Subject: Re: sysbench throughput degradation in 4.13+
To: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>,
        =?UTF-8?B?546L6YeR5rWm?= <jinpuwang@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@redhat.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        "KVM-ML (kvm@vger.kernel.org)" <kvm@vger.kernel.org>,
        vcaputo@pengaru.com, Matthew Rosato <mjrosato@linux.vnet.ibm.com>
References: <95edafb1-5e9d-8461-db73-bcb002b7ebef@linux.vnet.ibm.com>
 <CAD9gYJJ9nSAbznEn80hfY3=+YjA8cKw6RztpgW6iDm7rQ0EsFg@mail.gmail.com>
 <50a279d3-84eb-3403-f2f0-854934778037@linux.vnet.ibm.com>
 <20170922155348.zujigkn3o5eylctn@hirez.programming.kicks-ass.net>
 <754f5a9f-5332-148d-2631-918fc7a7cfe9@linux.vnet.ibm.com>
 <20170927093530.s3sgdz2vamc5ka4w@hirez.programming.kicks-ass.net>
From: Eric Farman <farman@linux.vnet.ibm.com>
Date: Wed, 27 Sep 2017 12:27:50 -0400
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On 09/27/2017 05:35 AM, Peter Zijlstra wrote:
> 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?
> 

Ran a quick test this morning with 4.13.0 + 90001d67be2f + a731ebe6f17b 
and then with/without this patch.  An oltp sysbench run shows that guest 
cpu migrations decreased significantly, from ~27K to ~2K over 5 seconds.

So, we applied this patch to linux-next (next-20170926) and ran it 
against a couple sysbench tests:

--test=oltp
Baseline:	5655.26 transactions/second
Patched:	9618.13 transactions/second

--test=threads
Baseline:	25482.9 events/sec
Patched:	29577.9 events/sec

That's good!  With that...

Tested-by: Eric Farman <farman@linux.vnet.ibm.com>

Thanks!

  - Eric

> 
> 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);
>   }
> 
>   /**
>