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>

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.


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:

"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.


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

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);
}