Following patch avoids taking the global tasklist_lock when possible,
if a process is single threaded during getrusage(). Any avoidance of
tasklist_lock is good for NUMA boxes (and possibly for large SMPs). We found
that this optimization reduces the runtime of a certain scientific application
by half on a 16 cpu NUMA box.
This optimization is similar to the sys_times tasklist_lock optimization.
Signed-off-by: Nippun Goel <[email protected]>
Signed-off-by: Ravikiran Thirumalai <[email protected]>
Signed-off-by: Shai Fultheim <[email protected]>
Index: linux-2.6.15-rc6/kernel/sys.c
===================================================================
--- linux-2.6.15-rc6.orig/kernel/sys.c 2005-12-20 14:10:52.000000000 -0800
+++ linux-2.6.15-rc6/kernel/sys.c 2005-12-21 00:39:41.000000000 -0800
@@ -1664,8 +1664,9 @@
* a lot simpler! (Which we're not doing right now because we're not
* measuring them yet).
*
- * This expects to be called with tasklist_lock read-locked or better,
- * and the siglock not locked. It may momentarily take the siglock.
+ * This function was earlier called with tasklist_lock lock taken for read.
+ * Now, we take tasklist_lock for read (and the siglock) only when required.
+ * See notes below.
*
* When sampling multiple threads for RUSAGE_SELF, under SMP we might have
* races with threads incrementing their own counters. But since word
@@ -1674,6 +1675,25 @@
* the c* fields from p->signal from races with exit.c updating those
* fields when reaping, so a sample either gets all the additions of a
* given child after it's reaped, or none so this sample is before reaping.
+ *
+ * Locking:
+ * If we have a multithreaded process, we need to take tasklist read lock
+ * for RUSAGE_SELF and RUSAGE_BOTH. We don't need to take the tasklist lock
+ * for RUSAGE_CHILDREN and just the siglock should suffice there.
+ *
+ * If we are a single threaded process, we donot need to take the tasklist_lock
+ * for read. However, we need to take siglock for the RUSAGE_BOTH case.
+ * RUSAGE_SELF and RUSAGE_CHILDREN is invoked via the syscall, and is
+ * for the current process -- unlike RUSAGE_BOTH. So not taking the siglock
+ * for RUSAGE_SELF and RUSAGE_CHILDREN is safe.
+ *
+ * In the multithreaded scenaio, while we have the tasklist_lock held for
+ * read, the non c* p->signal field updates cannot take place as the
+ * __exit_signal thread is called with write lock taken on tasklist_lock.
+ * Reads on the p->signal c* fields however can race if a child is being reaped.
+ * we avoid the race by taking the siglock to read the c* fileds.
+ *
+ * Hence, tasklist lock for read is sufficient for RUSAGE_SELF.
*/
static void k_getrusage(struct task_struct *p, int who, struct rusage *r)
@@ -1681,6 +1701,7 @@
struct task_struct *t;
unsigned long flags;
cputime_t utime, stime;
+ int lockflag = 0;
memset((char *) r, 0, sizeof *r);
@@ -1689,22 +1710,33 @@
switch (who) {
case RUSAGE_CHILDREN:
- spin_lock_irqsave(&p->sighand->siglock, flags);
+ if (!thread_group_empty(p)) {
+ spin_lock_irqsave(&p->sighand->siglock, flags);
+ lockflag = 1;
+ }
utime = p->signal->cutime;
stime = p->signal->cstime;
r->ru_nvcsw = p->signal->cnvcsw;
r->ru_nivcsw = p->signal->cnivcsw;
r->ru_minflt = p->signal->cmin_flt;
r->ru_majflt = p->signal->cmaj_flt;
- spin_unlock_irqrestore(&p->sighand->siglock, flags);
+ if (lockflag)
+ spin_unlock_irqrestore(&p->sighand->siglock, flags);
cputime_to_timeval(utime, &r->ru_utime);
cputime_to_timeval(stime, &r->ru_stime);
break;
case RUSAGE_SELF:
- spin_lock_irqsave(&p->sighand->siglock, flags);
+ if (!thread_group_empty(p)) {
+ read_lock(&tasklist_lock);
+ lockflag = 1;
+ }
utime = stime = cputime_zero;
goto sum_group;
case RUSAGE_BOTH:
+ if (!thread_group_empty(p)) {
+ read_lock(&tasklist_lock);
+ lockflag = 1;
+ }
spin_lock_irqsave(&p->sighand->siglock, flags);
utime = p->signal->cutime;
stime = p->signal->cstime;
@@ -1712,6 +1744,7 @@
r->ru_nivcsw = p->signal->cnivcsw;
r->ru_minflt = p->signal->cmin_flt;
r->ru_majflt = p->signal->cmaj_flt;
+ spin_unlock_irqrestore(&p->sighand->siglock, flags);
sum_group:
utime = cputime_add(utime, p->signal->utime);
stime = cputime_add(stime, p->signal->stime);
@@ -1729,7 +1762,8 @@
r->ru_majflt += t->maj_flt;
t = next_thread(t);
} while (t != p);
- spin_unlock_irqrestore(&p->sighand->siglock, flags);
+ if (lockflag)
+ read_unlock(&tasklist_lock);
cputime_to_timeval(utime, &r->ru_utime);
cputime_to_timeval(stime, &r->ru_stime);
break;
@@ -1741,9 +1775,7 @@
int getrusage(struct task_struct *p, int who, struct rusage __user *ru)
{
struct rusage r;
- read_lock(&tasklist_lock);
k_getrusage(p, who, &r);
- read_unlock(&tasklist_lock);
return copy_to_user(ru, &r, sizeof(r)) ? -EFAULT : 0;
}
-
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