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

[Ravikiran G Thirumalai]

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
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