Subject: Re: [PATCH 1/3] sched: Fix nohz_kick_needed to consider the nr_busy of the parent domain's group

From: Preeti U Murthy <[email-masked]>
Date: Tue, 29 Oct 2013 09:00:52 +0530

Hi Peter,

On 10/28/2013 07:20 PM, Peter Zijlstra wrote:
> On Thu, Oct 24, 2013 at 01:37:38PM +0530, Preeti U Murthy wrote:
>>  kernel/sched/core.c  |    5 +++++
>>  kernel/sched/fair.c  |   38 ++++++++++++++++++++------------------
>>  kernel/sched/sched.h |    1 +
>>  3 files changed, 26 insertions(+), 18 deletions(-)
>>
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index c06b8d3..c540392 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -5271,6 +5271,7 @@ DEFINE_PER_CPU(struct sched_domain *, sd_llc);
>>  DEFINE_PER_CPU(int, sd_llc_size);
>>  DEFINE_PER_CPU(int, sd_llc_id);
>>  DEFINE_PER_CPU(struct sched_domain *, sd_numa);
>> +DEFINE_PER_CPU(struct sched_domain *, sd_busy);
>>  
>>  static void update_top_cache_domain(int cpu)
>>  {
>> @@ -5290,6 +5291,10 @@ static void update_top_cache_domain(int cpu)
>>  
>>  	sd = lowest_flag_domain(cpu, SD_NUMA);
>>  	rcu_assign_pointer(per_cpu(sd_numa, cpu), sd);
>> +
>> +	sd = highest_flag_domain(cpu, SD_SHARE_PKG_RESOURCES);
>> +	if (sd)
>> +		rcu_assign_pointer(per_cpu(sd_busy, cpu), sd->parent);
>>  }
>>  
>>  /*
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index e9c9549..f66cfd9 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -6515,16 +6515,16 @@ static inline void nohz_balance_exit_idle(int cpu)
>>  static inline void set_cpu_sd_state_busy(void)
>>  {
>>  	struct sched_domain *sd;
>> +	int cpu = smp_processor_id();
>>  
>>  	rcu_read_lock();
>> +	sd = rcu_dereference(per_cpu(sd_busy, cpu));
>>  
>>  	if (!sd || !sd->nohz_idle)
>>  		goto unlock;
>>  	sd->nohz_idle = 0;
>>  
>> +	atomic_inc(&sd->groups->sgp->nr_busy_cpus);
>>  unlock:
>>  	rcu_read_unlock();
>>  }
>> @@ -6532,16 +6532,16 @@ unlock:
>>  void set_cpu_sd_state_idle(void)
>>  {
>>  	struct sched_domain *sd;
>> +	int cpu = smp_processor_id();
>>  
>>  	rcu_read_lock();
>> +	sd = rcu_dereference(per_cpu(sd_busy, cpu));
>>  
>>  	if (!sd || sd->nohz_idle)
>>  		goto unlock;
>>  	sd->nohz_idle = 1;
>>  
>> +	atomic_dec(&sd->groups->sgp->nr_busy_cpus);
>>  unlock:
>>  	rcu_read_unlock();
>>  }
> 
> Oh nice, that gets rid of the multiple atomics, and it nicely splits
> this nohz logic into per topology groups -- now if only we could split
> the rest too :-)

I am sorry, I don't get you here. By the 'rest', do you refer to
nohz_kick_needed() as below? Or am I missing something?

> 
>> @@ -6748,6 +6748,8 @@ static inline int nohz_kick_needed(struct rq *rq, int cpu)
>>  {
>>  	unsigned long now = jiffies;
>>  	struct sched_domain *sd;
>> +	struct sched_group_power *sgp;
>> +	int nr_busy;
>>  
>>  	if (unlikely(idle_cpu(cpu)))
>>  		return 0;
>> @@ -6773,22 +6775,22 @@ static inline int nohz_kick_needed(struct rq *rq, int cpu)
>>  		goto need_kick;
>>  
>>  	rcu_read_lock();
>> +	sd = rcu_dereference(per_cpu(sd_busy, cpu));
>>  
>> +	if (sd) {
>> +		sgp = sd->groups->sgp;
>> +		nr_busy = atomic_read(&sgp->nr_busy_cpus);
>>  
>> +		if (nr_busy > 1)
>>  			goto need_kick_unlock;
>>  	}
> 
> OK, so far so good.
> 
>> +
>> +	sd = highest_flag_domain(cpu, SD_ASYM_PACKING);
>> +
>> +	if (sd && (cpumask_first_and(nohz.idle_cpus_mask,
>> +				  sched_domain_span(sd)) < cpu))
>> +		goto need_kick_unlock;
>> +
>>  	rcu_read_unlock();
>>  	return 0;
> 
> This again is a bit sad; most archs will not have SD_ASYM_PACKING set at
> all; this means that they all will do a complete (and pointless) sched
> domain tree walk here.

There will not be a 'complete' sched domain tree walk right? The
iteration will break at the first level of the sched domain for those
archs which do not have SD_ASYM_PACKING set at all.

But it is true that doing a sched domain tree walk regularly is a bad
idea, might as well update the domain with SD_ASYM_PACKING flag set once
and query this domain when required.

I will send out the patch with sd_asym domain introduced rather than the
above.

Thanks

Regards
Preeti U Murthy

> 
> It would be much better to also introduce sd_asym and do the analogous
> thing to the new sd_busy.
>