. The kernel uses the smallest number of
			CLOSIDs of all enabled resources as limit.

"cbm_mask":     	The bitmask which is valid for this resource.
			This mask is equivalent to 100%.

"min_cbm_bits": 	The minimum number of consecutive bits which
			must be set when writing a mask.

"shareable_bits":	Bitmask of shareable resource with other executing
			entities (e.g. I/O). User can use this when
			setting up exclusive cache partitions. Note that
			some platforms support devices that have their
			own settings for cache use which can over-ride
			these bits.

Memory bandwitdh(MB) subdirectory contains the following files
with respect to allocation:

"min_bandwidth":	The minimum memory bandwidth percentage which
			user can request.

"bandwidth_gran":	The granularity in which the memory bandwidth
			percentage is allocated. The allocated
			b/w percentage is rounded off to the next
			control step available on the hardware. The
			available bandwidth control steps are:
			min_bandwidth + N * bandwidth_gran.

"delay_linear": 	Indicates if the delay scale is linear or
			non-linear. This field is purely informational
			only.

If RDT monitoring is available there will be an "L3_MON" directory
with the following files:

"num_rmids":		The number of RMIDs available. This is the
			upper bound for how many "CTRL_MON" + "MON"
			groups can be created.

"mon_features":	Lists the monitoring events if
			monitoring is enabled for the resource.

"max_threshold_occupancy":
			Read/write file provides the largest value (in
			bytes) at which a previously used LLC_occupancy
			counter can be considered for re-use.

Finally, in the top level of the "info" directory there is a file
named "last_cmd_status". This is reset with every "command" issued
via the file system (making new directories or writing to any of the
control files). If the command was successful, it will read as "ok".
If the command failed, it will provide more information that can be
conveyed in the error returns from file operations. E.g.

	# echo L3:0=f7 > schemata
	bash: echo: write error: Invalid argument
	# cat info/last_cmd_status
	mask f7 has non-consecutive 1-bits

Resource alloc and monitor groups
---------------------------------

Resource groups are represented as directories in the resctrl file
system.  The default group is the root directory which, immediately
after mounting, owns all the tasks and cpus in the system and can make
full use of all resources.

On a system with RDT control features additional directories can be
created in the root directory that specify different amounts of each
resource (see "schemata" below). The root and these additional top level
directories are referred to as "CTRL_MON" groups below.

On a system with RDT monitoring the root directory and other top level
directories contain a directory named "mon_groups" in which additional
directories can be created to monitor subsets of tasks in the CTRL_MON
group that is their ancestor. These are called "MON" groups in the rest
of this document.

Removing a directory will move all tasks and cpus owned by the group it
represents to the parent. Removing one of the created CTRL_MON groups
will automatically remove all MON groups below it.

All groups contain the following files:

"tasks":
	Reading this file shows the list of all tasks that belong to
	this group. Writing a task id to the file will add a task to the
	group. If the group is a CTRL_MON group the task is removed from
	whichever previous CTRL_MON group owned the task and also from
	any MON group that owned the task. If the group is a MON group,
	then the task must already belong to the CTRL_MON parent of this
	group. The task is removed from any previous MON group.


"cpus":
	Reading this file shows a bitmask of the logical CPUs owned by
	this group. Writing a mask to this file will add and remove
	CPUs to/from this group. As with the tasks file a hierarchy is
	maintained where MON groups may only include CPUs owned by the
	parent CTRL_MON group.


"cpus_list":
	Just like "cpus", only using ranges of CPUs instead of bitmasks.


When control is enabled all CTRL_MON groups will also contain:

"schemata":
	A list of all the resources available to this group.
	Each resource has its own line and format - see below for details.

When monitoring is enabled all MON groups will also contain:

"mon_data":
	This contains a set of files organized by L3 domain and by
	RDT event. E.g. on a system with two L3 domains there will
	be subdirectories "mon_L3_00" and "mon_L3_01".	Each of these
	directories have one file per event (e.g. "llc_occupancy",
	"mbm_total_bytes", and "mbm_local_bytes"). In a MON group these
	files provide a read out of the current value of the event for
	all tasks in the group. In CTRL_MON groups these files provide
	the sum for all tasks in the CTRL_MON group and all tasks in
	MON groups. Please see example section for more details on usage.

Resource allocation rules
-------------------------
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.

Resource monitoring rules
-------------------------
1) If a task is a member of a MON group, or non-default CTRL_MON group
   then RDT events for the task will be reported in that group.

2) If a task is a member of the default CTRL_MON group, but is running
   on a CPU that is assigned to some specific group, then the RDT events
   for the task will be reported in that group.

3) Otherwise RDT events for the task will be reported in the root level
   "mon_data" group.


