Subject: [PATCH] sched: define TIF_ALLOW_RESCHED
From: Thomas Gleixner <[email-masked]>
Date: Sat, 23 Sep 2023 03:11:05 +0200

On Fri, Sep 22 2023 at 00:55, Thomas Gleixner wrote:
> On Thu, Sep 21 2023 at 09:00, Linus Torvalds wrote:
>> That said - I think as a proof of concept and "look, with this we get
>> the expected scheduling event counts", that patch is perfect. I think
>> you more than proved the concept.
>
> There is certainly quite some analyis work to do to make this a one to
> one replacement.
>
> With a handful of benchmarks the PoC (tweaked with some obvious fixes)
> is pretty much on par with the current mainline variants (NONE/FULL),
> but the memtier benchmark makes a massive dent.
>
> It sports a whopping 10% regression with the LAZY mode versus the mainline
> NONE model. Non-LAZY and FULL behave unsurprisingly in the same way.
>
> That benchmark is really sensitive to the preemption model. With current
> mainline (DYNAMIC_PREEMPT enabled) the preempt=FULL model has ~20%
> performance drop versus preempt=NONE.

That 20% was a tired pilot error. The real number is in the 5% ballpark.

> I have no clue what's going on there yet, but that shows that there is
> obviously quite some work ahead to get this sorted.

It took some head scratching to figure that out. The initial fix broke
the handling of the hog issue, i.e. the problem that Ankur tried to
solve, but I hacked up a "solution" for that too.

With that the memtier benchmark is roughly back to the mainline numbers,
but my throughput benchmark know how is pretty close to zero, so that
should be looked at by people who actually understand these things.

Likewise the hog prevention is just at the PoC level and clearly beyond
my knowledge of scheduler details: It unconditionally forces a
reschedule when the looping task is not responding to a lazy reschedule
request before the next tick. IOW it forces a reschedule on the second
tick, which is obviously different from the cond_resched()/might_sleep()
behaviour.

The changes vs. the original PoC aside of the bug and thinko fixes:

    1) A hack to utilize the TRACE_FLAG_IRQS_NOSUPPORT flag to trace the
       lazy preempt bit as the trace_entry::flags field is full already.

       That obviously breaks the tracer ABI, but if we go there then
       this needs to be fixed. Steven?

    2) debugfs file to validate that loops can be force preempted w/o
       cond_resched()

       The usage is:

       # taskset -c 1 bash
       # echo 1 > /sys/kernel/debug/sched/hog &
       # echo 1 > /sys/kernel/debug/sched/hog &
       # echo 1 > /sys/kernel/debug/sched/hog &

       top shows ~33% CPU for each of the hogs and tracing confirms that
       the crude hack in the scheduler tick works:

            bash-4559    [001] dlh2.  2253.331202: resched_curr <-__update_curr
            bash-4560    [001] dlh2.  2253.340199: resched_curr <-__update_curr
            bash-4561    [001] dlh2.  2253.346199: resched_curr <-__update_curr
            bash-4559    [001] dlh2.  2253.353199: resched_curr <-__update_curr
            bash-4561    [001] dlh2.  2253.358199: resched_curr <-__update_curr
            bash-4560    [001] dlh2.  2253.370202: resched_curr <-__update_curr
            bash-4559    [001] dlh2.  2253.378198: resched_curr <-__update_curr
            bash-4561    [001] dlh2.  2253.389199: resched_curr <-__update_curr

       The 'l' instead of the usual 'N' reflects that the lazy resched
       bit is set. That makes __update_curr() invoke resched_curr()
       instead of the lazy variant. resched_curr() sets TIF_NEED_RESCHED
       and folds it into preempt_count so that preemption happens at the
       next possible point, i.e. either in return from interrupt or at
       the next preempt_enable().

That's as much as I wanted to demonstrate and I'm not going to spend
more cycles on it as I have already too many other things on flight and
the resulting scheduler woes are clearly outside of my expertice.

Though definitely I'm putting a permanent NAK in place for any attempts
to duct tape the preempt=NONE model any further by sprinkling more
cond*() and whatever warts around.

