Re: [PATCH] sched: Consolidated and improved smpnice patch

[Peter Williams <pwil3058@bigpond.net.au>]

Con Kolivas wrote:
> On Tuesday 21 February 2006 09:35, Peter Williams wrote:
>
> > Con Kolivas wrote:
> >
> > > On Monday 20 February 2006 16:02, Peter Williams wrote:
> > > [snip description]
> > >
> > > Hi peter, I've had a good look and have just a couple of comments:
> > >
> > > ---
> > > #endif
> > >        int prio, static_prio;
> > > +#ifdef CONFIG_SMP
> > > +       int load_weight;        /* for load balancing purposes */
> > > +#endif
> > > ---
> > >
> > > Can this be moved up to be part of the other ifdef CONFIG_SMP? Not highly
> > > significant since it's in a .h file but looks a tiny bit nicer.
> >
> > I originally put it where it is to be near prio and static_prio which
> > are referenced at the same time as it BUT that doesn't happen often
> > enough to justify it anymore so I guess it can be moved.
>
>
> Well it is just before prio instead of just after it now and I understand the 
> legacy of the position.
>
>
> > > ---
> > > +/*
> > > + * Priority weight for load balancing ranges from 1/20 (nice==19) to
> > > 459/20 (RT
> > > + * priority of 100).
> > > + */
> > > +#define NICE_TO_LOAD_PRIO(nice) \
> > > +       ((nice >= 0) ? (20 - (nice)) : (20 + (nice) * (nice)))
> > > +#define LOAD_WEIGHT(lp) \
> > > +       (((lp) * SCHED_LOAD_SCALE) / NICE_TO_LOAD_PRIO(0))
> > > +#define NICE_TO_LOAD_WEIGHT(nice)     
> > > LOAD_WEIGHT(NICE_TO_LOAD_PRIO(nice)) +#define PRIO_TO_LOAD_WEIGHT(prio)
> > > NICE_TO_LOAD_WEIGHT(PRIO_TO_NICE(prio))
> > > +#define RTPRIO_TO_LOAD_WEIGHT(rp) \
> > > +       LOAD_WEIGHT(NICE_TO_LOAD_PRIO(-20) + (rp))
> > > ---
> > >
> > > The weighting seems not related to anything in particular apart from
> > > saying that -nice values are more heavily weighted.
> >
> > The idea (for the change from the earlier model) was to actually give
> > equal weight to negative and positive nices.  Under the old (purely
> > linear) model a nice=19 task has 1/20th the weight of a nice==0 task but
> > a nice==-20 task only has twice the weight of a nice==0 so that system
> > is heavily weighted against negative nices.  With this new mapping a
> > nice=19 has 1/20th and a nice==-19 has 20 times the weight of a nice==0
> > task and to me that is symmetric.  Does that make sense to you?
>
>
> Yes but what I meant is it's still an arbitrary algorithm which is why I 
> suggested proportional to tasks' timeslice because then it should scale with 
> the theoretically allocated cpu resource.
>
>
> > Should I add a comment to explain the mapping?
> >
> >
> > > Since you only do this when
> > > setting the priority of tasks can you link it to the scale of
> > > (SCHED_NORMAL) tasks' timeslice instead even though that will take a
> > > fraction more calculation? RTPRIO_TO_LOAD_WEIGHT is fine since there
> > > isn't any obvious cpu proportion relationship to rt_prio level.
> >
> > Interesting idea.  I'll look at this more closely.
>
>
> Would be just a matter of using task_timeslice(p) and making it proportional 
> to some baseline ensuring the baseline works at any HZ.

How does the attached patch grab you?  It's independent of HZ.

Peter
--
Peter Williams                                   pwil3058@bigpond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
 -- Ambrose Bierce

Index: MM-2.6.X/kernel/sched.c
===================================================================
--- MM-2.6.X.orig/kernel/sched.c	2006-02-21 16:27:32.000000000 +1100
+++ MM-2.6.X/kernel/sched.c	2006-02-21 17:01:12.000000000 +1100
@@ -169,13 +169,13 @@
 
 #define SCALE_PRIO(x, prio) \
 	max(x * (MAX_PRIO - prio) / (MAX_USER_PRIO/2), MIN_TIMESLICE)
+#define PRIO_TO_TS(sp) \
+	((sp) < NICE_TO_PRIO(0) ? SCALE_PRIO(DEF_TIMESLICE*4, (sp)) : \
+	 SCALE_PRIO(DEF_TIMESLICE, (sp)))
 
 static unsigned int task_timeslice(task_t *p)
 {
-	if (p->static_prio < NICE_TO_PRIO(0))
-		return SCALE_PRIO(DEF_TIMESLICE*4, p->static_prio);
-	else
-		return SCALE_PRIO(DEF_TIMESLICE, p->static_prio);
+	return PRIO_TO_TS(p->static_prio);
 }
 #define task_hot(p, now, sd) ((long long) ((now) - (p)->last_ran)	\
 				< (long long) (sd)->cache_hot_time)
@@ -680,11 +680,9 @@ static int effective_prio(task_t *p)
  */
 
 /*
- * Priority weight for load balancing ranges from 1/20 (nice==19) to 459/20 (RT
- * priority of 100).
+ * Priority weight for load balancing ranges (based on time slice allocations).
  */
-#define NICE_TO_LOAD_PRIO(nice) \
-	((nice >= 0) ? (20 - (nice)) : (20 + (nice) * (nice)))
+#define NICE_TO_LOAD_PRIO(nice) 	PRIO_TO_TS(NICE_TO_PRIO(nice))
 #define LOAD_WEIGHT(lp) \
 	(((lp) * SCHED_LOAD_SCALE) / NICE_TO_LOAD_PRIO(0))
 #define NICE_TO_LOAD_WEIGHT(nice)	LOAD_WEIGHT(NICE_TO_LOAD_PRIO(nice))