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 <pet...@infradead.org> > AuthorDate: Mon, 20 Aug 2012 11:26:57 +0200 > Committer: Paul E. McKenney <paul...@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.mull...@gmail.com> > Reported-by: Paul E. McKenney <paul...@linux.vnet.ibm.com> > Signed-off-by: Peter Zijlstra <a.p.zijls...@chello.nl> > Signed-off-by: Paul E. McKenney <paul.mcken...@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? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