Notes on cache occupancy monitoring and control
-----------------------------------------------
When moving a task from one group to another you should remember that
this only affects *new* cache allocations by the task. E.g. you may have
a task in a monitor group showing 3 MB of cache occupancy. If you move
to a new group and immediately check the occupancy of the old and new
groups you will likely see that the old group is still showing 3 MB and
the new group zero. When the task accesses locations still in cache from
before the move, the h/w does not update any counters. On a busy system
you will likely see the occupancy in the old group go down as cache lines
are evicted and re-used while the occupancy in the new group rises as
the task accesses memory and loads into the cache are counted based on
membership in the new group.

The same applies to cache allocation control. Moving a task to a group
with a smaller cache partition will not evict any cache lines. The
process may continue to use them from the old partition.

Hardware uses CLOSid(Class of service ID) and an RMID(Resource monitoring ID)
to identify a control group and a monitoring group respectively. Each of
the resource groups are mapped to these IDs based on the kind of group. The
number of CLOSid and RMID are limited by the hardware and hence the creation of
a "CTRL_MON" directory may fail if we run out of either CLOSID or RMID
and creation of "MON" group may fail if we run out of RMIDs.

max_threshold_occupancy - generic concepts
------------------------------------------

Note that an RMID once freed may not be immediately available for use as
the RMID is still tagged the cache lines of the previous user of RMID.
Hence such RMIDs are placed on limbo list and checked back if the cache
occupancy has gone down. If there is a time when system has a lot of
limbo RMIDs but which are not ready to be used, user may see an -EBUSY
during mkdir.

max_threshold_occupancy is a user configurable value to determine the
occupancy at which an RMID can be freed.

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.

Memory bandwidth Allocation and monitoring
------------------------------------------

For Memory bandwidth resource, by default the user controls the resource
by indicating the percentage of total memory bandwidth.

The minimum bandwidth percentage value for each cpu model is predefined
and can be looked up through "info/MB/min_bandwidth". The bandwidth
granularity that is allocated is also dependent on the cpu model and can
be looked up at "info/MB/bandwidth_gran". The available bandwidth
control steps are: min_bw + N * bw_gran. Intermediate values are rounded
to the next control step available on the hardware.

The bandwidth throttling is a core specific mechanism on some of Intel
SKUs. Using a high bandwidth and a low bandwidth setting on two threads
sharing a core will result in both threads being throttled to use the
low bandwidth. The fact that Memory bandwidth allocation(MBA) is a core
specific mechanism where as memory bandwidth monitoring(MBM) is done at
the package level may lead to confusion when users try to apply control
via the MBA and then monitor the bandwidth to see if the controls are
effective. Below are such scenarios:

1. User may *not* see increase in actual bandwidth when percentage
   values are increased:

This can occur when aggregate L2 external bandwidth is more than L3
external bandwidth. Consider an SKL SKU with 24 cores on a package and
where L2 external  is 10GBps (hence aggregate L2 external bandwidth is
240GBps) and L3 external bandwidth is 100GBps. Now a workload with '20
threads, having 50% bandwidth, each consuming 5GBps' consumes the max L3
bandwidth of 100GBps although the percentage value specified is only 50%
<< 100%. Hence increasing the bandwidth percentage will not yeild any
more bandwidth. This is because although the L2 external bandwidth still
has capacity, the L3 external bandwidth is fully used. Also note that
this would be dependent on number of cores the benchmark is run on.

2. Same bandwidth percentage may mean different actual bandwidth
   depending on # of threads:

For the same SKU in #1, a 'single thread, with 10% bandwidth' and '4
thread, with 10% bandwidth' can consume upto 10GBps and 40GBps although
they have same percentage bandwidth of 10%. This is simply because as
threads start using more cores in an rdtgroup, the actual bandwidth may
increase or vary although user specified bandwidth percentage is same.

In order to mitigate this and make the interface more user friendly,
resctrl added support for specifying the bandwidth in MBps as well.  The
kernel underneath would use a software feedback mechanism or a "Software
Controller(mba_sc)" which reads the actual bandwidth using MBM counters
and adjust the memowy bandwidth percentages to ensure

	"actual bandwidth < user specified bandwidth".

By default, the schemata would take the bandwidth percentage values
where as user can switch to the "MBA software controller" mode using
a mount option 'mba_MBps'. The schemata format is specified in the below
sections.

L3 schemata file details (code and data prioritization disabled)
----------------------------------------------------------------
With CDP disabled the L3 schemata format is:

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

L3 schemata file 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 schemata file details
------------------------
L2 cache does not support code and data prioritization, so the
schemata format is always:

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

Memory bandwidth Allocation (default mode)
------------------------------------------

Memory b/w domain is L3 cache.

	MB:<cache_id0>=bandwidth0;<cache_id1>=bandwidth1;...

