User Interface for Resource Allocation in Intel Resource Director Technology Copyright (C) 2016 Intel Corporation Fenghua Yu Tony Luck 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:=;=;... 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:=;=;... L3code:=;=;... L2 details ---------- L2 cache does not support code and data prioritization, so the schemata format is always: L2:=;=;... 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 #include 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); }