Commit 3c2a769d authored by Marcelo Tosatti's avatar Marcelo Tosatti Committed by Thomas Gleixner

Documentation, x86, resctrl: Recommend locking for resctrlfs

Concurrent write or read/write access from applications to the resctrlfs
directory can result in incorrect readouts or setups.

Recommend a standard locking scheme for applications to use.
Signed-off-by: default avatarMarcelo Tosatti <mtosatti@redhat.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Link: http://lkml.kernel.org/r/20161214170835.GA16924@amt.cnetSigned-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
parent 76ae054c
......@@ -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);
}
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