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Date: Thu, 27 Sep 2012 05:24:08 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org, mingo@...nel.org, hpa@...or.com,
paul.mckenney@...aro.org, rakib.mullick@...il.com,
tglx@...utronix.de, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:core/rcu] sched: Fix load avg vs cpu-hotplug
On Thu, Sep 27, 2012 at 10:22:51AM +0200, Peter Zijlstra wrote:
> 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@...radead.org>
> > AuthorDate: Mon, 20 Aug 2012 11:26:57 +0200
> > Committer: Paul E. McKenney <paulmck@...ux.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@...il.com>
> > Reported-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > Signed-off-by: Paul E. McKenney <paul.mckenney@...aro.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?
Yep, it all got straightened out in the merge commit 593d1006
(Merge remote-tracking branch 'tip/core/rcu' into next.2012.09.25b).
After this merge commit, the code looks as follows:
migrate_tasks(cpu);
BUG_ON(rq->nr_running != 1); /* the migration thread */
raw_spin_unlock_irqrestore(&rq->lock, flags);
break;
case CPU_DEAD:
calc_load_migrate(rq);
break;
Which is what you had in https://lkml.org/lkml/2012/9/5/585. I am
not sure what happened to that patch, but as you can see from the
merge commit, it had not made it yet.
Thanx, Paul
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