Re: sysbench throughput degradation in 4.13+

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Sep 22, 2017 at 12:12:45PM -0400, Eric Farman wrote:
> 
> MySQL.  We've tried a few different configs with both test=oltp and
> test=threads, but both show the same behavior.  What I have settled on for
> my repro is the following:
> 

Right, didn't even need to run it in a guest to observe a regression.

So the below cures native sysbench and NAS bench for me, does it also
work for you virt thingy?


PRE (current tip/master):

ivb-ex sysbench:

  2: [30 secs]     transactions:                        64110  (2136.94 per sec.)
  5: [30 secs]     transactions:                        143644 (4787.99 per sec.)
 10: [30 secs]     transactions:                        274298 (9142.93 per sec.)
 20: [30 secs]     transactions:                        418683 (13955.45 per sec.)
 40: [30 secs]     transactions:                        320731 (10690.15 per sec.)
 80: [30 secs]     transactions:                        355096 (11834.28 per sec.)

hsw-ex NAS:

OMP_PROC_BIND/lu.C.x_threads_144_run_1.log: Time in seconds =                    18.01
OMP_PROC_BIND/lu.C.x_threads_144_run_2.log: Time in seconds =                    17.89
OMP_PROC_BIND/lu.C.x_threads_144_run_3.log: Time in seconds =                    17.93
lu.C.x_threads_144_run_1.log: Time in seconds =                   434.68
lu.C.x_threads_144_run_2.log: Time in seconds =                   405.36
lu.C.x_threads_144_run_3.log: Time in seconds =                   433.83


POST (+patch):

ivb-ex sysbench:

  2: [30 secs]     transactions:                        64494  (2149.75 per sec.)
  5: [30 secs]     transactions:                        145114 (4836.99 per sec.)
 10: [30 secs]     transactions:                        278311 (9276.69 per sec.)
 20: [30 secs]     transactions:                        437169 (14571.60 per sec.)
 40: [30 secs]     transactions:                        669837 (22326.73 per sec.)
 80: [30 secs]     transactions:                        631739 (21055.88 per sec.)

hsw-ex NAS:

lu.C.x_threads_144_run_1.log: Time in seconds =                    23.36
lu.C.x_threads_144_run_2.log: Time in seconds =                    22.96
lu.C.x_threads_144_run_3.log: Time in seconds =                    22.52


This patch takes out all the shiny wake_affine stuff and goes back to
utter basics. Rik was there another NUMA benchmark that wanted your
fancy stuff? Because NAS isn't it.

(the previous, slightly fancier wake_affine was basically a !idle
extension of this, in that it would pick the 'shortest' of the 2 queues
and thereby run quickest, in approximation)

I'll try and run a number of other benchmarks I have around to see if
there's anything that shows a difference between the below trivial
wake_affine and the old 2-cpu-load one.

---
 include/linux/sched/topology.h |   8 ---
 kernel/sched/fair.c            | 125 ++---------------------------------------
 2 files changed, 6 insertions(+), 127 deletions(-)

diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
index d7b6dab956ec..7d065abc7a47 100644
--- a/include/linux/sched/topology.h
+++ b/include/linux/sched/topology.h
@@ -71,14 +71,6 @@ struct sched_domain_shared {
 	atomic_t	ref;
 	atomic_t	nr_busy_cpus;
 	int		has_idle_cores;
-
-	/*
-	 * Some variables from the most recent sd_lb_stats for this domain,
-	 * used by wake_affine().
-	 */
-	unsigned long	nr_running;
-	unsigned long	load;
-	unsigned long	capacity;
 };
 
 struct sched_domain {
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 70ba32e08a23..66930ce338af 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5356,115 +5356,19 @@ static int wake_wide(struct task_struct *p)
 	return 1;
 }
 
