[PATCH V3 15/39] x86/intel_rdt: Documentation for Cache Pseudo-Locking
From: Reinette Chatre
Date: Wed Apr 25 2018 - 14:26:02 EST
Add description of Cache Pseudo-Locking feature, its interface,
as well as an example of its usage.
Signed-off-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>
---
Documentation/x86/intel_rdt_ui.txt | 278 ++++++++++++++++++++++++++++++++++++-
1 file changed, 276 insertions(+), 2 deletions(-)
diff --git a/Documentation/x86/intel_rdt_ui.txt b/Documentation/x86/intel_rdt_ui.txt
index f1210ac40e3d..da74fc34dfc0 100644
--- a/Documentation/x86/intel_rdt_ui.txt
+++ b/Documentation/x86/intel_rdt_ui.txt
@@ -27,7 +27,11 @@ mount options are:
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.
+monitoring, only control, or both monitoring and control. Cache
+pseudo-locking is a unique way of using cache control to "pin" or
+"lock" data in the cache. Details can be found in
+"Cache Pseudo-Locking".
+
The mount succeeds if either of allocation or monitoring is present, but
only those files and directories supported by the system will be created.
@@ -84,6 +88,8 @@ related to allocation:
and available for sharing.
"E" - Corresponding region is used exclusively by
one resource group. No sharing allowed.
+ "P" - Corresponding region is pseudo-locked. No
+ sharing allowed.
Memory bandwitdh(MB) subdirectory contains the following files
with respect to allocation:
@@ -190,7 +196,12 @@ When control is enabled all CTRL_MON groups will also contain:
"mode":
The "mode" of the resource group dictates the sharing of its
allocations. A "shareable" resource group allows sharing of its
- allocations while an "exclusive" resource group does not.
+ allocations while an "exclusive" resource group does not. A
+ cache pseudo-locked region is created by first writing
+ "pseudo-locksetup" to the "mode" file before writing the cache
+ pseudo-locked region's schemata to the resource group's "schemata"
+ file. On successful pseudo-locked region creation the mode will
+ automatically change to "pseudo-locked".
When monitoring is enabled all MON groups will also contain:
@@ -360,6 +371,168 @@ L3CODE:0=fffff;1=fffff;2=fffff;3=fffff
L3DATA:0=fffff;1=fffff;2=3c0;3=fffff
L3CODE:0=fffff;1=fffff;2=fffff;3=fffff
+Cache Pseudo-Locking
+--------------------
+CAT enables a user to specify the amount of cache space that an
+application can fill. Cache pseudo-locking builds on the fact that a
+CPU can still read and write data pre-allocated outside its current
+allocated area on a cache hit. With cache pseudo-locking, data can be
+preloaded into a reserved portion of cache that no application can
+fill, and from that point on will only serve cache hits. The cache
+pseudo-locked memory is made accessible to user space where an
+application can map it into its virtual address space and thus have
+a region of memory with reduced average read latency.
+
+The creation of a cache pseudo-locked region is triggered by a request
+from the user to do so that is accompanied by a schemata of the region
+to be pseudo-locked. The cache pseudo-locked region is created as follows:
+- Create a CAT allocation CLOSNEW with a CBM matching the schemata
+ from the user of the cache region that will contain the pseudo-locked
+ memory. This region must not overlap with any current CAT allocation/CLOS
+ on the system and no future overlap with this cache region is allowed
+ while the pseudo-locked region exists.
+- Create a contiguous region of memory of the same size as the cache
+ region.
+- Flush the cache, disable hardware prefetchers, disable preemption.
+- Make CLOSNEW the active CLOS and touch the allocated memory to load
+ it into the cache.
+- Set the previous CLOS as active.
+- At this point the closid CLOSNEW can be released - the cache
+ pseudo-locked region is protected as long as its CBM does not appear in
+ any CAT allocation. Even though the cache pseudo-locked region will from
+ this point on not appear in any CBM of any CLOS an application running with
+ any CLOS will be able to access the memory in the pseudo-locked region since
+ the region continues to serve cache hits.
+- The contiguous region of memory loaded into the cache is exposed to
+ user-space as a character device.
+
+Cache pseudo-locking increases the probability that data will remain
+in the cache via carefully configuring the CAT feature and controlling
+application behavior. There is no guarantee that data is placed in
+cache. Instructions like INVD, WBINVD, CLFLUSH, etc. can still evict
+âlockedâ data from cache. Power management C-states may shrink or
+power off cache. It is thus recommended to limit the processor maximum
+C-state, for example, by setting the processor.max_cstate kernel parameter.
