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@oracle.com>
---
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