Re: [tip:core/rcu] sched: Fix load avg vs cpu-hotplug

[Peter Zijlstra <peterz@infradead.org>]

On Wed, 2012-09-26 at 22:12 -0700, tip-bot for Peter Zijlstra wrote:
> Commit-ID:  5d18023294abc22984886bd7185344e0c2be0daf
> Gitweb:     http://git.kernel.org/tip/5d18023294abc22984886bd7185344e0c2be0daf
> Author:     Peter Zijlstra <peterz@infradead.org>
> AuthorDate: Mon, 20 Aug 2012 11:26:57 +0200
> Committer:  Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> CommitDate: Sun, 23 Sep 2012 07:43:56 -0700
> 
> sched: Fix load avg vs cpu-hotplug
> 
> Rabik and Paul reported two different issues related to the same few
> lines of code.
> 
> Rabik's issue is that the nr_uninterruptible migration code is wrong in
> that he sees artifacts due to this (Rabik please do expand in more
> detail).
> 
> Paul's issue is that this code as it stands relies on us using
> stop_machine() for unplug, we all would like to remove this assumption
> so that eventually we can remove this stop_machine() usage altogether.
> 
> The only reason we'd have to migrate nr_uninterruptible is so that we
> could use for_each_online_cpu() loops in favour of
> for_each_possible_cpu() loops, however since nr_uninterruptible() is the
> only such loop and its using possible lets not bother at all.
> 
> The problem Rabik sees is (probably) caused by the fact that by
> migrating nr_uninterruptible we screw rq->calc_load_active for both rqs
> involved.
> 
> So don't bother with fancy migration schemes (meaning we now have to
> keep using for_each_possible_cpu()) and instead fold any nr_active delta
> after we migrate all tasks away to make sure we don't have any skewed
> nr_active accounting.
> 
> [ paulmck: Move call to calc_load_migration to CPU_DEAD to avoid
> miscounting noted by Rakib. ]
> 
> Reported-by: Rakib Mullick <rakib.mullick@gmail.com>
> Reported-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
> ---
>  kernel/sched/core.c |   41 ++++++++++++++++++++---------------------
>  1 files changed, 20 insertions(+), 21 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index fbf1fd0..8c38b5e 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5304,27 +5304,17 @@ void idle_task_exit(void)
>  }
>  
>  /*
> - * While a dead CPU has no uninterruptible tasks queued at this point,
> - * it might still have a nonzero ->nr_uninterruptible counter, because
> - * for performance reasons the counter is not stricly tracking tasks to
> - * their home CPUs. So we just add the counter to another CPU's counter,
> - * to keep the global sum constant after CPU-down:
> - */
> -static void migrate_nr_uninterruptible(struct rq *rq_src)
> -{
> -	struct rq *rq_dest = cpu_rq(cpumask_any(cpu_active_mask));
> -
> -	rq_dest->nr_uninterruptible += rq_src->nr_uninterruptible;
> -	rq_src->nr_uninterruptible = 0;
> -}
> -
> -/*
> - * remove the tasks which were accounted by rq from calc_load_tasks.
> + * Since this CPU is going 'away' for a while, fold any nr_active delta
> + * we might have. Assumes we're called after migrate_tasks() so that the
> + * nr_active count is stable.
> + *
> + * Also see the comment "Global load-average calculations".
>   */
> -static void calc_global_load_remove(struct rq *rq)
> +static void calc_load_migrate(struct rq *rq)
>  {
> -	atomic_long_sub(rq->calc_load_active, &calc_load_tasks);
> -	rq->calc_load_active = 0;
> +	long delta = calc_load_fold_active(rq);
> +	if (delta)
> +		atomic_long_add(delta, &calc_load_tasks);
>  }
>  
>  /*
> @@ -5617,9 +5607,18 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
>  		migrate_tasks(cpu);
>  		BUG_ON(rq->nr_running != 1); /* the migration thread */
>  		raw_spin_unlock_irqrestore(&rq->lock, flags);
> +		break;
>  
> -		migrate_nr_uninterruptible(rq);
> -		calc_global_load_remove(rq);
> +	case CPU_DEAD:
> +		{
> +			struct rq *dest_rq;
> +
> +			local_irq_save(flags);
> +			dest_rq = cpu_rq(smp_processor_id());
> +			raw_spin_lock(&dest_rq->lock);
> +			calc_load_migrate(rq);
> +			raw_spin_unlock_irqrestore(&dest_rq->lock, flags);
> +		}
>  		break;
>  #endif
>  	}


Huh, what is this patch doing??! Didn't we merge my version of this?