[PATCH v6 13/21] sched: Admit forcefully-affined tasks into SCHED_DEADLINE

[Will Deacon <will@kernel.org>]

On asymmetric systems where the affinity of a task is restricted to
contain only the CPUs capable of running it, admission to the deadline
scheduler is likely to fail because the span of the sched domain
contains incompatible CPUs. Although this is arguably the right thing to
do, it is inconsistent with the case where the affinity of a task is
restricted after already having been admitted to the deadline scheduler.

For example, on an arm64 system where not all CPUs support 32-bit
applications, a 64-bit deadline task can exec() a 32-bit image and have
its affinity forcefully restricted.

Rather than reject these tasks altogether, favour the requested user
affinity saved in 'task_struct::user_cpus_ptr' over the actual affinity
of the task which has been restricted by the kernel.

Signed-off-by: Will Deacon <will@kernel.org>
---
 kernel/sched/core.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index ba66bcf8e812..d7d058fc012e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6403,13 +6403,14 @@ static int __sched_setscheduler(struct task_struct *p,
 		if (dl_bandwidth_enabled() && dl_policy(policy) &&
 				!(attr->sched_flags & SCHED_FLAG_SUGOV)) {
 			cpumask_t *span = rq->rd->span;
+			const cpumask_t *aff = p->user_cpus_ptr ?: p->cpus_ptr;
 
 			/*
 			 * Don't allow tasks with an affinity mask smaller than
 			 * the entire root_domain to become SCHED_DEADLINE. We
 			 * will also fail if there's no bandwidth available.
 			 */
-			if (!cpumask_subset(span, p->cpus_ptr) ||
+			if (!cpumask_subset(span, aff) ||
 			    rq->rd->dl_bw.bw == 0) {
 				retval = -EPERM;
 				goto unlock;
-- 
2.31.1.751.gd2f1c929bd-goog