Add the basic infrastructure to split the TIF_NEED_RESCHED bit in two.
Either bit will cause a resched on return-to-user, but only
TIF_NEED_RESCHED will drive IRQ preemption.

No behavioural change intended.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 include/linux/entry-common.h |    3 ++-
 include/linux/entry-kvm.h    |    5 +++--
 include/linux/sched.h        |    3 ++-
 include/linux/thread_info.h  |   21 +++++++++++++++++----
 kernel/entry/common.c        |    2 +-
 kernel/entry/kvm.c           |    4 ++--
 kernel/sched/core.c          |   34 +++++++++++++++++++++-------------
 7 files changed, 48 insertions(+), 24 deletions(-)

--- a/include/linux/entry-common.h
+++ b/include/linux/entry-common.h
@@ -64,7 +64,8 @@
 
 #define EXIT_TO_USER_MODE_WORK						\
 	(_TIF_SIGPENDING | _TIF_NOTIFY_RESUME | _TIF_UPROBE |		\
-	 _TIF_NEED_RESCHED | _TIF_PATCH_PENDING | _TIF_NOTIFY_SIGNAL |	\
+	 _TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY |			\
+	 _TIF_PATCH_PENDING | _TIF_NOTIFY_SIGNAL |			\
 	 ARCH_EXIT_TO_USER_MODE_WORK)
 
 /**
--- a/include/linux/entry-kvm.h
+++ b/include/linux/entry-kvm.h
@@ -17,8 +17,9 @@
 #endif
 
 #define XFER_TO_GUEST_MODE_WORK						\
-	(_TIF_NEED_RESCHED | _TIF_SIGPENDING | _TIF_NOTIFY_SIGNAL |	\
-	 _TIF_NOTIFY_RESUME | ARCH_XFER_TO_GUEST_MODE_WORK)
+	(_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY | _TIF_SIGPENDING | \
+	 _TIF_NOTIFY_SIGNAL | _TIF_NOTIFY_RESUME |			\
+	 ARCH_XFER_TO_GUEST_MODE_WORK)
 
 struct kvm_vcpu;
 
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2002,7 +2002,8 @@ static inline void set_tsk_need_resched(
 
 static inline void clear_tsk_need_resched(struct task_struct *tsk)
 {
-	clear_tsk_thread_flag(tsk,TIF_NEED_RESCHED);
+	atomic_long_andnot(_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY,
+			   (atomic_long_t *)&task_thread_info(tsk)->flags);
 }
 
 static inline int test_tsk_need_resched(struct task_struct *tsk)
--- a/include/linux/thread_info.h
+++ b/include/linux/thread_info.h
@@ -59,6 +59,14 @@ enum syscall_work_bit {
 
 #include <asm/thread_info.h>
 
+#ifndef TIF_NEED_RESCHED_LAZY
+#ifdef CONFIG_ARCH_HAS_PREEMPT_LAZY
+#error Inconsistent PREEMPT_LAZY
+#endif
+#define TIF_NEED_RESCHED_LAZY TIF_NEED_RESCHED
+#define _TIF_NEED_RESCHED_LAZY _TIF_NEED_RESCHED
+#endif
+
 #ifdef __KERNEL__
 
 #ifndef arch_set_restart_data
@@ -179,22 +187,27 @@ static __always_inline unsigned long rea
 
 #ifdef _ASM_GENERIC_BITOPS_INSTRUMENTED_NON_ATOMIC_H
 
-static __always_inline bool tif_need_resched(void)
+static __always_inline bool tif_test_bit(int bit)
 {
-	return arch_test_bit(TIF_NEED_RESCHED,
+	return arch_test_bit(bit,
 			     (unsigned long *)(&current_thread_info()->flags));
 }
 
 #else
 
-static __always_inline bool tif_need_resched(void)
+static __always_inline bool tif_test_bit(int bit)
 {
-	return test_bit(TIF_NEED_RESCHED,
+	return test_bit(bit,
 			(unsigned long *)(&current_thread_info()->flags));
 }
 