Thanks,

        tglx

[tglx: s@CONFIG_PREEMPT_AUTO@CONFIG_PREEMPT_BUILD_AUTO@ ]

Link: https://lore.kernel.org/all/87jzshhexi.ffs@tglx/
Signed-off-by: Thomas Gleixner <[email-masked]>
Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
---
 arch/x86/Kconfig                   |    1 
 arch/x86/include/asm/thread_info.h |    6 ++--
 drivers/acpi/processor_idle.c      |    2 -
 include/linux/entry-common.h       |    2 -
 include/linux/entry-kvm.h          |    2 -
 include/linux/sched.h              |   12 +++++---
 include/linux/sched/idle.h         |    8 ++---
 include/linux/thread_info.h        |   24 +++++++++++++++++
 include/linux/trace_events.h       |    8 ++---
 kernel/Kconfig.preempt             |   17 +++++++++++-
 kernel/entry/common.c              |    4 +-
 kernel/entry/kvm.c                 |    2 -
 kernel/sched/core.c                |   50 +++++++++++++++++++++++++------------
 kernel/sched/debug.c               |   19 ++++++++++++++
 kernel/sched/fair.c                |   46 ++++++++++++++++++++++------------
 kernel/sched/features.h            |    2 +
 kernel/sched/idle.c                |    3 --
 kernel/sched/sched.h               |    1 
 kernel/trace/trace.c               |    2 +
 kernel/trace/trace_output.c        |   16 ++++++++++-
 20 files changed, 171 insertions(+), 56 deletions(-)

Index: linux-6.8.2-rt10/arch/x86/Kconfig
===================================================================
--- linux-6.8.2-rt10.orig/arch/x86/Kconfig
+++ linux-6.8.2-rt10/arch/x86/Kconfig
@@ -277,6 +277,7 @@ config X86
 	select HAVE_STATIC_CALL
 	select HAVE_STATIC_CALL_INLINE		if HAVE_OBJTOOL
 	select HAVE_PREEMPT_DYNAMIC_CALL
+	select HAVE_PREEMPT_AUTO
 	select HAVE_RSEQ
 	select HAVE_RUST			if X86_64
 	select HAVE_SYSCALL_TRACEPOINTS
Index: linux-6.8.2-rt10/arch/x86/include/asm/thread_info.h
===================================================================
--- linux-6.8.2-rt10.orig/arch/x86/include/asm/thread_info.h
+++ linux-6.8.2-rt10/arch/x86/include/asm/thread_info.h
@@ -81,8 +81,9 @@ struct thread_info {
 #define TIF_NOTIFY_RESUME	1	/* callback before returning to user */
 #define TIF_SIGPENDING		2	/* signal pending */
 #define TIF_NEED_RESCHED	3	/* rescheduling necessary */
-#define TIF_SINGLESTEP		4	/* reenable singlestep on user return*/
-#define TIF_SSBD		5	/* Speculative store bypass disable */
+#define TIF_ARCH_RESCHED_LAZY	4	/* Lazy rescheduling */
+#define TIF_SINGLESTEP		5	/* reenable singlestep on user return*/
+#define TIF_SSBD		6	/* Speculative store bypass disable */
 #define TIF_SPEC_IB		9	/* Indirect branch speculation mitigation */
 #define TIF_SPEC_L1D_FLUSH	10	/* Flush L1D on mm switches (processes) */
 #define TIF_USER_RETURN_NOTIFY	11	/* notify kernel of userspace return */
@@ -104,6 +105,7 @@ struct thread_info {
 #define _TIF_NOTIFY_RESUME	(1 << TIF_NOTIFY_RESUME)
 #define _TIF_SIGPENDING		(1 << TIF_SIGPENDING)
 #define _TIF_NEED_RESCHED	(1 << TIF_NEED_RESCHED)
+#define _TIF_ARCH_RESCHED_LAZY	(1 << TIF_ARCH_RESCHED_LAZY)
 #define _TIF_SINGLESTEP		(1 << TIF_SINGLESTEP)
 #define _TIF_SSBD		(1 << TIF_SSBD)
 #define _TIF_SPEC_IB		(1 << TIF_SPEC_IB)
Index: linux-6.8.2-rt10/drivers/acpi/processor_idle.c
===================================================================
--- linux-6.8.2-rt10.orig/drivers/acpi/processor_idle.c
+++ linux-6.8.2-rt10/drivers/acpi/processor_idle.c
@@ -108,7 +108,7 @@ static const struct dmi_system_id proces
  */
 static void __cpuidle acpi_safe_halt(void)
 {
-	if (!tif_need_resched()) {
+	if (!need_resched()) {
 		raw_safe_halt();
 		raw_local_irq_disable();
 	}
Index: linux-6.8.2-rt10/include/linux/entry-common.h
===================================================================
--- linux-6.8.2-rt10.orig/include/linux/entry-common.h
+++ linux-6.8.2-rt10/include/linux/entry-common.h
@@ -65,7 +65,7 @@
 #define EXIT_TO_USER_MODE_WORK						\
 	(_TIF_SIGPENDING | _TIF_NOTIFY_RESUME | _TIF_UPROBE |		\
 	 _TIF_NEED_RESCHED | _TIF_PATCH_PENDING | _TIF_NOTIFY_SIGNAL |	\
-	 ARCH_EXIT_TO_USER_MODE_WORK)
+	 _TIF_NEED_RESCHED_LAZY | ARCH_EXIT_TO_USER_MODE_WORK)
 
