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Subjectsem_otime trashing
Hi Rik,

I finally managed to get EFI boot, i.e. I'm now able to test on my i3
(2core+HT).

With semscale (i.e.: just overhead, perform semop=0 operations), the
scalability from 1 to 2 cores is good, but not linear:
# semscale 10 | grep "interleave 2"
> Cpus 1, interleave 2 delay 0: 35502103 in 10 secs
> Cpus 2, interleave 2 delay 0: 53990954 in 10 secs
---
+53% when adding the 2nd core
(interleave 2 to force to use different cores)

Did you consider moving sem_otime into the individual semaphores?
I did that (gross patch attached), and the performance is significantly
better:

# semscale 10 | grep "interleave 2"
Cpus 1, interleave 2 delay 0: 35585634 in 10 secs
Cpus 2, interleave 2 delay 0: 70410230 in 10 secs
---
+99% scalability when adding the 2nd core

Unfortunately I won't be able to read my mails next week, but the effect
was too significant not to share it immediately.

--
Manfred
diff --git a/Makefile b/Makefile
index 73e20db..42137ab 100644
--- a/Makefile
+++ b/Makefile
@@ -1,7 +1,7 @@
VERSION = 3
PATCHLEVEL = 10
SUBLEVEL = 0
-EXTRAVERSION = -rc3
+EXTRAVERSION = -rc3-otime
NAME = Unicycling Gorilla

# *DOCUMENTATION*
diff --git a/include/linux/sem.h b/include/linux/sem.h
index 55e17f6..976ce3a 100644
--- a/include/linux/sem.h
+++ b/include/linux/sem.h
@@ -12,7 +12,6 @@ struct task_struct;
struct sem_array {
struct kern_ipc_perm ____cacheline_aligned_in_smp
sem_perm; /* permissions .. see ipc.h */
- time_t sem_otime; /* last semop time */
time_t sem_ctime; /* last change time */
struct sem *sem_base; /* ptr to first semaphore in array */
struct list_head pending_alter; /* pending operations */
diff --git a/ipc/sem.c b/ipc/sem.c
index 1dbb2fa..e5f000f 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -92,6 +92,7 @@

/* One semaphore structure for each semaphore in the system. */
struct sem {
+ char filler[64];
int semval; /* current value */
int sempid; /* pid of last operation */
spinlock_t lock; /* spinlock for fine-grained semtimedop */
@@ -99,7 +100,8 @@ struct sem {
/* that alter the semaphore */
struct list_head pending_const; /* pending single-sop operations */
/* that do not alter the semaphore*/
-};
+ time_t sem_otime; /* candidate for sem_otime */
+} ____cacheline_aligned_in_smp;

/* One queue for each sleeping process in the system. */
struct sem_queue {
@@ -919,8 +921,14 @@ static void do_smart_update(struct sem_array *sma, struct sembuf *sops, int nsop
}
}
}
- if (otime)
- sma->sem_otime = get_seconds();
+ if (otime) {
+ if (sops == NULL) {
+ sma->sem_base[0].sem_otime = get_seconds();
+ } else {
+ sma->sem_base[sops[0].sem_num].sem_otime =
+ get_seconds();
+ }
+ }
}


@@ -1066,6 +1074,21 @@ static unsigned long copy_semid_to_user(void __user *buf, struct semid64_ds *in,
}
}

+static time_t get_semotime(struct sem_array *sma)
+{
+ int i;
+ time_t res;
+
+ res = sma->sem_base[0].sem_otime;
+ for (i = 1; i < sma->sem_nsems; i++) {
+ time_t to = sma->sem_base[i].sem_otime;
+
+ if (to > res)
+ res = to;
+ }
+ return res;
+}
+
static int semctl_nolock(struct ipc_namespace *ns, int semid,
int cmd, int version, void __user *p)
{
@@ -1139,9 +1162,9 @@ static int semctl_nolock(struct ipc_namespace *ns, int semid,
goto out_unlock;

kernel_to_ipc64_perm(&sma->sem_perm, &tbuf.sem_perm);
- tbuf.sem_otime = sma->sem_otime;
- tbuf.sem_ctime = sma->sem_ctime;
- tbuf.sem_nsems = sma->sem_nsems;
+ tbuf.sem_otime = get_semotime(sma);
+ tbuf.sem_ctime = sma->sem_ctime;
+ tbuf.sem_nsems = sma->sem_nsems;
rcu_read_unlock();
if (copy_semid_to_user(p, &tbuf, version))
return -EFAULT;
@@ -2029,6 +2052,9 @@ static int sysvipc_sem_proc_show(struct seq_file *s, void *it)
{
struct user_namespace *user_ns = seq_user_ns(s);
struct sem_array *sma = it;
+ time_t sem_otime;
+
+ sem_otime = get_semotime(sma);

return seq_printf(s,
"%10d %10d %4o %10u %5u %5u %5u %5u %10lu %10lu\n",
@@ -2040,7 +2066,7 @@ static int sysvipc_sem_proc_show(struct seq_file *s, void *it)
from_kgid_munged(user_ns, sma->sem_perm.gid),
from_kuid_munged(user_ns, sma->sem_perm.cuid),
from_kgid_munged(user_ns, sma->sem_perm.cgid),
- sma->sem_otime,
+ sem_otime,
sma->sem_ctime);
}
#endif
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