Patch "sched/core: Implement new approach to scale select_idle_cpu()" has been added to the 4.12-stable tree

Patch "sched/core: Implement new approach to scale select_idle_cpu()" has been added to the 4.12-stable tree

[gregkh]

This is a note to let you know that I've just added the patch titled

    sched/core: Implement new approach to scale select_idle_cpu()

to the 4.12-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     sched-core-implement-new-approach-to-scale-select_idle_cpu.patch
and it can be found in the queue-4.12 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From 1ad3aaf3fcd2444406628a19a9b9e0922b95e2d4 Mon Sep 17 00:00:00 2001
From: Peter Zijlstra <peterz@infradead.org>
Date: Wed, 17 May 2017 12:53:50 +0200
Subject: sched/core: Implement new approach to scale select_idle_cpu()

From: Peter Zijlstra <peterz@infradead.org>

commit 1ad3aaf3fcd2444406628a19a9b9e0922b95e2d4 upstream.

Hackbench recently suffered a bunch of pain, first by commit:

  4c77b18cf8b7 ("sched/fair: Make select_idle_cpu() more aggressive")

and then by commit:

  c743f0a5c50f ("sched/fair, cpumask: Export for_each_cpu_wrap()")

which fixed a bug in the initial for_each_cpu_wrap() implementation
that made select_idle_cpu() even more expensive. The bug was that it
would skip over CPUs when bits were consequtive in the bitmask.

This however gave me an idea to fix select_idle_cpu(); where the old
scheme was a cliff-edge throttle on idle scanning, this introduces a
more gradual approach. Instead of stopping to scan entirely, we limit
how many CPUs we scan.

Initial benchmarks show that it mostly recovers hackbench while not
hurting anything else, except Mason's schbench, but not as bad as the
old thing.

It also appears to recover the tbench high-end, which also suffered like
hackbench.

Tested-by: Matt Fleming <matt@codeblueprint.co.uk>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Chris Mason <clm@fb.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: hpa@zytor.com
Cc: kitsunyan <kitsunyan@inbox.ru>
Cc: linux-kernel@vger.kernel.org
Cc: lvenanci@redhat.com
Cc: riel@redhat.com
Cc: xiaolong.ye@intel.com
Link: http://lkml.kernel.org/r/20170517105350.hk5m4h4jb6dfr65a@hirez.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 kernel/sched/fair.c     |   21 ++++++++++++++++-----
 kernel/sched/features.h |    1 +
 2 files changed, 17 insertions(+), 5 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5772,27 +5772,38 @@ static inline int select_idle_smt(struct
 static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int target)
 {
 	struct sched_domain *this_sd;
-	u64 avg_cost, avg_idle = this_rq()->avg_idle;
+	u64 avg_cost, avg_idle;
 	u64 time, cost;
 	s64 delta;
-	int cpu;
+	int cpu, nr = INT_MAX;
 
 	this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
 	if (!this_sd)
 		return -1;
 
-	avg_cost = this_sd->avg_scan_cost;
-
 	/*
 	 * Due to large variance we need a large fuzz factor; hackbench in
 	 * particularly is sensitive here.
 	 */
-	if (sched_feat(SIS_AVG_CPU) && (avg_idle / 512) < avg_cost)
+	avg_idle = this_rq()->avg_idle / 512;
+	avg_cost = this_sd->avg_scan_cost + 1;
+
+	if (sched_feat(SIS_AVG_CPU) && avg_idle < avg_cost)
 		return -1;
 
+	if (sched_feat(SIS_PROP)) {
+		u64 span_avg = sd->span_weight * avg_idle;
+		if (span_avg > 4*avg_cost)
+			nr = div_u64(span_avg, avg_cost);
+		else
+			nr = 4;
+	}
+
 	time = local_clock();
 
 	for_each_cpu_wrap(cpu, sched_domain_span(sd), target) {
+		if (!--nr)
+			return -1;
 		if (!cpumask_test_cpu(cpu, &p->cpus_allowed))
 			continue;
 		if (idle_cpu(cpu))
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -55,6 +55,7 @@ SCHED_FEAT(TTWU_QUEUE, true)
  * When doing wakeups, attempt to limit superfluous scans of the LLC domain.
  */
 SCHED_FEAT(SIS_AVG_CPU, false)
+SCHED_FEAT(SIS_PROP, true)
 
 /*
  * Issue a WARN when we do multiple update_rq_clock() calls


Patches currently in stable-queue which might be from peterz@infradead.org are

queue-4.12/sched-numa-hide-numa_wake_affine-from-up-build.patch
queue-4.12/sched-numa-use-down_read_trylock-for-the-mmap_sem.patch
queue-4.12/x86-uaccess-optimize-copy_user_enhanced_fast_string-for-short-strings.patch
queue-4.12/sched-core-implement-new-approach-to-scale-select_idle_cpu.patch
queue-4.12/sched-fair-remove-effective_load.patch
queue-4.12/sched-numa-implement-numa-node-level-wake_affine.patch
queue-4.12/sched-fair-simplify-wake_affine-for-the-single-socket-case.patch
queue-4.12/sched-fair-cpumask-export-for_each_cpu_wrap.patch
queue-4.12/sched-numa-override-part-of-migrate_degrades_locality-when-idle-balancing.patch