+
+It is required that an application using a pseudo-locked region runs
+with affinity to the cores (or a subset of the cores) associated
+with the cache on which the pseudo-locked region resides. A sanity check
+within the code will not allow an application to map pseudo-locked memory
+unless it runs with affinity to cores associated with the cache on which the
+pseudo-locked region resides. The sanity check is only done during the
+initial mmap() handling, there is no enforcement afterwards and the
+application self needs to ensure it remains affine to the correct cores.
+
+Pseudo-locking is accomplished in two stages:
+1) During the first stage the system administrator allocates a portion
+ of cache that should be dedicated to pseudo-locking. At this time an
+ equivalent portion of memory is allocated, loaded into allocated
+ cache portion, and exposed as a character device.
+2) During the second stage a user-space application maps (mmap()) the
+ pseudo-locked memory into its address space.
+
+Cache Pseudo-Locking Interface
+------------------------------
+A pseudo-locked region is created using the resctrl interface as follows:
+
+1) Create a new resource group by creating a new directory in /sys/fs/resctrl.
+2) Change the new resource group's mode to "pseudo-locksetup" by writing
+ "pseudo-locksetup" to the "mode" file.
+3) Write the schemata of the pseudo-locked region to the "schemata" file. All
+ bits within the schemata should be "unused" according to the "bit_usage"
+ file.
+
+On successful pseudo-locked region creation the "mode" file will contain
+"pseudo-locked" and a new character device with the same name as the resource
+group will exist in /dev/pseudo_lock. This character device can be mmap()'ed
+by user space in order to obtain access to the pseudo-locked memory region.
+
+An example of cache pseudo-locked region creation and usage can be found below.
+
+Cache Pseudo-Locking Debugging Interface
+---------------------------------------
+The pseudo-locking debugging interface is enabled with
+CONFIG_INTEL_RDT_DEBUGFS and can be found in /sys/kernel/debug/resctrl.
+
+There is no explicit way for the kernel to test if a provided memory
+location is present in the cache. The pseudo-locking debugging interface uses
+the tracing infrastructure to provide two ways to measure cache residency of
+the pseudo-locked region:
+1) Memory access latency using the pseudo_lock_mem_latency tracepoint. Data
+ from these measurements are best visualized using a hist trigger (see
+ example below). In this test the pseudo-locked region is traversed at
+ a stride of 32 bytes while hardware prefetchers and preemption
+ are disabled. This also provides a substitute visualization of cache
+ hits and misses.
+2) Cache hit and miss measurements using model specific precision counters if
+ available. Depending on the levels of cache on the system the pseudo_lock_l2
+ and pseudo_lock_l3 tracepoints are available.
+ WARNING: triggering this measurement uses from two (for just L2
+ measurements) to four (for L2 and L3 measurements) precision counters on
+ the system, if any other measurements are in progress the counters and
+ their corresponding event registers will be clobbered.
+
+When a pseudo-locked region is created a new debugfs directory is created for
+it in debugfs as /sys/kernel/debug/resctrl/<newdir>. A single
+write-only file, pseudo_lock_measure, is present in this directory. The
+measurement on the pseudo-locked region depends on the number, 1 or 2,
+written to this debugfs file. Since the measurements are recorded with the
+tracing infrastructure the relevant tracepoints need to be enabled before the
+measurement is triggered.
+
+Example of latency debugging interface:
+In this example a pseudo-locked region named "newlock" was created. Here is
+how we can measure the latency in cycles of reading from this region:
+# :> /sys/kernel/debug/tracing/trace
+# echo 'hist:keys=latency' > /sys/kernel/debug/tracing/events/resctrl/pseudo_lock_mem_latency/trigger
+# echo 1 > /sys/kernel/debug/tracing/events/resctrl/pseudo_lock_mem_latency/enable
+# echo 1 > /sys/kernel/debug/resctrl/newlock/pseudo_lock_measure
+# echo 0 > /sys/kernel/debug/tracing/events/resctrl/pseudo_lock_mem_latency/enable
+# cat /sys/kernel/debug/tracing/events/resctrl/pseudo_lock_mem_latency/hist
+
+# event histogram
+#
+# trigger info: hist:keys=latency:vals=hitcount:sort=hitcount:size=2048 [active]
+#
+
+{ latency: 456 } hitcount: 1
+{ latency: 50 } hitcount: 83
+{ latency: 36 } hitcount: 96
+{ latency: 44 } hitcount: 174
+{ latency: 48 } hitcount: 195
+{ latency: 46 } hitcount: 262
+{ latency: 42 } hitcount: 693
+{ latency: 40 } hitcount: 3204
+{ latency: 38 } hitcount: 3484
+
+Totals:
+ Hits: 8192
+ Entries: 9
+ Dropped: 0
+
+Example of cache hits/misses debugging:
+In this example a pseudo-locked region named "newlock" was created on the L2
+cache of a platform. Here is how we can obtain details of the cache hits
+and misses using the platform's precision counters.
