329 lines
9.9 KiB
Plaintext
329 lines
9.9 KiB
Plaintext
User Interface for Resource Allocation in Intel Resource Director Technology
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Copyright (C) 2016 Intel Corporation
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Fenghua Yu <fenghua.yu@intel.com>
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Tony Luck <tony.luck@intel.com>
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This feature is enabled by the CONFIG_INTEL_RDT_A Kconfig and the
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X86 /proc/cpuinfo flag bits "rdt", "cat_l3" and "cdp_l3".
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To use the feature mount the file system:
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# mount -t resctrl resctrl [-o cdp] /sys/fs/resctrl
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mount options are:
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"cdp": Enable code/data prioritization in L3 cache allocations.
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Info directory
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--------------
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The 'info' directory contains information about the enabled
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resources. Each resource has its own subdirectory. The subdirectory
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names reflect the resource names. Each subdirectory contains the
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following files:
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"num_closids": The number of CLOSIDs which are valid for this
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resource. The kernel uses the smallest number of
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CLOSIDs of all enabled resources as limit.
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"cbm_mask": The bitmask which is valid for this resource. This
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mask is equivalent to 100%.
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"min_cbm_bits": The minimum number of consecutive bits which must be
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set when writing a mask.
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Resource groups
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---------------
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Resource groups are represented as directories in the resctrl file
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system. The default group is the root directory. Other groups may be
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created as desired by the system administrator using the "mkdir(1)"
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command, and removed using "rmdir(1)".
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There are three files associated with each group:
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"tasks": A list of tasks that belongs to this group. Tasks can be
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added to a group by writing the task ID to the "tasks" file
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(which will automatically remove them from the previous
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group to which they belonged). New tasks created by fork(2)
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and clone(2) are added to the same group as their parent.
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If a pid is not in any sub partition, it is in root partition
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(i.e. default partition).
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"cpus": A bitmask of logical CPUs assigned to this group. Writing
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a new mask can add/remove CPUs from this group. Added CPUs
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are removed from their previous group. Removed ones are
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given to the default (root) group. You cannot remove CPUs
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from the default group.
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"schemata": A list of all the resources available to this group.
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Each resource has its own line and format - see below for
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details.
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When a task is running the following rules define which resources
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are available to it:
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1) If the task is a member of a non-default group, then the schemata
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for that group is used.
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2) Else if the task belongs to the default group, but is running on a
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CPU that is assigned to some specific group, then the schemata for
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the CPU's group is used.
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3) Otherwise the schemata for the default group is used.
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Schemata files - general concepts
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---------------------------------
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Each line in the file describes one resource. The line starts with
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the name of the resource, followed by specific values to be applied
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in each of the instances of that resource on the system.
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Cache IDs
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---------
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On current generation systems there is one L3 cache per socket and L2
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caches are generally just shared by the hyperthreads on a core, but this
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isn't an architectural requirement. We could have multiple separate L3
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caches on a socket, multiple cores could share an L2 cache. So instead
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of using "socket" or "core" to define the set of logical cpus sharing
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a resource we use a "Cache ID". At a given cache level this will be a
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unique number across the whole system (but it isn't guaranteed to be a
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contiguous sequence, there may be gaps). To find the ID for each logical
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CPU look in /sys/devices/system/cpu/cpu*/cache/index*/id
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Cache Bit Masks (CBM)
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---------------------
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For cache resources we describe the portion of the cache that is available
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for allocation using a bitmask. The maximum value of the mask is defined
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by each cpu model (and may be different for different cache levels). It
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is found using CPUID, but is also provided in the "info" directory of
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the resctrl file system in "info/{resource}/cbm_mask". X86 hardware
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requires that these masks have all the '1' bits in a contiguous block. So
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0x3, 0x6 and 0xC are legal 4-bit masks with two bits set, but 0x5, 0x9
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and 0xA are not. On a system with a 20-bit mask each bit represents 5%
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of the capacity of the cache. You could partition the cache into four
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equal parts with masks: 0x1f, 0x3e0, 0x7c00, 0xf8000.
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L3 details (code and data prioritization disabled)
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--------------------------------------------------
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With CDP disabled the L3 schemata format is:
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L3:<cache_id0>=<cbm>;<cache_id1>=<cbm>;...
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L3 details (CDP enabled via mount option to resctrl)
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----------------------------------------------------
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When CDP is enabled L3 control is split into two separate resources
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so you can specify independent masks for code and data like this:
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L3data:<cache_id0>=<cbm>;<cache_id1>=<cbm>;...
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L3code:<cache_id0>=<cbm>;<cache_id1>=<cbm>;...
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L2 details
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----------
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L2 cache does not support code and data prioritization, so the
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schemata format is always:
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L2:<cache_id0>=<cbm>;<cache_id1>=<cbm>;...
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Example 1
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---------
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On a two socket machine (one L3 cache per socket) with just four bits
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for cache bit masks
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# mount -t resctrl resctrl /sys/fs/resctrl
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# cd /sys/fs/resctrl
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# mkdir p0 p1
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# echo "L3:0=3;1=c" > /sys/fs/resctrl/p0/schemata
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# echo "L3:0=3;1=3" > /sys/fs/resctrl/p1/schemata
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The default resource group is unmodified, so we have access to all parts
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of all caches (its schemata file reads "L3:0=f;1=f").
