[PATCH V2 0/2] x86/smpboot: tidy sched-topology and drop useless SMT level

[PATCH V2 0/2] x86/smpboot: tidy sched-topology and drop useless SMT level

[Li Chen]

From: Li Chen <chenl311@chinatelecom.cn>

This two–patch series cleans up sched-domain topology handling and
eliminates hundreds of pointless attach/destroy cycles when SMT is
not available.

Patch 1  (from Thomas, unchanged) gets rid of the #ifdef maze in
build_sched_topology() by statically initialising the topology array.

Patch 2  (mine) is a follow-up that simply memmoves the array when
cpu_smt_num_threads <= 1, so the SMT level never gets created and
immediately torn down again.

Tested on Qemu.

changelog:
v2: fix wording issue as suggested by Thomas [1]

[1]: https://www.spinics.net/lists/kernel/msg5732082.html

Li Chen (1):
  x86/smpboot: avoid SMT domain attach/destroy if SMT is not enabled

Thomas Gleixner (1):
  x86/smpboot: Decrapify build_sched_topology()

 arch/x86/kernel/smpboot.c | 59 +++++++++++++++++++++------------------
 1 file changed, 32 insertions(+), 27 deletions(-)

-- 
2.49.0

[PATCH V2 1/2] x86/smpboot: Decrapify build_sched_topology()

[Li Chen]

From: Thomas Gleixner <tglx@linutronix.de>

The #ifdeffery and the initializers in build_sched_topology() are just
disgusting. The SCHED_SMT #ifdef is also pointless because SCHED_SMT is
unconditionally enabled when SMP is enabled.

Statically initialize the domain levels in the topology array and let
build_sched_topology() invalidate the package domain level when NUMA in
package is available.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
changelog:
v2: fix wording issue as suggested by Thomas [1]

[1]: https://www.spinics.net/lists/kernel/msg5732082.html

 arch/x86/kernel/smpboot.c | 45 +++++++++++++++------------------------
 1 file changed, 17 insertions(+), 28 deletions(-)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index fc78c2325fd29..7d202f9785362 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -478,43 +478,32 @@ static int x86_cluster_flags(void)
  */
 static bool x86_has_numa_in_package;
 
-static struct sched_domain_topology_level x86_topology[6];
+#define DOMAIN(maskfn, flagsfn, dname) { .mask = maskfn, .sd_flags = flagsfn, .name = #dname }
 
-static void __init build_sched_topology(void)
-{
-	int i = 0;
-
-#ifdef CONFIG_SCHED_SMT
-	x86_topology[i++] = (struct sched_domain_topology_level){
-		cpu_smt_mask, cpu_smt_flags, SD_INIT_NAME(SMT)
-	};
-#endif
+static struct sched_domain_topology_level x86_topology[] = {
+	DOMAIN(cpu_smt_mask, cpu_smt_flags, SMT),
 #ifdef CONFIG_SCHED_CLUSTER
-	x86_topology[i++] = (struct sched_domain_topology_level){
-		cpu_clustergroup_mask, x86_cluster_flags, SD_INIT_NAME(CLS)
-	};
+	DOMAIN(cpu_clustergroup_mask, x86_cluster_flags, CLS),
 #endif
 #ifdef CONFIG_SCHED_MC
-	x86_topology[i++] = (struct sched_domain_topology_level){
-		cpu_coregroup_mask, x86_core_flags, SD_INIT_NAME(MC)
-	};
+	DOMAIN(cpu_coregroup_mask, x86_core_flags, MC),
 #endif
-	/*
-	 * When there is NUMA topology inside the package skip the PKG domain
-	 * since the NUMA domains will auto-magically create the right spanning
-	 * domains based on the SLIT.
-	 */
-	if (!x86_has_numa_in_package) {
-		x86_topology[i++] = (struct sched_domain_topology_level){
-			cpu_cpu_mask, x86_sched_itmt_flags, SD_INIT_NAME(PKG)
-		};
-	}
+	DOMAIN(cpu_cpu_mask, x86_sched_itmt_flags, PKG),
+	{ NULL },
+};
 
+static void __init build_sched_topology(void)
+{
 	/*
-	 * There must be one trailing NULL entry left.
+	 * When there is NUMA topology inside the package invalidate the
+	 * PKG domain since the NUMA domains will auto-magically create the
+	 * right spanning domains based on the SLIT.
 	 */
-	BUG_ON(i >= ARRAY_SIZE(x86_topology)-1);
+	if (x86_has_numa_in_package) {
+		unsigned int pkgdom = ARRAY_SIZE(x86_topology) - 2;
 
+		memset(&x86_topology[pkgdom], 0, sizeof(x86_topology[pkgdom]));
+	}
 	set_sched_topology(x86_topology);
 }
 
-- 
2.49.0

[PATCH V2 2/2] x86/smpboot: avoid SMT domain attach/destroy if SMT is not enabled

[Li Chen]

From: Li Chen <chenl311@chinatelecom.cn>

Currently, the SMT domain is added into sched_domain_topology
by default if CONFIG_SCHED_SMT is enabled.

If cpu_attach_domain finds that the CPU SMT domain’s cpumask_weight
is just 1, it will destroy it.

On a large machine, such as one with 512 cores, this results in
512 redundant domain attach/destroy operations.

Avoid these unnecessary operations by simply checking
cpu_smt_num_threads and not inserting SMT domain into x86_topology if
the SMT domain is not enabled.

Signed-off-by: Li Chen <chenl311@chinatelecom.cn>
---
 arch/x86/kernel/smpboot.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 7d202f9785362..9ff8b10715cc1 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -492,8 +492,24 @@ static struct sched_domain_topology_level x86_topology[] = {
 	{ NULL },
 };
 
+static void __init maybe_remove_smt_level(void)
+{
+	if (cpu_smt_num_threads <= 1) {
+		/*
+		 * SMT level is x86_topology[0].  Shift the array left by one,
+		 * keep the sentinel { NULL } at the end.
+		 */
+		memmove(&x86_topology[0], &x86_topology[1],
+			sizeof(x86_topology) - sizeof(x86_topology[0]));
+		memset(&x86_topology[ARRAY_SIZE(x86_topology) - 1], 0,
+		       sizeof(x86_topology[0]));
+	}
+}
+
 static void __init build_sched_topology(void)
 {
+	maybe_remove_smt_level();
+
 	/*
 	 * When there is NUMA topology inside the package invalidate the
 	 * PKG domain since the NUMA domains will auto-magically create the
-- 
2.49.0