Messages in this thread | | | Date | Fri, 23 Feb 2018 16:38:06 +0000 | From | Morten Rasmussen <> | Subject | Re: [PATCH 4/7] sched/fair: Avoid unnecessary balancing of asymmetric capacity groups |
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On Tue, Feb 20, 2018 at 07:26:05PM +0100, Peter Zijlstra wrote: > On Tue, Feb 20, 2018 at 04:33:52PM +0000, Morten Rasmussen wrote: > > On Mon, Feb 19, 2018 at 04:10:11PM +0100, Peter Zijlstra wrote: > > > On Thu, Feb 15, 2018 at 04:20:51PM +0000, Morten Rasmussen wrote: > > > > +/* > > > > + * group_similar_cpu_capacity: Returns true if the minimum capacity of the > > > > + * compared groups differ by less than 12.5%. > > > > + */ > > > > +static inline bool > > > > +group_similar_cpu_capacity(struct sched_group *sg, struct sched_group *ref) > > > > +{ > > > > + long diff = sg->sgc->min_capacity - ref->sgc->min_capacity; > > > > + long max = max(sg->sgc->min_capacity, ref->sgc->min_capacity); > > > > + > > > > + return abs(diff) < max >> 3; > > > > +} > > > > > > This seems a fairly random and dodgy heuristic. > > > > I can't deny that :-) > > > > We need to somehow figure out if we are doing asymmetric cpu capacity > > balancing or normal SMP balancing. We probably don't care about > > migrating tasks if the capacities are nearly identical. But how much is > > 'nearly'? > > > > We could make it strictly equal as long as sgc->min_capacity is based on > > capacity_orig. If we let things like rt-pressure influence > > sgc->min_capacity, it might become a mess. > > See, that is the problem, I think it this min_capacity thing is > influenced by rt-pressure and the like. > > See update_cpu_capacity(), min_capacity is set after we add the RT scale > factor thingy, and then update_group_capacity() filters the min of the > whole group. The thing only ever goes down. > > But this means that if a big CPU has a very high IRQ/RT load, its > capacity will dip below that of a little core and min_capacity for the > big group as a whole will appear smaller than that of the little group. > > Or am I now terminally confused again?
No, I think you are right, or I'm equally confused.
I don't think we can avoid having some sort of margin to decide when capacities are significantly different if we want to keep this patch.
Looking more into this, I realized that we do already have similar comparison and margin in group_smaller_cpu_capacity(). So I'm going to look using that instead if possible.
The commit message could use some clarification too as we do in fact already use group_smaller_cpu_capacity() to bail out of pulling big tasks from big cpus to little. However, there are cases where it is fine to have sum_nr_running > group_weight on the big side and cases where it is fine to have the bigs idling while the littles are lightly utilized which should be addressed by this patch.
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