[PATCH] sched: Avoid sched domain rebuilding when hotplugging an isolate cpu

[Xunlei Pang <xlpang@126.com>]

From: Xunlei Pang <pang.xunlei@linaro.org>

The sched domain is always rebuilt when hotplugging an isolate cpu,
as we can see in partition_sched_domains() with the NULL doms_new,
current sched domain is destroyed, then rebuilt. Worse still, the
rebuilding will reset all the parameters of the sched_domain. This
makes no sense and contradicts the isloate concept.

The patch avoids the rebuilding if the hotplug cpu is an isolate one.

Signed-off-by: Xunlei Pang <pang.xunlei@linaro.org>
---
 kernel/sched/core.c | 34 +++++++++++++++++++++++++++++-----
 1 file changed, 29 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index bb8f3ad..bae817e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6862,9 +6862,12 @@ error:
 }
 
 static cpumask_var_t *doms_cur;	/* current sched domains */
-static int ndoms_cur;		/* number of sched domains in 'doms_cur' */
+/* effective number of sched domains in 'doms_cur' */
+static int ndoms_cur;
+/* original number of sched domains in 'doms_cur' */
+static int ndoms_orig;
+/* attribues of custom domains in 'doms_cur' */
 static struct sched_domain_attr *dattr_cur;
-				/* attribues of custom domains in 'doms_cur' */
 
 /*
  * Special case: If a kmalloc of a doms_cur partition (array of
@@ -6918,6 +6921,7 @@ static int init_sched_domains(const struct cpumask *cpu_map)
 	int err;
 
 	arch_update_cpu_topology();
+	ndoms_orig = 1;
 	ndoms_cur = 1;
 	doms_cur = alloc_sched_domains(ndoms_cur);
 	if (!doms_cur)
@@ -6990,6 +6994,7 @@ void partition_sched_domains(int ndoms_new, cpumask_var_t doms_new[],
 {
 	int i, j, n;
 	int new_topology;
+	bool borrow_needed = false;
 
 	mutex_lock(&sched_domains_mutex);
 
@@ -6999,6 +7004,13 @@ void partition_sched_domains(int ndoms_new, cpumask_var_t doms_new[],
 	/* Let architecture update cpu core mappings. */
 	new_topology = arch_update_cpu_topology();
 
+	if (doms_new == NULL && doms_cur != &fallback_doms) {
+		doms_new = &fallback_doms;
+		cpumask_andnot(doms_new[0], cpu_active_mask, cpu_isolated_map);
+		borrow_needed = true;
+		WARN_ON_ONCE(dattr_new);
+	}
+
 	n = doms_new ? ndoms_new : 0;
 
 	/* Destroy deleted domains */
@@ -7036,10 +7048,22 @@ match2:
 	}
 
 	/* Remember the new sched domains */
-	if (doms_cur != &fallback_doms)
-		free_sched_domains(doms_cur, ndoms_cur);
+	if (doms_cur != &fallback_doms && !borrow_needed)
+		free_sched_domains(doms_cur, ndoms_orig);
 	kfree(dattr_cur);	/* kfree(NULL) is safe */
-	doms_cur = doms_new;
+
+	if (borrow_needed) {
+		/*
+		 * Borrow previous doms_cur as new storage, so that
+		 * fallback_doms can be used as the temporal storage
+		 * in the future rebuilding.
+		 */
+		cpumask_copy(doms_cur[0], doms_new[0]);
+	} else {
+		doms_cur = doms_new;
+		ndoms_orig = ndoms_new;
+	}
+
 	dattr_cur = dattr_new;
 	ndoms_cur = ndoms_new;
 
-- 
1.9.1