Re: [rfc][patch] Avoid taking global tasklist_lock for single threadedprocess at getrusage()

Ravikiran G Thirumalai
2006-01-06T09:46:22Z

On Thu, Jan 05, 2006 at 10:17:01PM +0300, Oleg Nesterov wrote:
> Ravikiran G Thirumalai wrote:
>
>
> I think we should cleanup k_getrusage() first, see the patch below.
>
> Do you agree with that patch? If yes, could you make the new one on
> top of it? (please send them both to Andrew).
>
> Note that after this cleanup we don't have code duplication.

Yes, nice patch. Thanks. Here is the tasklist lock optimization on top of
the simplify-k_getrusage.patch. Andrew, please apply.

Thanks,
Kiran

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 <nippung@calsoftinc.com>
Signed-off-by: Ravikiran Thirumalai <kiran@scalex86.org>
Signed-off-by: Shai Fultheim <shai@scalex86.org>

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;
}