Revert "sched: Improve scalability via 'CPU buddies', which withstand random perturbations"

This reverts commit 970e178985.

Nikolay Ulyanitsky reported thatthe 3.6-rc5 kernel has a 15-20%
performance drop on PostgreSQL 9.2 on his machine (running "pgbench").

Borislav Petkov was able to reproduce this, and bisected it to this
commit 970e178985 ("sched: Improve scalability via 'CPU buddies' ...")
apparently because the new single-idle-buddy model simply doesn't find
idle CPU's to reschedule on aggressively enough.

Mike Galbraith suspects that it is likely due to the user-mode spinlocks
in PostgreSQL not reacting well to preemption, but we don't really know
the details - I'll just revert the commit for now.

There are hopefully other approaches to improve scheduler scalability
without it causing these kinds of downsides.

Reported-by: Nikolay Ulyanitsky <lystor@gmail.com>
Bisected-by: Borislav Petkov <bp@alien8.de>
Acked-by: Mike Galbraith <efault@gmx.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/sched/core.c

@@ -6014,11 +6014,6 @@ static void destroy_sched_domains(struct sched_domain *sd, int cpu)
  * SD_SHARE_PKG_RESOURCE set (Last Level Cache Domain) for this
  * allows us to avoid some pointer chasing select_idle_sibling().
  *
- * Iterate domains and sched_groups downward, assigning CPUs to be
- * select_idle_sibling() hw buddy.  Cross-wiring hw makes bouncing
- * due to random perturbation self canceling, ie sw buddies pull
- * their counterpart to their CPU's hw counterpart.
- *
  * Also keep a unique ID per domain (we use the first cpu number in
  * the cpumask of the domain), this allows us to quickly tell if
  * two cpus are in the same cache domain, see cpus_share_cache().
@@ -6032,40 +6027,8 @@ static void update_top_cache_domain(int cpu)
 	int id = cpu;
 
 	sd = highest_flag_domain(cpu, SD_SHARE_PKG_RESOURCES);
-	if (sd) {
-		struct sched_domain *tmp = sd;
-		struct sched_group *sg, *prev;
-		bool right;
-
-		/*
-		 * Traverse to first CPU in group, and count hops
-		 * to cpu from there, switching direction on each
-		 * hop, never ever pointing the last CPU rightward.
-		 */
-		do {
-			id = cpumask_first(sched_domain_span(tmp));
-			prev = sg = tmp->groups;
-			right = 1;
-
-			while (cpumask_first(sched_group_cpus(sg)) != id)
-				sg = sg->next;
-
-			while (!cpumask_test_cpu(cpu, sched_group_cpus(sg))) {
-				prev = sg;
-				sg = sg->next;
-				right = !right;
-			}
-
-			/* A CPU went down, never point back to domain start. */
-			if (right && cpumask_first(sched_group_cpus(sg->next)) == id)
-				right = false;
-
-			sg = right ? sg->next : prev;
-			tmp->idle_buddy = cpumask_first(sched_group_cpus(sg));
-		} while ((tmp = tmp->child));
-
+	if (sd)
 		id = cpumask_first(sched_domain_span(sd));
-	}
 
 	rcu_assign_pointer(per_cpu(sd_llc, cpu), sd);
 	per_cpu(sd_llc_id, cpu) = id;