+
+# :> /sys/kernel/debug/tracing/trace
+# echo 1 > /sys/kernel/debug/tracing/events/resctrl/pseudo_lock_l2/enable
+# echo 2 > /sys/kernel/debug/resctrl/newlock/pseudo_lock_measure
+# echo 0 > /sys/kernel/debug/tracing/events/resctrl/pseudo_lock_l2/enable
+# cat /sys/kernel/debug/tracing/trace
+
+# tracer: nop
+#
+# _-----=> irqs-off
+# / _----=> need-resched
+# | / _---=> hardirq/softirq
+# || / _--=> preempt-depth
+# ||| / delay
+# TASK-PID CPU# |||| TIMESTAMP FUNCTION
+# | | | |||| | |
+ pseudo_lock_mea-1672 [002] .... 3132.860500: pseudo_lock_l2: hits=4097 miss=0
+
+
Examples for RDT allocation usage:
Example 1
@@ -537,6 +710,107 @@ A resource group cannot be forced to overlap with an exclusive resource group:
# cat info/last_cmd_status
overlaps with exclusive group
+Example of Cache Pseudo-Locking
+-------------------------------
+Lock portion of L2 cache from cache id 1 using CBM 0x3. Pseudo-locked
+region is exposed at /dev/pseudo_lock/newlock that can be provided to
+application for argument to mmap().
+
+# mount -t resctrl resctrl /sys/fs/resctrl/
+# cd /sys/fs/resctrl
+
+Ensure that there are bits available that can be pseudo-locked, since only
+unused bits can be pseudo-locked the bits to be pseudo-locked needs to be
+removed from the default resource group's schemata:
+# cat info/L2/bit_usage
+0=SSSSSSSS;1=SSSSSSSS
+# echo 'L2:1=0xfc' > schemata
+# cat info/L2/bit_usage
+0=SSSSSSSS;1=SSSSSS00
+
+Create a new resource group that will be associated with the pseudo-locked
+region, indicate that it will be used for a pseudo-locked region, and
+configure the requested pseudo-locked region capacity bitmask:
+
+# mkdir newlock
+# echo pseudo-locksetup > newlock/mode
+# echo 'L2:1=0x3' > newlock/schemata
+
+On success the resource group's mode will change to pseudo-locked, the
+bit_usage will reflect the pseudo-locked region, and the character device
+exposing the pseudo-locked region will exist:
+
+# cat newlock/mode
+pseudo-locked
+# cat info/L2/bit_usage
+0=SSSSSSSS;1=SSSSSSPP
+# ls -l /dev/pseudo_lock/newlock
+crw------- 1 root root 243, 0 Apr 3 05:01 /dev/pseudo_lock/newlock
+
+/*
+ * Example code to access one page of pseudo-locked cache region
+ * from user space.
+ */
+#define _GNU_SOURCE
+#include <fcntl.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <sys/mman.h>
+
+/*
+ * It is required that the application runs with affinity to only
+ * cores associated with the pseudo-locked region. Here the cpu
+ * is hardcoded for convenience of example.
+ */
+static int cpuid = 2;
+
+int main(int argc, char *argv[])
+{
+ cpu_set_t cpuset;
+ long page_size;
+ void *mapping;
+ int dev_fd;
+ int ret;
+
+ page_size = sysconf(_SC_PAGESIZE);
+
+ CPU_ZERO(&cpuset);
+ CPU_SET(cpuid, &cpuset);
+ ret = sched_setaffinity(0, sizeof(cpuset), &cpuset);
+ if (ret < 0) {
+ perror("sched_setaffinity");
+ exit(EXIT_FAILURE);
+ }
+
+ dev_fd = open("/dev/pseudo_lock/newlock", O_RDWR);
+ if (dev_fd < 0) {
+ perror("open");
+ exit(EXIT_FAILURE);
+ }
+
+ mapping = mmap(0, page_size, PROT_READ | PROT_WRITE, MAP_SHARED,
+ dev_fd, 0);
+ if (mapping == MAP_FAILED) {
+ perror("mmap");
+ close(dev_fd);
+ exit(EXIT_FAILURE);
+ }
+
+ /* Application interacts with pseudo-locked memory @mapping */
+
+ ret = munmap(mapping, page_size);
+ if (ret < 0) {
+ perror("munmap");
+ close(dev_fd);
+ exit(EXIT_FAILURE);
+ }
+
+ close(dev_fd);
+ exit(EXIT_SUCCESS);
+}
+
Locking between applications
----------------------------
--
2.13.6