Messages in this thread | | | Date | Thu, 10 Jul 2014 16:34:04 +0530 | From | Preeti U Murthy <> | Subject | Re: [PATCH v3 01/12] sched: fix imbalance flag reset |
| |
Hi Peter, Vincent,
On 07/10/2014 02:44 PM, Vincent Guittot wrote: > 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.
Peter,
The above paragraph describes my concern with regard to clearing the imbalance flag at a given level of sched domain in case of pinned tasks in the below conversation. https://lkml.org/lkml/2014/7/9/454.
You are right about iterating through all tasks including the current task during load balancing.
Thanks
Regards Preeti U Murthy > > I'm going to send a new revision of the patchset with the correction > > Vincent >
| |