 /**
  * arch_enter_from_user_mode - Architecture specific sanity check for user mode regs
Index: linux-6.8.2-rt10/include/linux/entry-kvm.h
===================================================================
--- linux-6.8.2-rt10.orig/include/linux/entry-kvm.h
+++ linux-6.8.2-rt10/include/linux/entry-kvm.h
@@ -18,7 +18,7 @@
 
 #define XFER_TO_GUEST_MODE_WORK						\
 	(_TIF_NEED_RESCHED | _TIF_SIGPENDING | _TIF_NOTIFY_SIGNAL |	\
-	 _TIF_NOTIFY_RESUME | ARCH_XFER_TO_GUEST_MODE_WORK)
+	 _TIF_NOTIFY_RESUME | _TIF_NEED_RESCHED_LAZY | ARCH_XFER_TO_GUEST_MODE_WORK)
 
 struct kvm_vcpu;
 
Index: linux-6.8.2-rt10/include/linux/sched.h
===================================================================
--- linux-6.8.2-rt10.orig/include/linux/sched.h
+++ linux-6.8.2-rt10/include/linux/sched.h
@@ -1934,17 +1934,17 @@ static inline void update_tsk_thread_fla
 	update_ti_thread_flag(task_thread_info(tsk), flag, value);
 }
 
-static inline int test_and_set_tsk_thread_flag(struct task_struct *tsk, int flag)
+static inline bool test_and_set_tsk_thread_flag(struct task_struct *tsk, int flag)
 {
 	return test_and_set_ti_thread_flag(task_thread_info(tsk), flag);
 }
 
-static inline int test_and_clear_tsk_thread_flag(struct task_struct *tsk, int flag)
+static inline bool test_and_clear_tsk_thread_flag(struct task_struct *tsk, int flag)
 {
 	return test_and_clear_ti_thread_flag(task_thread_info(tsk), flag);
 }
 
-static inline int test_tsk_thread_flag(struct task_struct *tsk, int flag)
+static inline bool test_tsk_thread_flag(struct task_struct *tsk, int flag)
 {
 	return test_ti_thread_flag(task_thread_info(tsk), flag);
 }
@@ -1957,9 +1957,11 @@ 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);
+	if (IS_ENABLED(CONFIG_PREEMPT_BUILD_AUTO))
+		clear_tsk_thread_flag(tsk, TIF_NEED_RESCHED_LAZY);
 }
 
-static inline int test_tsk_need_resched(struct task_struct *tsk)
+static inline bool test_tsk_need_resched(struct task_struct *tsk)
 {
 	return unlikely(test_tsk_thread_flag(tsk,TIF_NEED_RESCHED));
 }
@@ -2100,7 +2102,7 @@ static inline bool preempt_model_preempt
 
 static __always_inline bool need_resched(void)
 {
-	return unlikely(tif_need_resched());
+	return unlikely(tif_need_resched_lazy() || tif_need_resched());
 }
 
