Re: sysbench throughput degradation in 4.13+

Peter Zijlstra
2017-09-27T09:35:39Z

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);
}

/**