Messages in this thread | | | Date | Wed, 9 Jul 2014 12:43:32 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v3 01/12] sched: fix imbalance flag reset |
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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. [unhandled content-type:application/pgp-signature] | |