Subject: Re: [PATCHv4 2/2] powerpc: implement arch_scale_smt_power for Power7

From: Peter Zijlstra <[email-masked]>
Date: Thu, 18 Feb 2010 14:17:03 +0100

On Thu, 2010-02-18 at 09:20 +1100, Michael Neuling wrote:
> > Suppose for a moment we have 2 threads (hot-unplugged thread 1 and 3, we
> > can construct an equivalent but more complex example for 4 threads), and
> > we have 4 tasks, 3 SCHED_OTHER of equal nice level and 1 SCHED_FIFO, the
> > SCHED_FIFO task will consume exactly 50% walltime of whatever cpu it
> > ends up on.
> > 
> > In that situation, provided that each cpu's cpu_power is of equal
> > measure, scale_rt_power() ensures that we run 2 SCHED_OTHER tasks on the
> > cpu that doesn't run the RT task, and 1 SCHED_OTHER task next to the RT
> > task, so that each task consumes 50%, which is all fair and proper.
> > 
> > However, if you do the above, thread 0 will have +75% = 1.75 and thread
> > 2 will have -75% = 0.25, then if the RT task will land on thread 0,
> > we'll be having: 0.875 vs 0.25, or on thread 3, 1.75 vs 0.125. In either
> > case thread 0 will receive too many (if not all) SCHED_OTHER tasks.
> > 
> > That is, unless these threads 2 and 3 really are _that_ weak, at which
> > point one wonders why IBM bothered with the silicon ;-)
> 
> Peter,
> 
> 2 & 3 aren't weaker than 0 & 1 but.... 
> 
> The core has dynamic SMT mode switching which is controlled by the
> hypervisor (IBM's PHYP).  There are 3 SMT modes:
> 	SMT1 uses thread  0
> 	SMT2 uses threads 0 & 1
> 	SMT4 uses threads 0, 1, 2 & 3
> When in any particular SMT mode, all threads have the same performance
> as each other (ie. at any moment in time, all threads perform the same).  
> 
> The SMT mode switching works such that when linux has threads 2 & 3 idle
> and 0 & 1 active, it will cede (H_CEDE hypercall) threads 2 and 3 in the
> idle loop and the hypervisor will automatically switch to SMT2 for that
> core (independent of other cores).  The opposite is not true, so if
> threads 0 & 1 are idle and 2 & 3 are active, we will stay in SMT4 mode.
> 
> Similarly if thread 0 is active and threads 1, 2 & 3 are idle, we'll go
> into SMT1 mode.  
> 
> If we can get the core into a lower SMT mode (SMT1 is best), the threads
> will perform better (since they share less core resources).  Hence when
> we have idle threads, we want them to be the higher ones.

Just out of curiosity, is this a hardware constraint or a hypervisor
constraint?

> So to answer your question, threads 2 and 3 aren't weaker than the other
> threads when in SMT4 mode.  It's that if we idle threads 2 & 3, threads
> 0 & 1 will speed up since we'll move to SMT2 mode.
>
> I'm pretty vague on linux scheduler details, so I'm a bit at sea as to
> how to solve this.  Can you suggest any mechanisms we currently have in
> the kernel to reflect these properties, or do you think we need to
> develop something new?  If so, any pointers as to where we should look?

Well there currently isn't one, and I've been telling people to create a
new SD_flag to reflect this and influence the f_b_g() behaviour.

Something like the below perhaps, totally untested and without comments
so that you'll have to reverse engineer and validate my thinking.

There's one fundamental assumption, and one weakness in the
implementation.

---

 include/linux/sched.h |    2 +-
 kernel/sched_fair.c   |   61 +++++++++++++++++++++++++++++++++++++++++++++---
 2 files changed, 58 insertions(+), 5 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 0eef87b..42fa5c6 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -849,7 +849,7 @@ enum cpu_idle_type {
 #define SD_POWERSAVINGS_BALANCE	0x0100	/* Balance for power savings */
 #define SD_SHARE_PKG_RESOURCES	0x0200	/* Domain members share cpu pkg resources */
 #define SD_SERIALIZE		0x0400	/* Only a single load balancing instance */
-
+#define SD_ASYM_PACKING		0x0800
 #define SD_PREFER_SIBLING	0x1000	/* Prefer to place tasks in a sibling domain */
 
 enum powersavings_balance_level {
diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index ff7692c..7e42bfe 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -2086,6 +2086,7 @@ struct sd_lb_stats {
 	struct sched_group *this;  /* Local group in this sd */
 	unsigned long total_load;  /* Total load of all groups in sd */
 	unsigned long total_pwr;   /*	Total power of all groups in sd */
+	unsigned long total_nr_running;
 	unsigned long avg_load;	   /* Average load across all groups in sd */
 