 /*
Index: linux-6.8.2-rt10/include/linux/sched/idle.h
===================================================================
--- linux-6.8.2-rt10.orig/include/linux/sched/idle.h
+++ linux-6.8.2-rt10/include/linux/sched/idle.h
@@ -63,7 +63,7 @@ static __always_inline bool __must_check
 	 */
 	smp_mb__after_atomic();
 
-	return unlikely(tif_need_resched());
+	return unlikely(need_resched());
 }
 
 static __always_inline bool __must_check current_clr_polling_and_test(void)
@@ -76,7 +76,7 @@ static __always_inline bool __must_check
 	 */
 	smp_mb__after_atomic();
 
-	return unlikely(tif_need_resched());
+	return unlikely(need_resched());
 }
 
 #else
@@ -85,11 +85,11 @@ static inline void __current_clr_polling
 
 static inline bool __must_check current_set_polling_and_test(void)
 {
-	return unlikely(tif_need_resched());
+	return unlikely(need_resched());
 }
 static inline bool __must_check current_clr_polling_and_test(void)
 {
-	return unlikely(tif_need_resched());
+	return unlikely(need_resched());
 }
 #endif
 
Index: linux-6.8.2-rt10/include/linux/thread_info.h
===================================================================
--- linux-6.8.2-rt10.orig/include/linux/thread_info.h
+++ linux-6.8.2-rt10/include/linux/thread_info.h
@@ -59,6 +59,16 @@ enum syscall_work_bit {
 
 #include <asm/thread_info.h>
 
+#ifdef CONFIG_PREEMPT_BUILD_AUTO
+# define TIF_NEED_RESCHED_LAZY		TIF_ARCH_RESCHED_LAZY
+# define _TIF_NEED_RESCHED_LAZY		_TIF_ARCH_RESCHED_LAZY
+# define TIF_NEED_RESCHED_LAZY_OFFSET	(TIF_NEED_RESCHED_LAZY - TIF_NEED_RESCHED)
+#else
+# define TIF_NEED_RESCHED_LAZY		TIF_NEED_RESCHED
+# define _TIF_NEED_RESCHED_LAZY		_TIF_NEED_RESCHED
+# define TIF_NEED_RESCHED_LAZY_OFFSET	0
+#endif
+
 #ifdef __KERNEL__
 
 #ifndef arch_set_restart_data
@@ -185,6 +195,13 @@ static __always_inline bool tif_need_res
 			     (unsigned long *)(&current_thread_info()->flags));
 }
 
+static __always_inline bool tif_need_resched_lazy(void)
+{
+	return IS_ENABLED(CONFIG_PREEMPT_BUILD_AUTO) &&
+		arch_test_bit(TIF_NEED_RESCHED_LAZY,
+			      (unsigned long *)(&current_thread_info()->flags));
+}
+
 #else
 
 static __always_inline bool tif_need_resched(void)
@@ -193,6 +210,13 @@ static __always_inline bool tif_need_res
 			(unsigned long *)(&current_thread_info()->flags));
 }
 
+static __always_inline bool tif_need_resched_lazy(void)
+{
+	return IS_ENABLED(CONFIG_PREEMPT_BUILD_AUTO) &&
+		test_bit(TIF_NEED_RESCHED_LAZY,
+			 (unsigned long *)(&current_thread_info()->flags));
+}
+
 #endif /* _ASM_GENERIC_BITOPS_INSTRUMENTED_NON_ATOMIC_H */
 
