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

This reverts commit 970e178985cadbca660feb02f4d2ee3a09f7fdda.

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 970e178985ca ("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>

1 files changed, 1 insertions, 38 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index a4ea245f3d85..649c9f876cb1 100644
--- a/kernel/sched/core.c
+++ b/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;