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

Ravikiran G Thirumalai
2006-01-08T19:58:53Z

On Sun, Jan 08, 2006 at 02:49:31PM +0300, Oleg Nesterov wrote:
> Sorry for delay,
>
> Ravikiran G Thirumalai wrote:
> >
> > 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;
>
> Unneeded initialization

akpm changed the condition statement below with an if test. So it is needed now.

>
> > 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();
>
> This patch doesn't try to optimize ->sighand.siglock locking,
> and I think this is right. But this also means we don't need
> rmb() here. It was needed to protect against "another thread
> just exited, cpu can read ->c* values before thread_group_empty()
> without taking siglock" case, now it is not possible.

Don't we still need rmb for the RUSAGE_SELF case? we do not take the
siglock for rusage self and the non c* signal fields are written to
at __exit_signal...

What is wrong with optimizing by not taking the siglock in RUSAGE_BOTH
and RUSAGE_CHILDREN? I would like to add that in too unless I am
missing something and the optimization is incorrect.

Thanks,
Kiran