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

[Ravikiran G Thirumalai <kiran@scalex86.org>]

On Mon, Jan 09, 2006 at 09:55:51PM +0300, Oleg Nesterov wrote:
> Ravikiran G Thirumalai wrote:
> >
> >
> > 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...
> 
> I think it is unneeded because RUSAGE_SELF case is "racy" anyway even
> if we held both locks, task_struct->xxx counters can change at any
> moment.
> 
> But may be you are right.

Hmm...access to task_struct->xxx has been racy, but accessing the 
signal->* counters were not.  What if read of the signal->utime  was  a 
hoisted read and signal->stime was a read after the counter is updated?  
This was not a possibility earlier no? 
 
> 
> > 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.
> 
> We can't have contention on ->siglock when need_lock == 0, so why should
> we optimize this case?

We would be saving 1 buslocked operation in that case (on some arches), a 
cacheline fetch for exclusive (since signal and sighand are on different memory
locations), and disabling/enabling onchip interrupts.  But yes, this would be a 
smaller optimization....Unless you have strong objections this can also 
go in?

Thanks,
Kiran

Signed-off-by: Ravikiran Thirumalai <kiran@scalex86.org>

Index: linux-2.6.15-mm2/kernel/sys.c
===================================================================
--- linux-2.6.15-mm2.orig/kernel/sys.c	2006-01-09 12:44:30.000000000 -0800
+++ linux-2.6.15-mm2/kernel/sys.c	2006-01-09 12:45:07.000000000 -0800
@@ -1730,14 +1730,16 @@ static void k_getrusage(struct task_stru
 	switch (who) {
 		case RUSAGE_BOTH:
 		case RUSAGE_CHILDREN:
-			spin_lock_irqsave(&p->sighand->siglock, flags);
+			if (need_lock)
+				spin_lock_irqsave(&p->sighand->siglock, flags);
 			utime = p->signal->cutime;
 			stime = p->signal->cstime;
 			r->ru_nvcsw = p->signal->cnvcsw;
 			r->ru_nivcsw = p->signal->cnivcsw;
 			r->ru_minflt = p->signal->cmin_flt;
 			r->ru_majflt = p->signal->cmaj_flt;
-			spin_unlock_irqrestore(&p->sighand->siglock, flags);
+			if (need_lock)
+				spin_unlock_irqrestore(&p->sighand->siglock, flags);
 
 			if (who == RUSAGE_CHILDREN)
 				break;