On Fri, May 21, 2021 at 05:25:24PM +0100, Qais Yousef wrote:

On 05/18/21 10:47, Will Deacon wrote:

quoted

Asymmetric systems may not offer the same level of userspace ISA support
across all CPUs, meaning that some applications cannot be executed by
some CPUs. As a concrete example, upcoming arm64 big.LITTLE designs do
not feature support for 32-bit applications on both clusters.

Modify guarantee_online_cpus() to take task_cpu_possible_mask() into
account when trying to find a suitable set of online CPUs for a given
task. This will avoid passing an invalid mask to set_cpus_allowed_ptr()
during ->attach() and will subsequently allow the cpuset hierarchy to be
taken into account when forcefully overriding the affinity mask for a
task which requires migration to a compatible CPU.

Cc: Li Zefan <redacted>
Cc: Tejun Heo <tj@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Will Deacon <will@kernel.org>
---
 include/linux/cpuset.h |  2 +-
 kernel/cgroup/cpuset.c | 33 +++++++++++++++++++--------------
 2 files changed, 20 insertions(+), 15 deletions(-)

diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
index ed6ec677dd6b..414a8e694413 100644
--- a/include/linux/cpuset.h
+++ b/include/linux/cpuset.h

@@ -185,7 +185,7 @@ static inline void cpuset_read_unlock(void) { }
 static inline void cpuset_cpus_allowed(struct task_struct *p,
 				       struct cpumask *mask)
 {
-	cpumask_copy(mask, cpu_possible_mask);
+	cpumask_copy(mask, task_cpu_possible_mask(p));
 }
 
 static inline void cpuset_cpus_allowed_fallback(struct task_struct *p)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 8c799260a4a2..b532a5333ff9 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c

@@ -372,18 +372,26 @@ static inline bool is_in_v2_mode(void)
 }
 
 /*
- * Return in pmask the portion of a cpusets's cpus_allowed that
- * are online.  If none are online, walk up the cpuset hierarchy
- * until we find one that does have some online cpus.
+ * Return in pmask the portion of a task's cpusets's cpus_allowed that
+ * are online and are capable of running the task.  If none are found,
+ * walk up the cpuset hierarchy until we find one that does have some
+ * appropriate cpus.
  *
  * One way or another, we guarantee to return some non-empty subset
  * of cpu_online_mask.
  *
  * Call with callback_lock or cpuset_mutex held.
  */
-static void guarantee_online_cpus(struct cpuset *cs, struct cpumask *pmask)
+static void guarantee_online_cpus(struct task_struct *tsk,
+				  struct cpumask *pmask)
 {
-	while (!cpumask_intersects(cs->effective_cpus, cpu_online_mask)) {
+	struct cpuset *cs = task_cs(tsk);

task_cs() requires rcu_read_lock(), but I can't see how the lock is obtained
from cpuset_attach() path, did I miss it? Running with lockdep should spill
suspicious RCU usage warning.

Maybe it makes more sense to move the rcu_read_lock() inside the function now
with task_cs()?

Well spotted! I'll add the rcu_read_[un]lock() calls to
guarantee_online_cpus().

Will