[PATCH 3/4] sysv,ipc: cacheline align kern_ipc_perm

Davidlohr Bueso
2017-02-09T20:53:35Z

Assign 'struct kern_ipc_perm' its own cacheline to avoid false
sharing with sysv ipc calls. While the structure itself is rather
read-mostly throughout the lifespan of ipc, the spinlock causes
most of the invalidations. One example is 31a7c4746e9 (ipc/sem.c:
cacheline align the ipc spinlock for semaphores). Therefore,
extend this to all ipc.

The effect of cacheline alignment on sems can be seen in sembench,
which deals mostly with semtimedop wait/wakes is seen to improve raw
throughput (worker loops) between 8 to 12% on a 24-core x86 with
over 4 threads.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
include/linux/ipc.h | 7 +++----
include/linux/sem.h | 3 +--
2 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/include/linux/ipc.h b/include/linux/ipc.h
index 9d84942ae2e5..71fd92d81b26 100644
--- a/include/linux/ipc.h
+++ b/include/linux/ipc.h
@@ -8,8 +8,7 @@
#define IPCMNI 32768 /* <= MAX_INT limit for ipc arrays (including sysctl changes) */

/* used by in-kernel data structures */
-struct kern_ipc_perm
-{
+struct kern_ipc_perm {
spinlock_t lock;
bool deleted;
int id;
@@ -18,9 +17,9 @@ struct kern_ipc_perm
kgid_t gid;
kuid_t cuid;
kgid_t cgid;
- umode_t mode;
+ umode_t mode;
unsigned long seq;
void *security;
-};
+} ____cacheline_aligned_in_smp;

#endif /* _LINUX_IPC_H */
diff --git a/include/linux/sem.h b/include/linux/sem.h
index 4fc222f8755d..9edec926e9d9 100644
--- a/include/linux/sem.h
+++ b/include/linux/sem.h
@@ -10,8 +10,7 @@ struct task_struct;

/* One sem_array data structure for each set of semaphores in the system. */
struct sem_array {
- struct kern_ipc_perm ____cacheline_aligned_in_smp
- sem_perm; /* permissions .. see ipc.h */
+ struct kern_ipc_perm sem_perm; /* permissions .. see ipc.h */
time_t sem_ctime; /* last change time */
struct sem *sem_base; /* ptr to first semaphore in array */
struct list_head pending_alter; /* pending operations */
--
2.6.6