redonkable
/
alistair23-linux
1
0
Fork 0
Code Releases Activity

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: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Link: http://lkml.kernel.org/r/20161214170835.GA16924@amt.cnet
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
hifive-unleashed-5.1
Marcelo Tosatti 2016-12-14 15:08:37 -02:00 committed by Thomas Gleixner
parent 76ae054c69
commit 3c2a769de7
1 changed files with 114 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

114
Documentation/x86/intel_rdt_ui.txt
View File

@ -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);
}