 	/** Statistics of this group */
@@ -2414,10 +2415,10 @@ static inline void update_sg_lb_stats(struct sched_domain *sd,
 			int *balance, struct sg_lb_stats *sgs)
 {
 	unsigned long load, max_cpu_load, min_cpu_load;
-	int i;
 	unsigned int balance_cpu = -1, first_idle_cpu = 0;
 	unsigned long sum_avg_load_per_task;
 	unsigned long avg_load_per_task;
+	int i;
 
 	if (local_group)
 		balance_cpu = group_first_cpu(group);
@@ -2493,6 +2494,28 @@ static inline void update_sg_lb_stats(struct sched_domain *sd,
 		DIV_ROUND_CLOSEST(group->cpu_power, SCHED_LOAD_SCALE);
 }
 
+static int update_sd_pick_busiest(struct sched_domain *sd,
+	       			  struct sd_lb_stats *sds,
+				  struct sched_group *sg,
+			  	  struct sg_lb_stats *sgs)
+{
+	if (sgs->sum_nr_running > sgs->group_capacity)
+		return 1;
+
+	if (sgs->group_imb)
+		return 1;
+
+	if ((sd->flags & SD_ASYM_PACKING) && sgs->sum_nr_running) {
+		if (!sds->busiest)
+			return 1;
+
+		if (group_first_cpu(sds->busiest) < group_first_cpu(group))
+			return 1;
+	}
+
+	return 0;
+}
+
 /**
  * update_sd_lb_stats - Update sched_group's statistics for load balancing.
  * @sd: sched_domain whose statistics are to be updated.
@@ -2533,6 +2556,7 @@ static inline void update_sd_lb_stats(struct sched_domain *sd, int this_cpu,
 
 		sds->total_load += sgs.group_load;
 		sds->total_pwr += group->cpu_power;
+		sds->total_nr_running += sgs.sum_nr_running;
 
 		/*
 		 * In case the child domain prefers tasks go to siblings
@@ -2547,9 +2571,8 @@ static inline void update_sd_lb_stats(struct sched_domain *sd, int this_cpu,
 			sds->this = group;
 			sds->this_nr_running = sgs.sum_nr_running;
 			sds->this_load_per_task = sgs.sum_weighted_load;
-		} else if (sgs.avg_load > sds->max_load &&
-			   (sgs.sum_nr_running > sgs.group_capacity ||
-				sgs.group_imb)) {
+		} else if (sgs.avg_load >= sds->max_load &&
+			   update_sd_pick_busiest(sd, sds, group, &sgs)) {
 			sds->max_load = sgs.avg_load;
 			sds->busiest = group;
 			sds->busiest_nr_running = sgs.sum_nr_running;
@@ -2562,6 +2585,33 @@ static inline void update_sd_lb_stats(struct sched_domain *sd, int this_cpu,
 	} while (group != sd->groups);
 }
 
+static int check_asym_packing(struct sched_domain *sd,
+				    struct sd_lb_stats *sds, 
+				    int cpu, unsigned long *imbalance)
+{
+	int i, cpu, busiest_cpu;
+
+	if (!(sd->flags & SD_ASYM_PACKING))
+		return 0;
+
+	if (!sds->busiest)
+		return 0;
+
+	i = 0;
+	busiest_cpu = group_first_cpu(sds->busiest);
+	for_each_cpu(cpu, sched_domain_span(sd)) {
+		i++;
+		if (cpu == busiest_cpu)
+			break;
+	}
+
+	if (sds->total_nr_running > i)
+		return 0;
+
+	*imbalance = sds->max_load;
+	return 1;
+}
+
 /**
  * fix_small_imbalance - Calculate the minor imbalance that exists
  *			amongst the groups of a sched_domain, during
@@ -2761,6 +2811,9 @@ find_busiest_group(struct sched_domain *sd, int this_cpu,
 	return sds.busiest;
 
 out_balanced:
+	if (check_asym_packing(sd, &sds, this_cpu, imbalance))
+		return sds.busiest;
+
 	/*
 	 * There is no obvious imbalance. But check if we can do some balancing
 	 * to save power.