[RFC PATCH 45/86] preempt: ARCH_NO_PREEMPT only preempts lazily

[Ankur Arora]

Note: this commit is badly broken. Only here for discussion.

Configurations with ARCH_NO_PREEMPT support preempt_count, but might
not be tested well enough under PREEMPTION to support it might not
be demarcating the necessary non-preemptible sections.

One way to handle this is by limiting them to PREEMPT_NONE mode, not
doing any tick enforcement and limiting preemption to happen only at
user boundary.

Unfortunately, this is only a partial solution because eager
rescheduling could still happen (say, due to RCU wanting an
expedited quiescent period.) And, because we do not trust the
preempt_count accounting, this would mean preemption inside an
unmarked critical section.

I suppose we could disable that (say by selecting PREEMPTION=n),
but then the only avenue for driving scheduling between kernel
contexts (when there is no ongoing userspace work) would be
explicit calls to schedule().

Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
---
 kernel/sched/core.c     | 12 ++++++++++--
 kernel/sched/features.h |  7 +++++++
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3fa78e8afb7d..bf5df2b866df 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1059,6 +1059,14 @@ void __resched_curr(struct rq *rq, resched_t rs)
 		trace_sched_wake_idle_without_ipi(cpu);
 }
 
+#ifndef CONFIG_ARCH_NO_PREEMPT
+#define force_preempt() sched_feat(FORCE_PREEMPT)
+#define preempt_priority() sched_feat(PREEMPT_PRIORITY)
+#else
+#define force_preempt() false
+#define preempt_priority() false
+#endif
+
 /*
  * resched_curr - mark rq's current task 'to be rescheduled' eagerly
  * or lazily according to the current policy.
@@ -1084,7 +1092,7 @@ void resched_curr(struct rq *rq, bool above)
 	resched_t rs = RESCHED_lazy;
 	int context;
 
-	if (sched_feat(FORCE_PREEMPT) ||
+	if (force_preempt() ||
 	    (rq->curr->sched_class == &idle_sched_class)) {
 		rs = RESCHED_eager;
 		goto resched;
@@ -1115,7 +1123,7 @@ void resched_curr(struct rq *rq, bool above)
 		goto resched;
 	}
 
-	if (sched_feat(PREEMPT_PRIORITY) && above)
+	if (preempt_priority() && above)
 		rs = RESCHED_eager;
 
 resched:
diff --git a/kernel/sched/features.h b/kernel/sched/features.h
index 9bf30732b03f..2575d018b181 100644
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -90,6 +90,12 @@ SCHED_FEAT(LATENCY_WARN, false)
 
 SCHED_FEAT(HZ_BW, true)
 
+#ifndef CONFIG_ARCH_NO_PREEMPT
+/*
+ * Architectures with CONFIG_ARCH_NO_PREEMPT cannot safely preempt.
+ * So even though they enable CONFIG_PREEMPTION, they never have the
+ * option to dynamically switch preemption models.
+ */
 #if defined(CONFIG_PREEMPT)
 SCHED_FEAT(FORCE_PREEMPT, true)
 SCHED_FEAT(PREEMPT_PRIORITY, true)
@@ -100,3 +106,4 @@ SCHED_FEAT(PREEMPT_PRIORITY, true)
 SCHED_FEAT(FORCE_PREEMPT, false)
 SCHED_FEAT(PREEMPT_PRIORITY, false)
 #endif
+#endif
-- 
2.31.1