 #ifndef CONFIG_HAVE_ARCH_WITHIN_STACK_FRAMES
Index: linux-6.8.2-rt10/include/linux/trace_events.h
===================================================================
--- linux-6.8.2-rt10.orig/include/linux/trace_events.h
+++ linux-6.8.2-rt10/include/linux/trace_events.h
@@ -178,8 +178,8 @@ unsigned int tracing_gen_ctx_irq_test(un
 
 enum trace_flag_type {
 	TRACE_FLAG_IRQS_OFF		= 0x01,
-	TRACE_FLAG_IRQS_NOSUPPORT	= 0x02,
-	TRACE_FLAG_NEED_RESCHED		= 0x04,
+	TRACE_FLAG_NEED_RESCHED		= 0x02,
+	TRACE_FLAG_NEED_RESCHED_LAZY	= 0x04,
 	TRACE_FLAG_HARDIRQ		= 0x08,
 	TRACE_FLAG_SOFTIRQ		= 0x10,
 	TRACE_FLAG_PREEMPT_RESCHED	= 0x20,
@@ -205,11 +205,11 @@ static inline unsigned int tracing_gen_c
 
 static inline unsigned int tracing_gen_ctx_flags(unsigned long irqflags)
 {
-	return tracing_gen_ctx_irq_test(TRACE_FLAG_IRQS_NOSUPPORT);
+	return tracing_gen_ctx_irq_test(0);
 }
 static inline unsigned int tracing_gen_ctx(void)
 {
-	return tracing_gen_ctx_irq_test(TRACE_FLAG_IRQS_NOSUPPORT);
+	return tracing_gen_ctx_irq_test(0);
 }
 #endif
 
Index: linux-6.8.2-rt10/kernel/Kconfig.preempt
===================================================================
--- linux-6.8.2-rt10.orig/kernel/Kconfig.preempt
+++ linux-6.8.2-rt10/kernel/Kconfig.preempt
@@ -11,6 +11,13 @@ config PREEMPT_BUILD
 	select PREEMPTION
 	select UNINLINE_SPIN_UNLOCK if !ARCH_INLINE_SPIN_UNLOCK
 
+config PREEMPT_BUILD_AUTO
+	bool
+	select PREEMPT_BUILD
+
+config HAVE_PREEMPT_AUTO
+	bool
+
 choice
 	prompt "Preemption Model"
 	default PREEMPT_NONE
@@ -67,9 +74,17 @@ config PREEMPT
 	  embedded system with latency requirements in the milliseconds
 	  range.
 
+config PREEMPT_AUTO
+	bool "Automagic preemption mode with runtime tweaking support"
+	depends on HAVE_PREEMPT_AUTO
+	select PREEMPT_BUILD_AUTO
+	help
+	  Add some sensible blurb here
+
 config PREEMPT_RT
 	bool "Fully Preemptible Kernel (Real-Time)"
 	depends on EXPERT && ARCH_SUPPORTS_RT
+	select PREEMPT_BUILD_AUTO if HAVE_PREEMPT_AUTO
 	select PREEMPTION
 	help
 	  This option turns the kernel into a real-time kernel by replacing
@@ -95,7 +110,7 @@ config PREEMPTION
 
 config PREEMPT_DYNAMIC
 	bool "Preemption behaviour defined on boot"
-	depends on HAVE_PREEMPT_DYNAMIC && !PREEMPT_RT
+	depends on HAVE_PREEMPT_DYNAMIC && !PREEMPT_RT && !PREEMPT_AUTO
 	select JUMP_LABEL if HAVE_PREEMPT_DYNAMIC_KEY
 	select PREEMPT_BUILD
 	default y if HAVE_PREEMPT_DYNAMIC_CALL
Index: linux-6.8.2-rt10/kernel/entry/common.c
===================================================================
--- linux-6.8.2-rt10.orig/kernel/entry/common.c
+++ linux-6.8.2-rt10/kernel/entry/common.c
@@ -92,7 +92,7 @@ __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)
@@ -301,7 +301,7 @@ void raw_irqentry_exit_cond_resched(void
 		rcu_irq_exit_check_preempt();
 		if (IS_ENABLED(CONFIG_DEBUG_ENTRY))
 			WARN_ON_ONCE(!on_thread_stack());
-		if (need_resched())
+		if (test_tsk_need_resched(current))
 			preempt_schedule_irq();
 	}
 }
Index: linux-6.8.2-rt10/kernel/entry/kvm.c
===================================================================
--- linux-6.8.2-rt10.orig/kernel/entry/kvm.c
+++ linux-6.8.2-rt10/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)
Index: linux-6.8.2-rt10/kernel/sched/core.c
===================================================================
--- linux-6.8.2-rt10.orig/kernel/sched/core.c
+++ linux-6.8.2-rt10/kernel/sched/core.c
@@ -899,14 +899,15 @@ static inline void hrtick_rq_init(struct
 