Memory bandwidth Allocation specified in MBps
---------------------------------------------

Memory bandwidth domain is L3 cache.

	MB:<cache_id0>=bw_MBps0;<cache_id1>=bw_MBps1;...

Reading/writing the schemata file
---------------------------------
Reading the schemata file will show the state of all resources
on all domains. When writing you only need to specify those values
which you wish to change.  E.g.

# cat schemata
L3DATA:0=fffff;1=fffff;2=fffff;3=fffff
L3CODE:0=fffff;1=fffff;2=fffff;3=fffff
# echo "L3DATA:2=3c0;" > schemata
# cat schemata
L3DATA:0=fffff;1=fffff;2=3c0;3=fffff
L3CODE:0=fffff;1=fffff;2=fffff;3=fffff

Examples for RDT allocation usage:

Example 1
---------
On a two socket machine (one L3 cache per socket) with just four bits
for cache bit masks, minimum b/w of 10% with a memory bandwidth
granularity of 10%

# mount -t resctrl resctrl /sys/fs/resctrl
# cd /sys/fs/resctrl
# mkdir p0 p1
# echo "L3:0=3;1=c\nMB:0=50;1=50" > /sys/fs/resctrl/p0/schemata
# echo "L3:0=3;1=3\nMB:0=50;1=50" > /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.

Similarly, tasks that are under the control of group "p0" may use a
maximum memory b/w of 50% on socket0 and 50% on socket 1.
Tasks in group "p1" may also use 50% memory b/w on both sockets.
Note that unlike cache masks, memory b/w cannot specify whether these
allocations can overlap or not. The allocations specifies the maximum
b/w that the group may be able to use and the system admin can configure
the b/w accordingly.

If the MBA is specified in MB(megabytes) then user can enter the max b/w in MB
rather than the percentage values.

# echo "L3:0=3;1=c\nMB:0=1024;1=500" > /sys/fs/resctrl/p0/schemata
# echo "L3:0=3;1=3\nMB:0=1024;1=500" > /sys/fs/resctrl/p1/schemata

In the above example the tasks in "p1" and "p0" on socket 0 would use a max b/w
of 1024MB where as on socket 1 they would use 500MB.

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 and 50% of memory b/w cannot be used by
ordinary tasks:

# echo "L3:0=3ff;1=fffff\nMB:0=50;1=100" > 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

For the same 2 socket system with memory b/w resource and CAT L3 the
schemata would look like(Assume min_bandwidth 10 and bandwidth_gran is
10):

For our first real time task this would request 20% memory b/w on socket
0.

# echo -e "L3:0=f8000;1=fffff\nMB:0=20;1=100" > p0/schemata

For our second real time task this would request an other 20% memory b/w
on socket 0.

# echo -e "L3:0=f8000;1=fffff\nMB:0=20;1=100" > p0/schemata

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, and 50% of memory bandwidth on socket 0
cannot be used by ordinary tasks:

# echo "L3:0=3ff\nMB:0=50" > 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 and 50% of memory bandwidth on
socket 0.

# mkdir p0
# echo "L3:0=ffc00\nMB:0=50" > 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. They should
also get 50% of memory bandwidth assuming that the cores 4-7 are SMT
siblings and only the real time threads are scheduled on the cores 4-7.

# echo F0 > p0/cpus

4) Locking between applications

Certain operations on the resctrl filesystem, composed of read/writes
to/from multiple files, must be atomic.

As an example, the allocation of an exclusive reservation of L3 cache
involves:

  1. Read the cbmmasks from each directory
  2. Find a contiguous set of bits in the global CBM bitmask that is clear
     in any of the directory cbmmasks
  3. Create a new directory
  4. Set the bits found in step 2 to the new directory "schemata" file

If two applications attempt to allocate space concurrently then they can
end up allocating the same bits so the reservations are shared instead of
exclusive.

To coordinate atomic operations on the resctrlfs and to avoid the problem
above, the following locking procedure is recommended:

Locking is based on flock, which is available in libc and also as a shell
script command

Write lock:

 A) Take flock(LOCK_EX) on /sys/fs/resctrl
 B) Read/write the directory structure.
 C) funlock

Read lock:

 A) Take flock(LOCK_SH) on /sys/fs/resctrl
 B) If success read the directory structure.
 C) funlock

Example with bash:

# Atomically read directory structure
$ flock -s /sys/fs/resctrl/ find /sys/fs/resctrl

# Read directory contents and create new subdirectory

$ cat create-dir.sh
find /sys/fs/resctrl/ > output.txt
mask = function-of(output.txt)
mkdir /sys/fs/resctrl/newres/
echo mask > /sys/fs/resctrl/newres/schemata

$ flock /sys/fs/resctrl/ ./create-dir.sh

Example with C:

