[PATCH] x86/topology: Fallback to SMT level only once

[Jiri Olsa]

Jirka, Peter and Jean-Pierre reported performance drop on
some cpus after making cpu offline and online again.

The reason is the kernel logic that falls back to SMT
level topology if more than one node is detected within
CPU package. During the system boot this logic cuts out
the DIE topology level and numa code adds NUMA level
on top of this.

After the boot if you reboot make the cpu offline and online
again, this logic resets the SMT level topology, removing
whole NUMA level stuff.

Ensuring the SMT topology fallback happens only once during
the boot so the NUMA topology level is kept once it's built.

This problem is one of the issues reported in the wastedcores
article [1]. There's similar patch to this issue attached
to the article [2].

[1] https://github.com/jplozi/wastedcores
[2] https://github.com/jplozi/wastedcores/blob/master/patches/missing_sched_domains_linux_4.1.patch

Cc: Jean-Pierre Lozi <jplozi@xxxxxxxx>
Cc: Jirka Hladky <jhladky@xxxxxxxxxx>
Cc: Petr Surà <psury@xxxxxxxxxx>
Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
---
 arch/x86/kernel/smpboot.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 2a6e84a30a54..f2a769b2b3fe 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -487,7 +487,17 @@ static struct sched_domain_topology_level numa_inside_package_topology[] = {
  */
 static void primarily_use_numa_for_topology(void)
 {
-	set_sched_topology(numa_inside_package_topology);
+	static bool once;
+
+	/*
+	 * We need to run it only during boot, once we are
+	 * here due to getting cpu online again we have already
+	 * NUMA topology setup done.
+	 */
+	if (!once) {
+		set_sched_topology(numa_inside_package_topology);
+		once = true;
+	}
 }
 
 void set_cpu_sibling_map(int cpu)
-- 
2.7.4