 #if defined(CONFIG_SMP) && defined(TIF_POLLING_NRFLAG)
 /*
- * Atomically set TIF_NEED_RESCHED and test for TIF_POLLING_NRFLAG,
+ * Atomically set TIF_NEED_RESCHED[_LAZY] and test for TIF_POLLING_NRFLAG,
  * 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 task_struct *p, int tif_bit)
 {
 	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_bit) & _TIF_POLLING_NRFLAG);
 }
 
 /*
@@ -923,7 +924,7 @@ static bool set_nr_if_polling(struct tas
 	do {
 		if (!(val & _TIF_POLLING_NRFLAG))
 			return false;
-		if (val & _TIF_NEED_RESCHED)
+		if (val & (_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY))
 			return true;
 	} while (!try_cmpxchg(&ti->flags, &val, val | _TIF_NEED_RESCHED));
 
@@ -931,9 +932,9 @@ 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 task_struct *p, int tif_bit)
 {
-	set_tsk_need_resched(p);
+	set_tsk_thread_flag(p, tif_bit);
 	return true;
 }
 
@@ -1038,28 +1039,47 @@ 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 lazy)
 {
+	int cpu, tif_bit = TIF_NEED_RESCHED + lazy;
 	struct task_struct *curr = rq->curr;
-	int cpu;
 
 	lockdep_assert_rq_held(rq);
 
-	if (test_tsk_need_resched(curr))
+	if (unlikely(test_tsk_thread_flag(curr, tif_bit)))
 		return;
 
 	cpu = cpu_of(rq);
 
 	if (cpu == smp_processor_id()) {
-		set_tsk_need_resched(curr);
-		set_preempt_need_resched();
+		set_tsk_thread_flag(curr, tif_bit);
+		if (!lazy)
+			set_preempt_need_resched();
 		return;
 	}
 
-	if (set_nr_and_not_polling(curr))
-		smp_send_reschedule(cpu);
-	else
+	if (set_nr_and_not_polling(curr, tif_bit)) {
+		if (!lazy)
+			smp_send_reschedule(cpu);
+	} else {
 		trace_sched_wake_idle_without_ipi(cpu);
+	}
+}
+
+void resched_curr(struct rq *rq)
+{
+	__resched_curr(rq, 0);
+}
+
+void resched_curr_lazy(struct rq *rq)
+{
+	int lazy = IS_ENABLED(CONFIG_PREEMPT_BUILD_AUTO) && !sched_feat(FORCE_NEED_RESCHED) ?
+		TIF_NEED_RESCHED_LAZY_OFFSET : 0;
+
+	if (lazy && unlikely(test_tsk_thread_flag(rq->curr, TIF_NEED_RESCHED)))
+		return;
+
+	__resched_curr(rq, lazy);
 }
 
 void resched_cpu(int cpu)
@@ -1154,7 +1174,7 @@ 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(rq->idle, TIF_NEED_RESCHED))
 		smp_send_reschedule(cpu);
 	else
 		trace_sched_wake_idle_without_ipi(cpu);
Index: linux-6.8.2-rt10/kernel/sched/debug.c
===================================================================
--- linux-6.8.2-rt10.orig/kernel/sched/debug.c
+++ linux-6.8.2-rt10/kernel/sched/debug.c
@@ -333,6 +333,23 @@ static const struct file_operations sche
 	.release	= seq_release,
 };
 
+static ssize_t sched_hog_write(struct file *filp, const char __user *ubuf,
+			       size_t cnt, loff_t *ppos)
+{
+	unsigned long end = jiffies + 60 * HZ;
+
+	for (; time_before(jiffies, end) && !signal_pending(current);)
+		cpu_relax();
+
+	return cnt;
+}
+
+static const struct file_operations sched_hog_fops = {
+	.write		= sched_hog_write,
+	.open		= simple_open,
+	.llseek		= default_llseek,
+};
+
 static struct dentry *debugfs_sched;
 
 static __init int sched_init_debug(void)
@@ -374,6 +391,8 @@ static __init int sched_init_debug(void)
 
 	debugfs_create_file("debug", 0444, debugfs_sched, NULL, &sched_debug_fops);
 
