Re: [PATCH v2 6/8] sched/idle: Move busy_cpu accounting to idle callback

[Srikar Dronamraju <srikar@linux.vnet.ibm.com>]

* Aubrey Li <aubrey.li@linux.intel.com> [2021-05-12 16:08:24]:

> On 5/7/21 12:45 AM, Srikar Dronamraju wrote:
> > Currently we account nr_busy_cpus in no_hz idle functions.
> > There is no reason why nr_busy_cpus should updated be in NO_HZ_COMMON
> > configs only. Also scheduler can mark a CPU as non-busy as soon as an
> > idle class task starts to run. Scheduler can then mark a CPU as busy
> > as soon as its woken up from idle or a new task is placed on it's
> > runqueue.
> 
> IIRC, we discussed this before, if a SCHED_IDLE task is placed on the
> CPU's runqueue, this CPU should be still taken as a wakeup target.
> 

Yes, this CPU is still a wakeup target, its only when this CPU is busy, that
we look at other CPUs

> Also, for those frequent context-switching tasks with very short idle,
> it's expensive for scheduler to mark idle/busy every time, that's why
> my patch only marks idle every time and marks busy ratelimited in
> scheduler tick.
> 

I have tried few tasks with very short idle times and updating nr_busy
everytime, doesnt seem to be impacting. Infact, it seems to help in picking
the idler-llc more often.

> Thanks,
> -Aubrey
> 
> > 
> > Cc: LKML <linux-kernel@vger.kernel.org>
> > Cc: Gautham R Shenoy <ego@linux.vnet.ibm.com>
> > Cc: Parth Shah <parth@linux.ibm.com>
> > Cc: Ingo Molnar <mingo@kernel.org>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Valentin Schneider <valentin.schneider@arm.com>
> > Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> > Cc: Mel Gorman <mgorman@techsingularity.net>
> > Cc: Vincent Guittot <vincent.guittot@linaro.org>
> > Cc: Rik van Riel <riel@surriel.com>
> > Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
> > ---
> >  kernel/sched/fair.c     |  6 ++++--
> >  kernel/sched/idle.c     | 29 +++++++++++++++++++++++++++--
> >  kernel/sched/sched.h    |  1 +
> >  kernel/sched/topology.c |  2 ++
> >  4 files changed, 34 insertions(+), 4 deletions(-)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index c30587631a24..4d3b0928fe98 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -10394,7 +10394,10 @@ static void set_cpu_sd_state_busy(int cpu)
> >  		goto unlock;
> >  	sd->nohz_idle = 0;
> >  
> > -	atomic_inc(&sd->shared->nr_busy_cpus);
> > +	if (sd && per_cpu(is_idle, cpu)) {
> > +		atomic_add_unless(&sd->shared->nr_busy_cpus, 1, per_cpu(sd_llc_size, cpu));
> > +		per_cpu(is_idle, cpu) = 0;
> > +	}
> >  unlock:
> >  	rcu_read_unlock();
> >  }
> > @@ -10424,7 +10427,6 @@ static void set_cpu_sd_state_idle(int cpu)
> >  		goto unlock;
> >  	sd->nohz_idle = 1;
> >  
> > -	atomic_dec(&sd->shared->nr_busy_cpus);
> >  unlock:
> >  	rcu_read_unlock();
> >  }
> > diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> > index cc828f3efe71..e624a05e48bd 100644
> > --- a/kernel/sched/idle.c
> > +++ b/kernel/sched/idle.c
> > @@ -425,12 +425,25 @@ static void check_preempt_curr_idle(struct rq *rq, struct task_struct *p, int fl
> >  
> >  static void put_prev_task_idle(struct rq *rq, struct task_struct *prev)
> >  {
> > -#ifdef CONFIG_SCHED_SMT
> > +#ifdef CONFIG_SMP
> > +	struct sched_domain_shared *sds;
> >  	int cpu = rq->cpu;
> >  
> > +#ifdef CONFIG_SCHED_SMT
> >  	if (static_branch_likely(&sched_smt_present))
> >  		set_core_busy(cpu);
> >  #endif
> > +
> > +	rcu_read_lock();
> > +	sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
> > +	if (sds) {
> > +		if (per_cpu(is_idle, cpu)) {
> > +			atomic_inc(&sds->nr_busy_cpus);
> > +			per_cpu(is_idle, cpu) = 0;
> > +		}
> > +	}
> > +	rcu_read_unlock();
> > +#endif
> >  }
> >  
> >  static void set_next_task_idle(struct rq *rq, struct task_struct *next, bool first)
> > @@ -442,9 +455,21 @@ static void set_next_task_idle(struct rq *rq, struct task_struct *next, bool fir
> >  struct task_struct *pick_next_task_idle(struct rq *rq)
> >  {
> >  	struct task_struct *next = rq->idle;
> > +#ifdef CONFIG_SMP
> > +	struct sched_domain_shared *sds;
> > +	int cpu = rq->cpu;
> >  
> > -	set_next_task_idle(rq, next, true);
> > +	rcu_read_lock();
> > +	sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
> > +	if (sds) {
> > +		atomic_add_unless(&sds->nr_busy_cpus, -1, 0);
> > +		per_cpu(is_idle, cpu) = 1;
> > +	}
> >  
> > +	rcu_read_unlock();
> > +#endif
> > +
> > +	set_next_task_idle(rq, next, true);
> >  	return next;
> >  }
> >  
> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index 5c0bd4b0e73a..baf8d9a4cb26 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -1483,6 +1483,7 @@ DECLARE_PER_CPU(int, sd_llc_id);
> >  #ifdef CONFIG_SCHED_SMT
> >  DECLARE_PER_CPU(int, smt_id);
> >  #endif
> > +DECLARE_PER_CPU(int, is_idle);
> >  DECLARE_PER_CPU(struct sched_domain_shared __rcu *, sd_llc_shared);
> >  DECLARE_PER_CPU(struct sched_domain __rcu *, sd_numa);
> >  DECLARE_PER_CPU(struct sched_domain __rcu *, sd_asym_packing);
> > diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> > index 8db40c8a6ad0..00e4669bb241 100644
> > --- a/kernel/sched/topology.c
> > +++ b/kernel/sched/topology.c
> > @@ -647,6 +647,7 @@ DEFINE_PER_CPU(int, sd_llc_id);
> >  #ifdef CONFIG_SCHED_SMT
> >  DEFINE_PER_CPU(int, smt_id);
> >  #endif
> > +DEFINE_PER_CPU(int, is_idle);
> >  DEFINE_PER_CPU(struct sched_domain_shared __rcu *, sd_llc_shared);
> >  DEFINE_PER_CPU(struct sched_domain __rcu *, sd_numa);
> >  DEFINE_PER_CPU(struct sched_domain __rcu *, sd_asym_packing);
> > @@ -673,6 +674,7 @@ static void update_top_cache_domain(int cpu)
> >  #ifdef CONFIG_SCHED_SMT
> >  	per_cpu(smt_id, cpu) = cpumask_first(cpu_smt_mask(cpu));
> >  #endif
> > +	per_cpu(is_idle, cpu) = 1;
> >  	rcu_assign_pointer(per_cpu(sd_llc_shared, cpu), sds);
> >  
> >  	sd = lowest_flag_domain(cpu, SD_NUMA);
> > 
> 

-- 
Thanks and Regards
Srikar Dronamraju