/*
 * Example code do take advisory locks
 * before accessing resctrl filesystem
 */
#include <sys/file.h>
#include <stdlib.h>

void resctrl_take_shared_lock(int fd)
{
	int ret;

	/* take shared lock on resctrl filesystem */
	ret = flock(fd, LOCK_SH);
	if (ret) {
		perror("flock");
		exit(-1);
	}
}

void resctrl_take_exclusive_lock(int fd)
{
	int ret;

	/* release lock on resctrl filesystem */
	ret = flock(fd, LOCK_EX);
	if (ret) {
		perror("flock");
		exit(-1);
	}
}

void resctrl_release_lock(int fd)
{
	int ret;

	/* take shared lock on resctrl filesystem */
	ret = flock(fd, LOCK_UN);
	if (ret) {
		perror("flock");
		exit(-1);
	}
}

void main(void)
{
	int fd, ret;

	fd = open("/sys/fs/resctrl", O_DIRECTORY);
	if (fd == -1) {
		perror("open");
		exit(-1);
	}
	resctrl_take_shared_lock(fd);
	/* code to read directory contents */
	resctrl_release_lock(fd);

	resctrl_take_exclusive_lock(fd);
	/* code to read and write directory contents */
	resctrl_release_lock(fd);
}

Examples for RDT Monitoring along with allocation usage:

Reading monitored data
----------------------
Reading an event file (for ex: mon_data/mon_L3_00/llc_occupancy) would
show the current snapshot of LLC occupancy of the corresponding MON
group or CTRL_MON group.


Example 1 (Monitor CTRL_MON group and subset of tasks in CTRL_MON group)
---------
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
# echo 5678 > p1/tasks
# echo 5679 > p1/tasks

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.

Create monitor groups and assign a subset of tasks to each monitor group.

# cd /sys/fs/resctrl/p1/mon_groups
# mkdir m11 m12
# echo 5678 > m11/tasks
# echo 5679 > m12/tasks

fetch data (data shown in bytes)

# cat m11/mon_data/mon_L3_00/llc_occupancy
16234000
# cat m11/mon_data/mon_L3_01/llc_occupancy
14789000
# cat m12/mon_data/mon_L3_00/llc_occupancy
16789000

The parent ctrl_mon group shows the aggregated data.

# cat /sys/fs/resctrl/p1/mon_data/mon_l3_00/llc_occupancy
31234000

Example 2 (Monitor a task from its creation)
---------
On a two socket machine (one L3 cache per socket)

# mount -t resctrl resctrl /sys/fs/resctrl
# cd /sys/fs/resctrl
# mkdir p0 p1

An RMID is allocated to the group once its created and hence the <cmd>
below is monitored from its creation.

# echo $$ > /sys/fs/resctrl/p1/tasks
# <cmd>

Fetch the data

# cat /sys/fs/resctrl/p1/mon_data/mon_l3_00/llc_occupancy
31789000

Example 3 (Monitor without CAT support or before creating CAT groups)
---------

Assume a system like HSW has only CQM and no CAT support. In this case
the resctrl will still mount but cannot create CTRL_MON directories.
But user can create different MON groups within the root group thereby
able to monitor all tasks including kernel threads.

This can also be used to profile jobs cache size footprint before being
able to allocate them to different allocation groups.

# mount -t resctrl resctrl /sys/fs/resctrl
# cd /sys/fs/resctrl
# mkdir mon_groups/m01
# mkdir mon_groups/m02

# echo 3478 > /sys/fs/resctrl/mon_groups/m01/tasks
# echo 2467 > /sys/fs/resctrl/mon_groups/m02/tasks

Monitor the groups separately and also get per domain data. From the
below its apparent that the tasks are mostly doing work on
domain(socket) 0.

# cat /sys/fs/resctrl/mon_groups/m01/mon_L3_00/llc_occupancy
31234000
# cat /sys/fs/resctrl/mon_groups/m01/mon_L3_01/llc_occupancy
34555
# cat /sys/fs/resctrl/mon_groups/m02/mon_L3_00/llc_occupancy
31234000
# cat /sys/fs/resctrl/mon_groups/m02/mon_L3_01/llc_occupancy
32789


Example 4 (Monitor real time tasks)
-----------------------------------

A single socket system which has real time tasks running on cores 4-7
and non real time tasks on other cpus. We want to monitor the cache
occupancy of the real time threads on these cores.

# mount -t resctrl resctrl /sys/fs/resctrl
# cd /sys/fs/resctrl
# mkdir p1

Move the cpus 4-7 over to p1
# echo f0 > p1/cpus

View the llc occupancy snapshot

# cat /sys/fs/resctrl/p1/mon_data/mon_L3_00/llc_occupancy
11234000