+	debugfs_create_file("hog", 0200, debugfs_sched, NULL, &sched_hog_fops);
+
 	return 0;
 }
 late_initcall(sched_init_debug);
Index: linux-6.8.2-rt10/kernel/sched/fair.c
===================================================================
--- linux-6.8.2-rt10.orig/kernel/sched/fair.c
+++ linux-6.8.2-rt10/kernel/sched/fair.c
@@ -975,8 +975,10 @@ static void clear_buddies(struct cfs_rq
  * XXX: strictly: vd_i += N*r_i/w_i such that: vd_i > ve_i
  * this is probably good enough.
  */
-static void update_deadline(struct cfs_rq *cfs_rq, struct sched_entity *se)
+static void update_deadline(struct cfs_rq *cfs_rq, struct sched_entity *se, bool tick)
 {
+	struct rq *rq = rq_of(cfs_rq);
+
 	if ((s64)(se->vruntime - se->deadline) < 0)
 		return;
 
@@ -995,10 +997,19 @@ static void update_deadline(struct cfs_r
 	/*
 	 * The task has consumed its request, reschedule.
 	 */
-	if (cfs_rq->nr_running > 1) {
-		resched_curr(rq_of(cfs_rq));
-		clear_buddies(cfs_rq, se);
+	if (cfs_rq->nr_running < 2)
+		return;
+
+	if (!IS_ENABLED(CONFIG_PREEMPT_BUILD_AUTO) || sched_feat(FORCE_NEED_RESCHED)) {
+		resched_curr(rq);
+	} else {
+		/* Did the task ignore the lazy reschedule request? */
+		if (tick && test_tsk_thread_flag(rq->curr, TIF_NEED_RESCHED_LAZY))
+			resched_curr(rq);
+		else
+			resched_curr_lazy(rq);
 	}
+	clear_buddies(cfs_rq, se);
 }
 
 #include "pelt.h"
@@ -1153,7 +1164,7 @@ s64 update_curr_common(struct rq *rq)
 /*
  * Update the current task's runtime statistics.
  */
-static void update_curr(struct cfs_rq *cfs_rq)
+static void __update_curr(struct cfs_rq *cfs_rq, bool tick)
 {
 	struct sched_entity *curr = cfs_rq->curr;
 	s64 delta_exec;
@@ -1166,7 +1177,7 @@ static void update_curr(struct cfs_rq *c
 		return;
 
 	curr->vruntime += calc_delta_fair(delta_exec, curr);
-	update_deadline(cfs_rq, curr);
+	update_deadline(cfs_rq, curr, tick);
 	update_min_vruntime(cfs_rq);
 
 	if (entity_is_task(curr))
@@ -1175,6 +1186,11 @@ static void update_curr(struct cfs_rq *c
 	account_cfs_rq_runtime(cfs_rq, delta_exec);
 }
 
+static inline void update_curr(struct cfs_rq *cfs_rq)
+{
+	__update_curr(cfs_rq, false);
+}
+
 static void update_curr_fair(struct rq *rq)
 {
 	update_curr(cfs_rq_of(&rq->curr->se));
@@ -5493,7 +5509,7 @@ entity_tick(struct cfs_rq *cfs_rq, struc
 	/*
 	 * Update run-time statistics of the 'current'.
 	 */
-	update_curr(cfs_rq);
+	__update_curr(cfs_rq, true);
 
