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

Oleg Nesterov
2006-03-20T18:08:12Z

Hello Ravikiran,

Ravikiran G Thirumalai wrote:
>
> 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).
>
> ...
>
> 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();
> +

I think now it is possible to improve this patch.

Could you look at these patches?

[PATCH] introduce lock_task_sighand() helper
http://marc.theaimsgroup.com/?l=linux-kernel&m=114028190927763

[PATCH 0/3] make threads traversal ->siglock safe
http://marc.theaimsgroup.com/?l=linux-kernel&m=114064825626496

I think we can forget about tasklist_lock in k_getrusage() completely
and just use lock_task_sighand().

What do you think?

Oleg.