Messages in this thread | | | Subject | Re: [PATCH 11/15] sched: Pass unlimited __cpu_power information to upper domain level groups | From | Peter Zijlstra <> | Date | Mon, 24 Aug 2009 19:26:50 +0200 |
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On Mon, 2009-08-24 at 22:14 +0530, Balbir Singh wrote: > * Peter Zijlstra <peterz@infradead.org> [2009-08-24 17:21:37]: > > > On Thu, 2009-08-20 at 15:41 +0200, Andreas Herrmann wrote: > > > For performance reasons __cpu_power in a sched_group might be limited > > > such that the group can handle only one task. To correctly calculate > > > the capacity in upper domain level groups the unlimited power > > > information is required. This patch stores unlimited __cpu_power > > > information in sched_groups.orig_power and uses this when calculating > > > __cpu_power in upper domain level groups. > > > > OK, so this tries to fix the cpu_power wreckage? > > > > ok, so let me try this with an example: > > > > > > Suppose we have a dual-core with shared cache and SMT > > > > 0-3 MC > > 0-1 2-3 SMT > > > > Then both levels fancy setting SHARED_RESOURCES and both levels end up > > normalizing the cpu_power to 1, so when we unplug cpu 2, load-balancing > > gets all screwy because the whole system doesn't get normalized > > properly. > > > > What you propose here is every time we muck with cpu_power we keep the > > real stuff in orig_power and use that to compute the level above. > > > > Except you don't use it in the load-balancer proper, so normalization is > > still hosed. > > > > Its a creative solution, but I'd rather see cpu_power returned to a > > straight sum of actual power to normalize the inter-cpu runqueue weights > > and do the placement decision using something else. > > The real solution is to find a way to solve asymmetric load balancing, > I suppose. The asymmetry might be due to cores being hot-plugged for > example
No, the solution is to not use cpu_power for placement and use it for normalization of the weight only. That would make the asym work by definition.
The real fun comes when we then introduce dynamic cpu_power based on feedback from things like aperf/mperf ratios for SMT and feedback from the RT scheduler.
The trouble is that cpu_power is now abused for placement decisions too, and that needs to be taken out.
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