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

[Oleg Nesterov]

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.
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