Commit 37407ea7 authored by Linus Torvalds's avatar Linus Torvalds
Browse files

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



This reverts commit 970e1789.

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 970e1789 ("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: default avatarNikolay Ulyanitsky <lystor@gmail.com>
Bisected-by: default avatarBorislav Petkov <bp@alien8.de>
Acked-by: default avatarMike 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: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 3f0c3c8f

include/linux/sched.h

+0 −1
Original line number Diff line number Diff line
@@ -954,7 +954,6 @@ struct sched_domain { 
	unsigned int smt_gain;

	int flags;			/* See SD_* */

	int level;

	int idle_buddy;			/* cpu assigned to select_idle_sibling() */



	/* Runtime fields. */

	unsigned long last_balance;	/* init to jiffies. units in jiffies */

kernel/sched/core.c

+1 −38
Original line number Diff line number Diff line
@@ -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;

kernel/sched/fair.c

+21 −7
Original line number Diff line number Diff line
@@ -2637,6 +2637,8 @@ static int select_idle_sibling(struct task_struct *p, int target) 
	int cpu = smp_processor_id();

	int prev_cpu = task_cpu(p);

	struct sched_domain *sd;

	struct sched_group *sg;

	int i;



	/*

	 * If the task is going to be woken-up on this cpu and if it is
@@ -2653,17 +2655,29 @@ static int select_idle_sibling(struct task_struct *p, int target) 
		return prev_cpu;



	/*

	 * Otherwise, check assigned siblings to find an elegible idle cpu.

	 * Otherwise, iterate the domains and find an elegible idle cpu.

	 */

	sd = rcu_dereference(per_cpu(sd_llc, target));



	for_each_lower_domain(sd) {

		if (!cpumask_test_cpu(sd->idle_buddy, tsk_cpus_allowed(p)))

			continue;

		if (idle_cpu(sd->idle_buddy))

			return sd->idle_buddy;

		sg = sd->groups;

		do {

			if (!cpumask_intersects(sched_group_cpus(sg),

						tsk_cpus_allowed(p)))

				goto next;



			for_each_cpu(i, sched_group_cpus(sg)) {

				if (!idle_cpu(i))

					goto next;

			}



			target = cpumask_first_and(sched_group_cpus(sg),

					tsk_cpus_allowed(p));

			goto done;

next:

			sg = sg->next;

		} while (sg != sd->groups);

	}

done:

	return target;

}

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