Re: [RFC][PATCH 4/7] sched: Replace sd_busy/nr_busy_cpus with sched_domain_shared

[Peter Zijlstra]

On Mon, May 16, 2016 at 04:00:32PM +0200, Peter Zijlstra wrote:
> On Fri, May 13, 2016 at 10:12:26AM +1000, Michael Neuling wrote:
> 
> > > Basically; and if so, if its cheap enough to shoot a task to an idle
> > > core to avoid queueing. Assuming there still is some cache residency on
> > > the old core, the inter-core fill should be much cheaper than fetching
> > > it off package (either remote cache or dram).
> > 
> > So I think that will apply on POWER8.
> > 
> > In 10.4.2 it says "The L3.1 ECO Caches will be snooped and provide
> > intervention data similar to the L2 and L3.0 caches on the
> > chip"  That should be much faster than going to another chip or DIMM.
> > 
> > So migrating to another core on the same chip should be faster than off
> > chip.
> 
> OK; so something like the below might be what you want to play with.
> 
> ---

Maybe even something like so; which would make Power <= 6 use the
default topology and result in a shared LLC between the on package cores
etc..

Power7 is then special for not having a shared L3 but having the
asymmetric SMT thing and Power8 again gains the shared L3 while
retaining the asymmetric SMT stuff.

---
 arch/powerpc/kernel/smp.c | 34 +++++++++++++++++++++++++++++++++-
 1 file changed, 33 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index 55c924b65f71..09ce422fc29c 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -782,6 +782,23 @@ static struct sched_domain_topology_level powerpc_topology[] = {
 	{ NULL, },
 };
 
+static struct sched_domain_topology_level powerpc8_topology[] = {
+#ifdef CONFIG_SCHED_SMT
+	{ cpu_smt_mask, powerpc_smt_flags, SD_INIT_NAME(SMT) },
+#endif
+#ifdef CONFIG_SCHED_MC
+	/*
+	 * Model the L3.1 cache and sets the LLC as the whole package.
+	 *
+	 * This also ensures we try and move woken tasks to idle cores inside
+	 * the package to avoid queueing.
+	 */
+	{ cpu_coregroup_mask, cpu_core_flags, SD_INIT_NAME(MC) },
+#endif
+	{ cpu_cpu_mask, SD_INIT_NAME(DIE) },
+	{ NULL, },
+};
+
 void __init smp_cpus_done(unsigned int max_cpus)
 {
 	cpumask_var_t old_mask;
@@ -806,7 +823,22 @@ void __init smp_cpus_done(unsigned int max_cpus)
 
 	dump_numa_cpu_topology();
 
-	set_sched_topology(powerpc_topology);
+	if (cpu_has_feature(CPU_FTRS_POWER7)) {
+		/*
+		 * Power7 topology is special because it doesn't have
+		 * a shared L3 between cores and has the ASYM(metric)
+		 * SMT thing.
+		 */
+		set_sched_topology(powerpc_topology);
+	}
+
+	if (cpu_has_feature(CPU_FTRS_POWER8)) {
+		/*
+		 * Power8 differs from Power7 in that it does sort-of
+		 * have a shared L3 again.
+		 */
+		set_sched_topology(powerpc8_topology);
+	}
 
 }