 #endif /* _ASM_GENERIC_BITOPS_INSTRUMENTED_NON_ATOMIC_H */
 
+static __always_inline bool tif_need_resched(void)
+{
+	return tif_test_bit(TIF_NEED_RESCHED);
+}
+
 #ifndef CONFIG_HAVE_ARCH_WITHIN_STACK_FRAMES
 static inline int arch_within_stack_frames(const void * const stack,
 					   const void * const stackend,
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -98,7 +98,7 @@ __always_inline unsigned long exit_to_us
 
 		local_irq_enable_exit_to_user(ti_work);
 
-		if (ti_work & _TIF_NEED_RESCHED)
+		if (ti_work & (_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY))
 			schedule();
 
 		if (ti_work & _TIF_UPROBE)
--- a/kernel/entry/kvm.c
+++ b/kernel/entry/kvm.c
@@ -13,7 +13,7 @@ static int xfer_to_guest_mode_work(struc
 			return -EINTR;
 		}
 
-		if (ti_work & _TIF_NEED_RESCHED)
+		if (ti_work & (_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY))
 			schedule();
 
 		if (ti_work & _TIF_NOTIFY_RESUME)
@@ -24,7 +24,7 @@ static int xfer_to_guest_mode_work(struc
 			return ret;
 
 		ti_work = read_thread_flags();
-	} while (ti_work & XFER_TO_GUEST_MODE_WORK || need_resched());
+	} while (ti_work & XFER_TO_GUEST_MODE_WORK);
 	return 0;
 }
 
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -936,10 +936,9 @@ static inline void hrtick_rq_init(struct
  * this avoids any races wrt polling state changes and thereby avoids
  * spurious IPIs.
  */
-static inline bool set_nr_and_not_polling(struct task_struct *p)
+static inline bool set_nr_and_not_polling(struct thread_info *ti, int tif)
 {
-	struct thread_info *ti = task_thread_info(p);
-	return !(fetch_or(&ti->flags, _TIF_NEED_RESCHED) & _TIF_POLLING_NRFLAG);
+	return !(fetch_or(&ti->flags, 1 << tif) & _TIF_POLLING_NRFLAG);
 }
 
 /*
@@ -964,9 +963,9 @@ static bool set_nr_if_polling(struct tas
 }
 
 #else
-static inline bool set_nr_and_not_polling(struct task_struct *p)
+static inline bool set_nr_and_not_polling(struct thread_info *ti, int tif)
 {
-	set_tsk_need_resched(p);
+	atomic_long_or(1 << tif, (atomic_long_t *)&ti->flags);
 	return true;
 }
 
@@ -1071,28 +1070,37 @@ void wake_up_q(struct wake_q_head *head)
  * might also involve a cross-CPU call to trigger the scheduler on
  * the target CPU.
  */
-void resched_curr(struct rq *rq)
+static void __resched_curr(struct rq *rq, int tif)
 {
 	struct task_struct *curr = rq->curr;
+	struct thread_info *cti = task_thread_info(curr);
 	int cpu;
 
 	lockdep_assert_rq_held(rq);
 
-	if (test_tsk_need_resched(curr))
+	if (cti->flags & ((1 << tif) | _TIF_NEED_RESCHED))
 		return;
 
 	cpu = cpu_of(rq);
 
 	if (cpu == smp_processor_id()) {
-		set_tsk_need_resched(curr);
-		set_preempt_need_resched();
+		set_ti_thread_flag(cti, tif);
+		if (tif == TIF_NEED_RESCHED)
+			set_preempt_need_resched();
 		return;
 	}
 
