On Wed, Jul 09, 2014 at 09:24:54AM +0530, Preeti U Murthy wrote:

In the example that I mention above, t1 and t2 are on the rq of cpu0;
while t1 is running on cpu0, t2 is on the rq but does not have cpu1 in
its cpus allowed mask. So during load balance, cpu1 tries to pull t2,
cannot do so, and hence LBF_ALL_PINNED flag is set and it jumps to
out_balanced. Note that there are only two sched groups at this level of
sched domain.one with cpu0 and the other with cpu1. In this scenario we
do not try to do active load balancing, atleast thats what the code does
now if LBF_ALL_PINNED flag is set.

I think Vince is right in saying that in this scenario ALL_PINNED won't
be set. move_tasks() will iterate cfs_rq::cfs_tasks, that list will also
include the current running task.

And can_migrate_task() only checks for current after the pinning bits.

Continuing with the above explanation; when LBF_ALL_PINNED flag is
set,and we jump to out_balanced, we clear the imbalance flag for the
sched_group comprising of cpu0 and cpu1,although there is actually an
imbalance. t2 could still be migrated to say cpu2/cpu3 (t2 has them in
its cpus allowed mask) in another sched group when load balancing is
done at the next sched domain level.

And this is where Vince is wrong; note how
update_sg_lb_stats()/sg_imbalance() uses group->sgc->imbalance, but
load_balance() sets: sd_parent->groups->sgc->imbalance, so explicitly
one level up.

So what we can do I suppose is clear 'group->sgc->imbalance' at
out_balanced.

In any case, the entirely of this group imbalance crap is just that,
crap. Its a terribly difficult situation and the current bits more or
less fudge around some of the common cases. Also see the comment near
sg_imbalanced(). Its not a solid and 'correct' anything. Its a bunch of
hacks trying to deal with hard cases.

A 'good' solution would be prohibitively expensive I fear.
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