On Fri, Jan 06, 2006 at 09:23:30AM -0800, Christoph Lameter wrote:
> On Fri, 6 Jan 2006, Ravikiran G Thirumalai wrote:
>
> > + need_lock = !(p == current && thread_group_empty(p));
>
> Isnt
>
> need_lock = (p != current || !thread_group_empty(b))
>
> clearer?
All the same I felt, and the comments were bold and clear. Also, the above was
c translation of what was said in the comment :)...I am OK either ways.
So Here goes...
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).
Thanks to Oleg Nesterov for review and suggestions.
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-delme/kernel/sys.c
===================================================================
--- linux-2.6.15-delme.orig/kernel/sys.c 2006-01-05 15:40:38.000000000 -0800
+++ linux-2.6.15-delme/kernel/sys.c 2006-01-06 00:43:08.000000000 -0800
@@ -1664,9 +1664,6 @@ asmlinkage long sys_setrlimit(unsigned i
* 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.
- *
* When sampling multiple threads for RUSAGE_SELF, under SMP we might have
* races with threads incrementing their own counters. But since word
* reads are atomic, we either get new values or old values and we don't
@@ -1674,6 +1671,25 @@ asmlinkage long sys_setrlimit(unsigned i
* 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.
+ *
+ * tasklist_lock locking optimisation:
+ * If we are current and single threaded, we do not need to take the tasklist
+ * lock or the siglock. No one else can take our signal_struct away,
+ * no one else can reap the children to update signal->c* counters, and
+ * no one else can race with the signal-> fields.
+ * If we do not take the tasklist_lock, the signal-> fields could be read
+ * out of order while another thread was just exiting. So we place a
+ * read memory barrier when we avoid the lock. On the writer side,
+ * write memory barrier is implied in __exit_signal as __exit_signal releases
+ * the siglock spinlock after updating the signal-> fields.
+ *
+ * We don't really need the siglock when we access the non c* fields
+ * of the signal_struct (for RUSAGE_SELF) even in multithreaded
+ * case, since we take the tasklist lock for read and the non c* signal->
+ * fields are updated only in __exit_signal, which is called with
+ * tasklist_lock taken for write, hence these two threads cannot execute
+ * concurrently.
+ *
*/
static void k_getrusage(struct task_struct *p, int who, struct rusage *r)
@@ -1681,14 +1697,22 @@ static void k_getrusage(struct task_stru
struct task_struct *t;
unsigned long flags;
cputime_t utime, stime;
+ int need_lock = 0;
memset((char *) r, 0, sizeof *r);
-
- if (unlikely(!p->signal))
- return;
-
utime = stime = cputime_zero;
+ need_lock = (p != current || !thread_group_empty(p));
+ if (need_lock) {
+ read_lock(&tasklist_lock);
+ if (unlikely(!p->signal)) {
+ read_unlock(&tasklist_lock);
+ return;
+ }
+ } else
+ /* See locking comments above */
+ smp_rmb();
+
switch (who) {
case RUSAGE_BOTH:
case RUSAGE_CHILDREN:
@@ -1727,6 +1751,8 @@ static void k_getrusage(struct task_stru
BUG();
}
+ if (need_lock)
+ read_unlock(&tasklist_lock);
cputime_to_timeval(utime, &r->ru_utime);
cputime_to_timeval(stime, &r->ru_stime);
}
@@ -1734,9 +1760,7 @@ static void k_getrusage(struct task_stru
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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