-	if (set_nr_and_not_polling(curr))
-		smp_send_reschedule(cpu);
-	else
+	if (set_nr_and_not_polling(cti, tif)) {
+		if (tif == TIF_NEED_RESCHED)
+			smp_send_reschedule(cpu);
+	} else {
 		trace_sched_wake_idle_without_ipi(cpu);
+	}
+}
+
+void resched_curr(struct rq *rq)
+{
+	__resched_curr(rq, TIF_NEED_RESCHED);
 }
 
 void resched_cpu(int cpu)
@@ -1187,7 +1195,7 @@ static void wake_up_idle_cpu(int cpu)
 	 * and testing of the above solutions didn't appear to report
 	 * much benefits.
 	 */
-	if (set_nr_and_not_polling(rq->idle))
+	if (set_nr_and_not_polling(task_thread_info(rq->idle), TIF_NEED_RESCHED))
 		smp_send_reschedule(cpu);
 	else
 		trace_sched_wake_idle_without_ipi(cpu);

On 2024-10-07 09:46:10 [+0200], Peter Zijlstra wrote:
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -964,9 +963,9 @@ static bool set_nr_if_polling(struct tas
>  }
>  
>  #else
> -static inline bool set_nr_and_not_polling(struct task_struct *p)
> +static inline bool set_nr_and_not_polling(struct thread_info *ti, int tif)
>  {
> -	set_tsk_need_resched(p);
> +	atomic_long_or(1 << tif, (atomic_long_t *)&ti->flags);

	set_ti_thread_flag(ti, tif);
?
Not sure if there is a benefit over atomic_long_or() but you could avoid
the left shift. I know. Performance critical.

>  	return true;
>  }
>  

Sebastian

On Wed, Oct 09, 2024 at 02:18:59PM +0200, Sebastian Andrzej Siewior wrote:
> On 2024-10-07 09:46:10 [+0200], Peter Zijlstra wrote:
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -964,9 +963,9 @@ static bool set_nr_if_polling(struct tas
> >  }
> >  
> >  #else
> > -static inline bool set_nr_and_not_polling(struct task_struct *p)
> > +static inline bool set_nr_and_not_polling(struct thread_info *ti, int tif)
> >  {
> > -	set_tsk_need_resched(p);
> > +	atomic_long_or(1 << tif, (atomic_long_t *)&ti->flags);
> 
> 	set_ti_thread_flag(ti, tif);
> ?
> Not sure if there is a benefit over atomic_long_or() but you could avoid
> the left shift. I know. Performance critical.

:-)

Yeah, I suppose set_ti_thread_flag() works just fine here.

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     26baa1f1c4bdc34b8d698c1900b407d863ad0e69
Gitweb:        https://git.kernel.org/tip/26baa1f1c4bdc34b8d698c1900b407d863ad0e69
Author:        Peter Zijlstra <peterz@infradead.org>
AuthorDate:    Fri, 04 Oct 2024 14:47:02 +02:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Tue, 05 Nov 2024 12:55:37 +01:00

sched: Add TIF_NEED_RESCHED_LAZY infrastructure

Add the basic infrastructure to split the TIF_NEED_RESCHED bit in two.
Either bit will cause a resched on return-to-user, but only
TIF_NEED_RESCHED will drive IRQ preemption.

No behavioural change intended.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lkml.kernel.org/r/20241007075055.219540785@infradead.org
---
 include/linux/entry-common.h |  3 ++-
 include/linux/entry-kvm.h    |  5 +++--
 include/linux/sched.h        |  3 ++-
 include/linux/thread_info.h  | 21 +++++++++++++++++----
 kernel/entry/common.c        |  2 +-
 kernel/entry/kvm.c           |  4 ++--
 kernel/sched/core.c          | 34 +++++++++++++++++++++-------------
 7 files changed, 48 insertions(+), 24 deletions(-)

diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h
index 1e50cdb..fc61d02 100644
--- a/include/linux/entry-common.h
+++ b/include/linux/entry-common.h
@@ -64,7 +64,8 @@
 
 #define EXIT_TO_USER_MODE_WORK						\
 	(_TIF_SIGPENDING | _TIF_NOTIFY_RESUME | _TIF_UPROBE |		\
-	 _TIF_NEED_RESCHED | _TIF_PATCH_PENDING | _TIF_NOTIFY_SIGNAL |	\
+	 _TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY |			\
+	 _TIF_PATCH_PENDING | _TIF_NOTIFY_SIGNAL |			\
 	 ARCH_EXIT_TO_USER_MODE_WORK)
 