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Tasks that are under the control of group "p0" may only allocate from the
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"lower" 50% on cache ID 0, and the "upper" 50% of cache ID 1.
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Tasks in group "p1" use the "lower" 50% of cache on both sockets.
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Example 2
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---------
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Again two sockets, but this time with a more realistic 20-bit mask.
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Two real time tasks pid=1234 running on processor 0 and pid=5678 running on
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processor 1 on socket 0 on a 2-socket and dual core machine. To avoid noisy
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neighbors, each of the two real-time tasks exclusively occupies one quarter
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of L3 cache on socket 0.
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# mount -t resctrl resctrl /sys/fs/resctrl
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# cd /sys/fs/resctrl
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First we reset the schemata for the default group so that the "upper"
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50% of the L3 cache on socket 0 cannot be used by ordinary tasks:
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# echo "L3:0=3ff;1=fffff" > schemata
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Next we make a resource group for our first real time task and give
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it access to the "top" 25% of the cache on socket 0.
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# mkdir p0
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# echo "L3:0=f8000;1=fffff" > p0/schemata
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Finally we move our first real time task into this resource group. We
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also use taskset(1) to ensure the task always runs on a dedicated CPU
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on socket 0. Most uses of resource groups will also constrain which
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processors tasks run on.
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# echo 1234 > p0/tasks
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# taskset -cp 1 1234
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Ditto for the second real time task (with the remaining 25% of cache):
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# mkdir p1
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# echo "L3:0=7c00;1=fffff" > p1/schemata
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# echo 5678 > p1/tasks
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# taskset -cp 2 5678
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Example 3
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---------
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A single socket system which has real-time tasks running on core 4-7 and
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non real-time workload assigned to core 0-3. The real-time tasks share text
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and data, so a per task association is not required and due to interaction
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with the kernel it's desired that the kernel on these cores shares L3 with
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the tasks.
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# mount -t resctrl resctrl /sys/fs/resctrl
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# cd /sys/fs/resctrl
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First we reset the schemata for the default group so that the "upper"
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50% of the L3 cache on socket 0 cannot be used by ordinary tasks:
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# echo "L3:0=3ff" > schemata
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Next we make a resource group for our real time cores and give
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it access to the "top" 50% of the cache on socket 0.
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# mkdir p0
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# echo "L3:0=ffc00;" > p0/schemata
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Finally we move core 4-7 over to the new group and make sure that the
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kernel and the tasks running there get 50% of the cache.
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# echo C0 > p0/cpus
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4) Locking between applications
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Certain operations on the resctrl filesystem, composed of read/writes
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to/from multiple files, must be atomic.
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As an example, the allocation of an exclusive reservation of L3 cache
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involves:
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1. Read the cbmmasks from each directory
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2. Find a contiguous set of bits in the global CBM bitmask that is clear
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in any of the directory cbmmasks
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3. Create a new directory
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4. Set the bits found in step 2 to the new directory "schemata" file
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If two applications attempt to allocate space concurrently then they can
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end up allocating the same bits so the reservations are shared instead of
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exclusive.
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To coordinate atomic operations on the resctrlfs and to avoid the problem
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above, the following locking procedure is recommended:
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Locking is based on flock, which is available in libc and also as a shell
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script command
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Write lock:
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A) Take flock(LOCK_EX) on /sys/fs/resctrl
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B) Read/write the directory structure.
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C) funlock
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Read lock:
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A) Take flock(LOCK_SH) on /sys/fs/resctrl
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B) If success read the directory structure.
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C) funlock
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Example with bash:
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# Atomically read directory structure
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$ flock -s /sys/fs/resctrl/ find /sys/fs/resctrl
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# Read directory contents and create new subdirectory
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$ cat create-dir.sh
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find /sys/fs/resctrl/ > output.txt
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mask = function-of(output.txt)
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mkdir /sys/fs/resctrl/newres/
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echo mask > /sys/fs/resctrl/newres/schemata
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$ flock /sys/fs/resctrl/ ./create-dir.sh
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Example with C:
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/*
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* Example code do take advisory locks
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* before accessing resctrl filesystem
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*/
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#include <sys/file.h>
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#include <stdlib.h>
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void resctrl_take_shared_lock(int fd)
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{
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int ret;
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/* take shared lock on resctrl filesystem */
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ret = flock(fd, LOCK_SH);
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if (ret) {
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perror("flock");
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exit(-1);
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}
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}
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void resctrl_take_exclusive_lock(int fd)
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{
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int ret;
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/* release lock on resctrl filesystem */
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ret = flock(fd, LOCK_EX);
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if (ret) {
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perror("flock");
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exit(-1);
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}
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}
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void resctrl_release_lock(int fd)
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{
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int ret;
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/* take shared lock on resctrl filesystem */
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ret = flock(fd, LOCK_UN);
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if (ret) {
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perror("flock");
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exit(-1);
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}
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}
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void main(void)
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{
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int fd, ret;
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fd = open("/sys/fs/resctrl", O_DIRECTORY);
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if (fd == -1) {
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perror("open");
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exit(-1);
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}
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resctrl_take_shared_lock(fd);
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/* code to read directory contents */
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resctrl_release_lock(fd);
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resctrl_take_exclusive_lock(fd);
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/* code to read and write directory contents */
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resctrl_release_lock(fd);
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}
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