[Bug 619] New: [perf][sdet]sched_best_cpu does not pick best cpu

[Martin J. Bligh]

http://bugme.osdl.org/show_bug.cgi?id=619

           Summary: [perf][sdet]sched_best_cpu does not pick best cpu
    Kernel Version: 2.6.65
            Status: NEW
          Severity: normal
             Owner: [e-mail]
         Submitter: [e-mail]

Hardware Environment:
pSeries NUMA 

Software Environment:
Linux 2.5

Steps to Reproduce:
1.Boot Linux 2.5 kernel with NUMA support
2.Run any workload with fork+exec

Actual Results:
sched_best_cpu does not pick best cpu

Expected Results:
sched_best_cpu does pick best cpu

This occurs when you have NUMA nodes with no cpus, like on p670: node0 has 8
cpus, node1 has 0 cpus, node2 has 8 cpus.  sched_best_cpu() uses
node_nr_running, which is initialized with "0" for each node.  Since there
are no cpus in node1, no tasks are mograted there, and node_nr_running[1]
does not go above 0.  So, with at least one task running, node1 is always
picked as the least loaded node to place a newly exec'd task.

Eventually the parent's task_cpu will be picked, making sched_best_cpu 
ineffective.

Also, ideally this code needs some attention in node load.  node load is
currently computed by summing up all the running tasks in a node.  This
does not compensate for nodes that have different cpu counts, which may be
a likely situation with cpu hot plug, dynamic partitions, etc.  This stuff
is probably a bit in the future and not highest priority, be we should
probably think about  it.

Anyway, to fix the basic problem, add a new function, node_online, which
functions simialr to cpu_online, and add  a statement "if
(!node_online(node) continue;" anywhere we do for(node=0; node<maxnumnodes;
node++)

Node_online for pSereis is easy, use numa_node_exists[] array.
I guess we need a asm-generic one, too.

I have a rough version of this running right now, I'll send a patch asap.