 /**
diff --git a/include/linux/entry-kvm.h b/include/linux/entry-kvm.h
index 6813171..16149f6 100644
--- a/include/linux/entry-kvm.h
+++ b/include/linux/entry-kvm.h
@@ -17,8 +17,9 @@
 #endif
 
 #define XFER_TO_GUEST_MODE_WORK						\
-	(_TIF_NEED_RESCHED | _TIF_SIGPENDING | _TIF_NOTIFY_SIGNAL |	\
-	 _TIF_NOTIFY_RESUME | ARCH_XFER_TO_GUEST_MODE_WORK)
+	(_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY | _TIF_SIGPENDING | \
+	 _TIF_NOTIFY_SIGNAL | _TIF_NOTIFY_RESUME |			\
+	 ARCH_XFER_TO_GUEST_MODE_WORK)
 
 struct kvm_vcpu;
 
diff --git a/include/linux/sched.h b/include/linux/sched.h
index a76e3d0..1d5cc3e 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2002,7 +2002,8 @@ static inline void set_tsk_need_resched(struct task_struct *tsk)
 
 static inline void clear_tsk_need_resched(struct task_struct *tsk)
 {
-	clear_tsk_thread_flag(tsk,TIF_NEED_RESCHED);
+	atomic_long_andnot(_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY,
+			   (atomic_long_t *)&task_thread_info(tsk)->flags);
 }
 
 static inline int test_tsk_need_resched(struct task_struct *tsk)
diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
index 9ea0b28..cf2446c 100644
--- a/include/linux/thread_info.h
+++ b/include/linux/thread_info.h
@@ -59,6 +59,14 @@ enum syscall_work_bit {
 
 #include <asm/thread_info.h>
 
+#ifndef TIF_NEED_RESCHED_LAZY
+#ifdef CONFIG_ARCH_HAS_PREEMPT_LAZY
+#error Inconsistent PREEMPT_LAZY
+#endif
+#define TIF_NEED_RESCHED_LAZY TIF_NEED_RESCHED
+#define _TIF_NEED_RESCHED_LAZY _TIF_NEED_RESCHED
+#endif
+
 #ifdef __KERNEL__
 
 #ifndef arch_set_restart_data
@@ -179,22 +187,27 @@ static __always_inline unsigned long read_ti_thread_flags(struct thread_info *ti
 
 #ifdef _ASM_GENERIC_BITOPS_INSTRUMENTED_NON_ATOMIC_H
 
-static __always_inline bool tif_need_resched(void)
+static __always_inline bool tif_test_bit(int bit)
 {
-	return arch_test_bit(TIF_NEED_RESCHED,
+	return arch_test_bit(bit,
 			     (unsigned long *)(&current_thread_info()->flags));
 }
 
 #else
 
-static __always_inline bool tif_need_resched(void)
+static __always_inline bool tif_test_bit(int bit)
 {
-	return test_bit(TIF_NEED_RESCHED,
+	return test_bit(bit,
 			(unsigned long *)(&current_thread_info()->flags));
 }
 
 #endif /* _ASM_GENERIC_BITOPS_INSTRUMENTED_NON_ATOMIC_H */
 
+static __always_inline bool tif_need_resched(void)
+{
+	return tif_test_bit(TIF_NEED_RESCHED);
+}
+
 #ifndef CONFIG_HAVE_ARCH_WITHIN_STACK_FRAMES
 static inline int arch_within_stack_frames(const void * const stack,
 					   const void * const stackend,
diff --git a/kernel/entry/common.c b/kernel/entry/common.c
index 5b6934e..e33691d 100644
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -98,7 +98,7 @@ __always_inline unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
 
 		local_irq_enable_exit_to_user(ti_work);
 
