Re: Subject: [PATCH] sched: fixed erroneous all_pinned logic.

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Fri, 2011-04-08 at 12:20 -0700, Ken Chen wrote:
> sched: fixed erroneous all_pinned logic.
> 
> The scheduler load balancer has specific code to deal with cases of
> unbalanced system due to lots of unmovable tasks (for example because
> of hard CPU affinity). In those situation, it exclude the busiest CPU
> that has pinned tasks for load balance consideration such that it can
> perform second 2nd load balance pass on the rest of the system.  This
> all works as designed if there is only one cgroup in the system.
> 
> However, when we have multiple cgroups, this logic has false positive
> and triggers multiple load balance passes despite there are actually
> no pinned tasks at all.
> 
> The reason it has false positive is that the all pinned logic is deep
> in the lowest function of can_migrate_task() and is too low level.
> load_balance_fair() iterate each task group and calls balance_tasks()
> to migrate target load.  Along the way, balance_tasks() will also set
> a all_pinned variable.  Given that task-groups are iterated, this
> all_pinned variable is essentially the status of last group in the
> scanning process.  Task group can have number of reasons that no load
> being migrated, none due to cpu affinity.  However, this status bit
> is being propagated back up to the higher level load_balance(), which
> incorrectly think that no tasks were moved.  It kick off the all pinned
> logic and start multiple passes attempt to move load onto puller CPU.
> 
> Moved the all_pinned aggregation up at the iterator level. This ensures
> that the status is aggregated over all task-groups, not just last one
> in the list.
> 
> Signed-off-by: Ken Chen <kenchen@google.com> 

Thanks Ken!