x86/CPU/AMD: Bring back Compute Unit ID

commit 79a8b9aa388b0620cc1d525d7c0f0d9a8a85e08e upstream.

Commit:

  a33d331761bc ("x86/CPU/AMD: Fix Bulldozer topology")

restored the initial approach we had with the Fam15h topology of
enumerating CU (Compute Unit) threads as cores. And this is still
correct - they're beefier than HT threads but still have some
shared functionality.

Our current approach has a problem with the Mad Max Steam game, for
example. Yves Dionne reported a certain "choppiness" while playing on
v4.9.5.

That problem stems most likely from the fact that the CU threads share
resources within one CU and when we schedule to a thread of a different
compute unit, this incurs latency due to migrating the working set to a
different CU through the caches.

When the thread siblings mask mirrors that aspect of the CUs and
threads, the scheduler pays attention to it and tries to schedule within
one CU first. Which takes care of the latency, of course.

Reported-by: Yves Dionne <yves.dionne@gmail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Brice Goglin <Brice.Goglin@inria.fr>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Yazen Ghannam <yazen.ghannam@amd.com>
Link: http://lkml.kernel.org/r/20170205105022.8705-1-bp@alien8.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 9 insertions, 3 deletions

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index e9bbe02950ad..36171bcd91f8 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -423,9 +423,15 @@ static bool match_smt(struct cpuinfo_x86 *c, struct cpuinfo_x86 *o)
                 int cpu1 = c->cpu_index, cpu2 = o->cpu_index;
 
                 if (c->phys_proc_id == o->phys_proc_id &&
-                    per_cpu(cpu_llc_id, cpu1) == per_cpu(cpu_llc_id, cpu2) &&
-                    c->cpu_core_id == o->cpu_core_id)
-                        return topology_sane(c, o, "smt");
+                    per_cpu(cpu_llc_id, cpu1) == per_cpu(cpu_llc_id, cpu2)) {
+                        if (c->cpu_core_id == o->cpu_core_id)
+                                return topology_sane(c, o, "smt");
+
+                        if ((c->cu_id != 0xff) &&
+                            (o->cu_id != 0xff) &&
+                            (c->cu_id == o->cu_id))
+                                return topology_sane(c, o, "smt");
+                }
 
         } else if (c->phys_proc_id == o->phys_proc_id &&
                    c->cpu_core_id == o->cpu_core_id) {