-		if (ti_work & _TIF_NEED_RESCHED)
+		if (ti_work & (_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY))
 			schedule();
 
 		if (ti_work & _TIF_UPROBE)
diff --git a/kernel/entry/kvm.c b/kernel/entry/kvm.c
index 2e0f75b..8485f63 100644
--- a/kernel/entry/kvm.c
+++ b/kernel/entry/kvm.c
@@ -13,7 +13,7 @@ static int xfer_to_guest_mode_work(struct kvm_vcpu *vcpu, unsigned long ti_work)
 			return -EINTR;
 		}
 
-		if (ti_work & _TIF_NEED_RESCHED)
+		if (ti_work & (_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY))
 			schedule();
 
 		if (ti_work & _TIF_NOTIFY_RESUME)
@@ -24,7 +24,7 @@ static int xfer_to_guest_mode_work(struct kvm_vcpu *vcpu, unsigned long ti_work)
 			return ret;
 
 		ti_work = read_thread_flags();
-	} while (ti_work & XFER_TO_GUEST_MODE_WORK || need_resched());
+	} while (ti_work & XFER_TO_GUEST_MODE_WORK);
 	return 0;
 }
 
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index aad4885..0cd05e3 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -941,10 +941,9 @@ static inline void hrtick_rq_init(struct rq *rq)
  * this avoids any races wrt polling state changes and thereby avoids
  * spurious IPIs.
  */
-static inline bool set_nr_and_not_polling(struct task_struct *p)
+static inline bool set_nr_and_not_polling(struct thread_info *ti, int tif)
 {
-	struct thread_info *ti = task_thread_info(p);
-	return !(fetch_or(&ti->flags, _TIF_NEED_RESCHED) & _TIF_POLLING_NRFLAG);
+	return !(fetch_or(&ti->flags, 1 << tif) & _TIF_POLLING_NRFLAG);
 }
 
 /*
@@ -969,9 +968,9 @@ static bool set_nr_if_polling(struct task_struct *p)
 }
 
 #else
-static inline bool set_nr_and_not_polling(struct task_struct *p)
+static inline bool set_nr_and_not_polling(struct thread_info *ti, int tif)
 {
-	set_tsk_need_resched(p);
+	set_ti_thread_flag(ti, tif);
 	return true;
 }
 
@@ -1076,28 +1075,37 @@ void wake_up_q(struct wake_q_head *head)
  * might also involve a cross-CPU call to trigger the scheduler on
  * the target CPU.
  */
-void resched_curr(struct rq *rq)
+static void __resched_curr(struct rq *rq, int tif)
 {
 	struct task_struct *curr = rq->curr;
+	struct thread_info *cti = task_thread_info(curr);
 	int cpu;
 
 	lockdep_assert_rq_held(rq);
 
-	if (test_tsk_need_resched(curr))
+	if (cti->flags & ((1 << tif) | _TIF_NEED_RESCHED))
 		return;
 
 	cpu = cpu_of(rq);
 
 	if (cpu == smp_processor_id()) {
-		set_tsk_need_resched(curr);
-		set_preempt_need_resched();
+		set_ti_thread_flag(cti, tif);
+		if (tif == TIF_NEED_RESCHED)
+			set_preempt_need_resched();
 		return;
 	}
 
-	if (set_nr_and_not_polling(curr))
-		smp_send_reschedule(cpu);
-	else
+	if (set_nr_and_not_polling(cti, tif)) {
+		if (tif == TIF_NEED_RESCHED)
+			smp_send_reschedule(cpu);
+	} else {
 		trace_sched_wake_idle_without_ipi(cpu);
+	}
+}
+
+void resched_curr(struct rq *rq)
+{
+	__resched_curr(rq, TIF_NEED_RESCHED);
 }
 
 void resched_cpu(int cpu)
@@ -1192,7 +1200,7 @@ static void wake_up_idle_cpu(int cpu)
 	 * and testing of the above solutions didn't appear to report
 	 * much benefits.
 	 */
-	if (set_nr_and_not_polling(rq->idle))
+	if (set_nr_and_not_polling(task_thread_info(rq->idle), TIF_NEED_RESCHED))
 		smp_send_reschedule(cpu);
 	else
 		trace_sched_wake_idle_without_ipi(cpu);