[PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED

Ankur Arora
2023-08-30T18:50:51Z

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@linux-foundation.org>
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---
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