-struct llc_stats {
-	unsigned long	nr_running;
-	unsigned long	load;
-	unsigned long	capacity;
-	int		has_capacity;
-};
-
-static bool get_llc_stats(struct llc_stats *stats, int cpu)
-{
-	struct sched_domain_shared *sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
-
-	if (!sds)
-		return false;
-
-	stats->nr_running	= READ_ONCE(sds->nr_running);
-	stats->load		= READ_ONCE(sds->load);
-	stats->capacity		= READ_ONCE(sds->capacity);
-	stats->has_capacity	= stats->nr_running < per_cpu(sd_llc_size, cpu);
-
-	return true;
-}
-
-/*
- * Can a task be moved from prev_cpu to this_cpu without causing a load
- * imbalance that would trigger the load balancer?
- *
- * Since we're running on 'stale' values, we might in fact create an imbalance
- * but recomputing these values is expensive, as that'd mean iteration 2 cache
- * domains worth of CPUs.
- */
-static bool
-wake_affine_llc(struct sched_domain *sd, struct task_struct *p,
-		int this_cpu, int prev_cpu, int sync)
-{
-	struct llc_stats prev_stats, this_stats;
-	s64 this_eff_load, prev_eff_load;
-	unsigned long task_load;
-
-	if (!get_llc_stats(&prev_stats, prev_cpu) ||
-	    !get_llc_stats(&this_stats, this_cpu))
-		return false;
-
-	/*
-	 * If sync wakeup then subtract the (maximum possible)
-	 * effect of the currently running task from the load
-	 * of the current LLC.
-	 */
-	if (sync) {
-		unsigned long current_load = task_h_load(current);
-
-		/* in this case load hits 0 and this LLC is considered 'idle' */
-		if (current_load > this_stats.load)
-			return true;
-
-		this_stats.load -= current_load;
-	}
-
-	/*
-	 * The has_capacity stuff is not SMT aware, but by trying to balance
-	 * the nr_running on both ends we try and fill the domain at equal
-	 * rates, thereby first consuming cores before siblings.
-	 */
-
-	/* if the old cache has capacity, stay there */
-	if (prev_stats.has_capacity && prev_stats.nr_running < this_stats.nr_running+1)
-		return false;
-
-	/* if this cache has capacity, come here */
-	if (this_stats.has_capacity && this_stats.nr_running+1 < prev_stats.nr_running)
-		return true;
-
-	/*
-	 * Check to see if we can move the load without causing too much
-	 * imbalance.
-	 */
-	task_load = task_h_load(p);
-
-	this_eff_load = 100;
-	this_eff_load *= prev_stats.capacity;
-
-	prev_eff_load = 100 + (sd->imbalance_pct - 100) / 2;
-	prev_eff_load *= this_stats.capacity;
-
-	this_eff_load *= this_stats.load + task_load;
-	prev_eff_load *= prev_stats.load - task_load;
-
-	return this_eff_load <= prev_eff_load;
-}
-
 static int wake_affine(struct sched_domain *sd, struct task_struct *p,
 		       int prev_cpu, int sync)
 {
 	int this_cpu = smp_processor_id();
-	bool affine;
-
-	/*
-	 * Default to no affine wakeups; wake_affine() should not effect a task
-	 * placement the load-balancer feels inclined to undo. The conservative
-	 * option is therefore to not move tasks when they wake up.
-	 */
-	affine = false;
+	bool affine = false;
 
 	/*
-	 * If the wakeup is across cache domains, try to evaluate if movement
-	 * makes sense, otherwise rely on select_idle_siblings() to do
-	 * placement inside the cache domain.
+	 * If we can run _now_ on the waking CPU, go there, otherwise meh.
 	 */
-	if (!cpus_share_cache(prev_cpu, this_cpu))
-		affine = wake_affine_llc(sd, p, this_cpu, prev_cpu, sync);
+	if (idle_cpu(this_cpu))
+		affine = true;
+	else if (sync && cpu_rq(this_cpu)->nr_running == 1)
+		affine = true;
 
 	schedstat_inc(p->se.statistics.nr_wakeups_affine_attempts);
 	if (affine) {
@@ -7600,7 +7504,6 @@ static inline enum fbq_type fbq_classify_rq(struct rq *rq)
  */
 static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sds)
 {
-	struct sched_domain_shared *shared = env->sd->shared;
 	struct sched_domain *child = env->sd->child;
 	struct sched_group *sg = env->sd->groups;
 	struct sg_lb_stats *local = &sds->local_stat;
@@ -7672,22 +7575,6 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
 		if (env->dst_rq->rd->overload != overload)
 			env->dst_rq->rd->overload = overload;
 	}
-
-	if (!shared)
-		return;
-
-	/*
-	 * Since these are sums over groups they can contain some CPUs
-	 * multiple times for the NUMA domains.
-	 *
-	 * Currently only wake_affine_llc() and find_busiest_group()
-	 * uses these numbers, only the last is affected by this problem.
-	 *
-	 * XXX fix that.
-	 */
-	WRITE_ONCE(shared->nr_running,	sds->total_running);
-	WRITE_ONCE(shared->load,	sds->total_load);
-	WRITE_ONCE(shared->capacity,	sds->total_capacity);
 }
 
 /**