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SubjectRe: [PATCH v3 01/12] sched: fix imbalance flag reset
On 9 July 2014 12:43, Peter Zijlstra <peterz@infradead.org> wrote:
> On Wed, Jul 09, 2014 at 09:24:54AM +0530, Preeti U Murthy wrote:

[snip]

>
>> 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.
>

I forgot this behavior when studying preeti use case

> 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.

I have tried to summarized several use cases that have been discussed
for this patch

The 1st use case is the one that i described in the commit message of
this patch: If we have a sporadic imbalance that set the imbalance
flag, we don't clear it after and it generates spurious and useless
active load balance

Then preeti came with the following use case :
we have a sched_domain made of CPU0 and CPU1 in 2 different sched_groups
2 tasks A and B are on CPU0, B can't run on CPU1, A is the running task.
When CPU1's sched_group is doing load balance, the imbalance should be
set. That's still happen with this patchset because the LBF_ALL_PINNED
flag will be cleared thanks to task A.

Preeti also explained me the following use cases on irc:

If we have both tasks A and B that can't run on CPU1, the
LBF_ALL_PINNED will stay set. As we can't do anything, we conclude
that we are balanced, we go to out_balanced and we clear the imbalance
flag. But we should not consider that as a balanced state but as a all
tasks pinned state instead and we should let the imbalance flag set.
If we now have 2 additional CPUs which are in the cpumask of task A
and/or B at the parent sched_domain level , we should migrate one task
in this group but this will not happen (with this patch) because the
sched_group made of CPU0 and CPU1 is not overloaded (2 tasks for 2
CPUs) and the imbalance flag has been cleared as described previously.

I'm going to send a new revision of the patchset with the correction

Vincent


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