[PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED

[Ankur Arora]

On preempt_model_none() or preempt_model_voluntary() configurations
rescheduling of kernel threads happens only when they allow it, and
only at explicit preemption points, via calls to cond_resched() or
similar.

That leaves out contexts where it is not convenient to periodically
call cond_resched() -- for instance when executing a potentially long
running primitive (such as REP; STOSB.)

This means that we either suffer high scheduling latency or avoid
certain constructs.

Define TIF_ALLOW_RESCHED to demarcate such sections.

Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
---
 arch/x86/include/asm/thread_info.h |  2 ++
 include/linux/sched.h              | 30 ++++++++++++++++++++++++++++++
 2 files changed, 32 insertions(+)

diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
index d63b02940747..fc6f4121b412 100644
--- a/arch/x86/include/asm/thread_info.h
+++ b/arch/x86/include/asm/thread_info.h
@@ -100,6 +100,7 @@ struct thread_info {
 #define TIF_BLOCKSTEP		25	/* set when we want DEBUGCTLMSR_BTF */
 #define TIF_LAZY_MMU_UPDATES	27	/* task is updating the mmu lazily */
 #define TIF_ADDR32		29	/* 32-bit address space on 64 bits */
+#define TIF_RESCHED_ALLOW	30	/* reschedule if needed */
 
 #define _TIF_NOTIFY_RESUME	(1 << TIF_NOTIFY_RESUME)
 #define _TIF_SIGPENDING		(1 << TIF_SIGPENDING)
@@ -122,6 +123,7 @@ struct thread_info {
 #define _TIF_BLOCKSTEP		(1 << TIF_BLOCKSTEP)
 #define _TIF_LAZY_MMU_UPDATES	(1 << TIF_LAZY_MMU_UPDATES)
 #define _TIF_ADDR32		(1 << TIF_ADDR32)
+#define _TIF_RESCHED_ALLOW	(1 << TIF_RESCHED_ALLOW)
 
 /* flags to check in __switch_to() */
 #define _TIF_WORK_CTXSW_BASE					\
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 177b3f3676ef..4dd3d91d990f 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2245,6 +2245,36 @@ static __always_inline bool need_resched(void)
 	return unlikely(tif_need_resched());
 }
 
+#ifdef TIF_RESCHED_ALLOW
+/*
+ * allow_resched() .. disallow_resched() demarcate a preemptible section.
+ *
+ * Used around primitives where it might not be convenient to periodically
+ * call cond_resched().
+ */
+static inline void allow_resched(void)
+{
+	might_sleep();
+	set_tsk_thread_flag(current, TIF_RESCHED_ALLOW);
+}
+
+static inline void disallow_resched(void)
+{
+	clear_tsk_thread_flag(current, TIF_RESCHED_ALLOW);
+}
+
+static __always_inline bool resched_allowed(void)
+{
+	return unlikely(test_tsk_thread_flag(current, TIF_RESCHED_ALLOW));
+}
+
+#else
+static __always_inline bool resched_allowed(void)
+{
+	return false;
+}
+#endif /* TIF_RESCHED_ALLOW */
+
 /*
  * Wrappers for p->thread_info->cpu access. No-op on UP.
  */
-- 
2.31.1