On 2/2/18 9:17 AM, Peter Zijlstra wrote:
On Fri, Feb 02, 2018 at 11:53:40AM -0500, Steven Sistare wrote:
+static int select_idle_smt(struct task_struct *p, struct sched_group *sg)
+       int i, rand_index, rand_cpu;
+       int this_cpu = smp_processor_id();
+ rand_index = CPU_PSEUDO_RANDOM(this_cpu) % sg->group_weight;
+       rand_cpu = sg->cp_array[rand_index];
Right, so yuck.. I know why you need that, but that extra array and
dereference is the reason I never went there.

How much difference does it really make vs the 'normal' wrapping search
from last CPU ?

This really should be a separate patch with separate performance numbers
For the benefit of other readers, if we always search and choose starting from
the first CPU in a core, then later searches will often need to traverse the 
N busy CPU's to find the first idle CPU.  Choosing a random starting point 
such bias.  It is probably a win for processors with 4 to 8 CPUs per core, and
a slight but hopefully negligible loss for 2 CPUs per core, and I agree we need
to see performance data for this as a separate patch to decide.  We have SPARC
systems with 8 CPUs per core.
Which is why the current code already doesn't start from the first cpu
in the mask. We start at whatever CPU the task ran last on, which is
effectively 'random' if the system is busy.

So how is a per-cpu rotor better than that?
In the scheme of SMT balance, if the idle cpu search is done _not_ in the last run core, then we need a random cpu to start from. If the idle cpu search is done in the last run core we can start the search from last run cpu. Since we need the random index for the first case I just did it for both.


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