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authorLinus Torvalds <torvalds@linux-foundation.org>2017-02-28 11:46:00 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2017-02-28 11:46:00 -0800
commitf89db789de2157441d3b5e879a742437ed69cbbc (patch)
treeab768dbb16c3c5c078d9721fee28d1b2b2508392 /Documentation
parent65314ed08e9c4a94ba85f7d52a7ad324050b152e (diff)
parent8312593a55941a0fae2b09731a4f91d87bd796db (diff)
downloadlinux-f89db789de2157441d3b5e879a742437ed69cbbc.tar.gz
Merge branch 'x86-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
Pull x86 fixes from Ingo Molnar: "Two documentation updates, plus a debugging annotation fix" * 'x86-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: x86/crash: Update the stale comment in reserve_crashkernel() x86/irq, trace: Add __irq_entry annotation to x86's platform IRQ handlers Documentation, x86, resctrl: Recommend locking for resctrlfs
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/x86/intel_rdt_ui.txt114
1 files changed, 114 insertions, 0 deletions
diff --git a/Documentation/x86/intel_rdt_ui.txt b/Documentation/x86/intel_rdt_ui.txt
index d918d26..51cf6fa 100644
--- a/Documentation/x86/intel_rdt_ui.txt
+++ b/Documentation/x86/intel_rdt_ui.txt
@@ -212,3 +212,117 @@ 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);
+}