x86/intel_rdt/cqm: Documentation for resctrl based RDT Monitoring

Tony Luck <tony.luck@intel.com>

Vikas Shivappa <vikas.shivappa@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".

This feature is enabled by the CONFIG_INTEL_RDT Kconfig and the

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

To use the feature mount the file system:

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

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.

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

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

"min_cbm_bits": 	The minimum number of consecutive bits which

			must be set when writing a mask.

Memory bandwitdh(MB) subdirectory contains the following files:

Memory bandwitdh(MB) subdirectory contains the following files

with respect to allocation:

"min_bandwidth":	The minimum memory bandwidth percentage which

			user can request.

			non-linear. This field is purely informational

			only.

Resource groups

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

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.

Resource alloc and monitor 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)".

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.

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).

When control is enabled all CTRL_MON groups will also contain:

"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.

"cpus_list": One or more CPU ranges of logical CPUs assigned to this

	     group. Same rules apply like for the "cpus" file.

When monitoring is enabled all 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.

"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.

When a task is running the following rules define which resources

are available to it:

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.

   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

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

sharing a core will result in both threads being throttled to use the

low bandwidth.

L3 details (code and data prioritization disabled)

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

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 details (CDP enabled via mount option to resctrl)

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

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 details

----------

L2 schemata file details

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

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

schemata format is always:

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

	/* 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 > p0/cpus

View the llc occupancy snapshot

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

11234000