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

From: Peter Zijlstra
Date: Fri Apr 08 2011 - 07:39:54 EST

On Thu, 2011-04-07 at 17:24 -0700, Ken Chen wrote:
> 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 logic up at the iterator level. This ensures
> that the logic is aggregated over all task-groups, not just the last
> one in the list. The core change is in the move_tasks() function:
> + if (total_load_moved)
> + *all_pinned = 0;
> The rest of the patch are code churn that removes parameter passing
> in the lower level functions.

Very nice!

This looks applicable to earlier kernels as well, so I stuck a stable
tag on it.

Also, your From: field is somewhat weird as it consists of and addr-spec
and comment instead of the regular name-addr.
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