Re: [Lse-tech] Re: NUMA scheduler (was: 2.5 merge candidate list 1.5)

[Martin J. Bligh]

>> Michael, the way I read the NR_CPUS loop, you walk every cpu
>> in the system, and take the best from all of them. In which case
>> what's the point of the last_exec_cpu stuff? On the other hand, 
>> I changed your NR_CPUS to 4 (ie just walk the cpus in that node), 
>> and it got worse. So perhaps I'm just misreading your code ...
>> and it does seem significantly cheaper to execute than Erich's.
>> 
> You are reading it correct.  The only thing that the last_exec_cpu
> does is to help spread the load across nodes.  Without that what was
> happening is that node 0 would get completely loaded, then node 1,
> etc.  With it, in cases where one or more runqueues have the same
> length, the one chosen tends to get spread out a bit.  Not the 
> greatest solution, but it helps.

OK. I made a simple boring optimisation to your patch. Shaved almost
a second off system time for kernbench, and seems idiotproof to me,
shouldn't change anything apart from touching fewer runqueues: if
we find a runqueue with nr_running == 0, stop searching ... we ain't
going to find anything better ;-)

Kernbench:
                                   Elapsed        User      System         CPU
                    2.5.44-mm4     19.676s    192.794s     42.678s     1197.4%
            2.5.44-mm4-hbaum-1     19.746s    189.232s     38.354s     1152.2%
           2.5.44-mm4-hbaum-12     19.322s    190.176s     40.354s     1192.6%
 2.5.44-mm4-hbaum-12-firstzero     19.292s     189.66s     39.428s     1187.4%

Patch is probably space-eaten, so just whack it in by hand.

--- 2.5.44-mm4-hbaum-12/kernel/sched.c  2002-10-27 19:54:25.000000000 -0800
+++ 2.5.44-mm4-hbaum-12-first_low/kernel/sched.c        2002-10-27 16:42:10.000000000 -0800
@@ -2206,6 +2206,8 @@
                if (minload > cpu_rq(cur_cpu)->nr_running) {
                        minload = cpu_rq(cur_cpu)->nr_running;
                        best_cpu = cur_cpu;
+                       if (minload == 0)
+                               break;
                }
                if (++cur_cpu >= NR_CPUS)
                        cur_cpu = 0;