[tip: sched/core] cpuset: Don't use the cpu_possible_mask as a last resort for cgroup v1

[tip-bot2 for Will Deacon]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     d4b96fb92ae7fe7533e11e662504d96161928575
Gitweb:        https://git.kernel.org/tip/d4b96fb92ae7fe7533e11e662504d96161928575
Author:        Will Deacon <will@xxxxxxxxxx>
AuthorDate:    Fri, 30 Jul 2021 12:24:29 +01:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Fri, 20 Aug 2021 12:32:58 +02:00

cpuset: Don't use the cpu_possible_mask as a last resort for cgroup v1

If the scheduler cannot find an allowed CPU for a task,
cpuset_cpus_allowed_fallback() will widen the affinity to cpu_possible_mask
if cgroup v1 is in use.

In preparation for allowing architectures to provide their own fallback
mask, just return early if we're either using cgroup v1 or we're using
cgroup v2 with a mask that contains invalid CPUs. This will allow
select_fallback_rq() to figure out the mask by itself.

Signed-off-by: Will Deacon <will@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Valentin Schneider <valentin.schneider@xxxxxxx>
Reviewed-by: Quentin Perret <qperret@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20210730112443.23245-3-will@xxxxxxxxxx
---
 include/linux/cpuset.h | 1 +
 kernel/cgroup/cpuset.c | 8 ++++++--
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
index 04c20de..ed6ec67 100644
--- a/include/linux/cpuset.h
+++ b/include/linux/cpuset.h
@@ -15,6 +15,7 @@
 #include <linux/cpumask.h>
 #include <linux/nodemask.h>
 #include <linux/mm.h>
+#include <linux/mmu_context.h>
 #include <linux/jump_label.h>
 
 #ifdef CONFIG_CPUSETS
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index adb5190..a869378 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -3322,9 +3322,13 @@ void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask)
 
 void cpuset_cpus_allowed_fallback(struct task_struct *tsk)
 {
+	const struct cpumask *possible_mask = task_cpu_possible_mask(tsk);
+	const struct cpumask *cs_mask;
+
 	rcu_read_lock();
-	do_set_cpus_allowed(tsk, is_in_v2_mode() ?
-		task_cs(tsk)->cpus_allowed : cpu_possible_mask);
+	cs_mask = task_cs(tsk)->cpus_allowed;
+	if (is_in_v2_mode() && cpumask_subset(cs_mask, possible_mask))
+		do_set_cpus_allowed(tsk, cs_mask);
 	rcu_read_unlock();
 
 	/*