Re: [PATCH 2.6.13-rc6] cpu_exclusive sched domains build fix

John Hawkes
2005-08-24T16:10:24Z

From: "Dinakar Guniguntala" <dino@in.ibm.com>
> Can we hold on to this patch for a while, as I reported yesterday,
> this hangs up my ppc64 box on doing rmdir on a exclusive cpuset.
> Still debugging the problem, hope to have a fix soon, Thanks

Paul's patch simply constrains the scope of cpuset configurations that will
invoke the "dynamic sched domains" functionality, which means that some
cpu-exclusive (a.k.a. "isolated") cpusets will continue to have the
2.6.12-and-earlier behavior of being periodically examined by the CPU
Scheduler in load-balancing activities. That is, Paul's patch simply reverts
cpuset/sched domain behavior to pre-2.6.13 status (for some cpusets).

The pre-2.6.13 non-"dynamic sched domains" behavior will in fact produce bad
load-balancing behavior if a cpu-exclusive cpuset is so heavily loaded with
executing processes, all pinned to the cpu(s) in the cpuset, that the other
cpus in the system see this cpu(s)/node as the most-heavily-loaded and just
focus on it during load-balancing -- which would be futile, of course, since
the processes pinned to this highest-load cpu (and node) cannot be offloaded.
Since load-balancing looks only at the most-heavily-loaded cpu as a cpu to
offload, this means that all system load-balancing would be effectively turned
off.

John Hawkes