 	/*
 	 * Ensure that runnable average is periodically updated.
@@ -5507,7 +5523,7 @@ entity_tick(struct cfs_rq *cfs_rq, struc
 	 * validating it and just reschedule.
 	 */
 	if (queued) {
-		resched_curr(rq_of(cfs_rq));
+		resched_curr_lazy(rq_of(cfs_rq));
 		return;
 	}
 	/*
@@ -5653,7 +5669,7 @@ static void __account_cfs_rq_runtime(str
 	 * hierarchy can be throttled
 	 */
 	if (!assign_cfs_rq_runtime(cfs_rq) && likely(cfs_rq->curr))
-		resched_curr(rq_of(cfs_rq));
+		resched_curr_lazy(rq_of(cfs_rq));
 }
 
 static __always_inline
@@ -5913,7 +5929,7 @@ unthrottle_throttle:
 
 	/* Determine whether we need to wake up potentially idle CPU: */
 	if (rq->curr == rq->idle && rq->cfs.nr_running)
-		resched_curr(rq);
+		resched_curr_lazy(rq);
 }
 
 #ifdef CONFIG_SMP
@@ -6628,7 +6644,7 @@ static void hrtick_start_fair(struct rq
 
 		if (delta < 0) {
 			if (task_current(rq, p))
-				resched_curr(rq);
+				resched_curr_lazy(rq);
 			return;
 		}
 		hrtick_start(rq, delta);
@@ -8304,7 +8320,7 @@ static void check_preempt_wakeup_fair(st
 	 * prevents us from potentially nominating it as a false LAST_BUDDY
 	 * below.
 	 */
-	if (test_tsk_need_resched(curr))
+	if (need_resched())
 		return;
 
 	/* Idle tasks are by definition preempted by non-idle tasks. */
@@ -8346,7 +8362,7 @@ static void check_preempt_wakeup_fair(st
 	return;
 
 preempt:
-	resched_curr(rq);
+	resched_curr_lazy(rq);
 }
 
 #ifdef CONFIG_SMP
@@ -12516,7 +12532,7 @@ static inline void task_tick_core(struct
 	 */
 	if (rq->core->core_forceidle_count && rq->cfs.nr_running == 1 &&
 	    __entity_slice_used(&curr->se, MIN_NR_TASKS_DURING_FORCEIDLE))
-		resched_curr(rq);
+		resched_curr_lazy(rq);
 }
 
 /*
@@ -12681,7 +12697,7 @@ prio_changed_fair(struct rq *rq, struct
 	 */
 	if (task_current(rq, p)) {
 		if (p->prio > oldprio)
-			resched_curr(rq);
+			resched_curr_lazy(rq);
 	} else
 		wakeup_preempt(rq, p, 0);
 }
Index: linux-6.8.2-rt10/kernel/sched/features.h
===================================================================
--- linux-6.8.2-rt10.orig/kernel/sched/features.h
+++ linux-6.8.2-rt10/kernel/sched/features.h
@@ -87,3 +87,5 @@ SCHED_FEAT(UTIL_EST, true)
 SCHED_FEAT(LATENCY_WARN, false)
 
 SCHED_FEAT(HZ_BW, true)
+
+SCHED_FEAT(FORCE_NEED_RESCHED, false)
Index: linux-6.8.2-rt10/kernel/sched/idle.c
===================================================================
--- linux-6.8.2-rt10.orig/kernel/sched/idle.c
+++ linux-6.8.2-rt10/kernel/sched/idle.c
@@ -57,8 +57,7 @@ static noinline int __cpuidle cpu_idle_p
 	ct_cpuidle_enter();
 
 	raw_local_irq_enable();
-	while (!tif_need_resched() &&
-	       (cpu_idle_force_poll || tick_check_broadcast_expired()))
+	while (!need_resched() && (cpu_idle_force_poll || tick_check_broadcast_expired()))
 		cpu_relax();
 	raw_local_irq_disable();
 
Index: linux-6.8.2-rt10/kernel/sched/sched.h
===================================================================
--- linux-6.8.2-rt10.orig/kernel/sched/sched.h
+++ linux-6.8.2-rt10/kernel/sched/sched.h
@@ -2463,6 +2463,7 @@ extern void init_sched_fair_class(void);
 extern void reweight_task(struct task_struct *p, int prio);
 
 extern void resched_curr(struct rq *rq);
+extern void resched_curr_lazy(struct rq *rq);
 extern void resched_cpu(int cpu);
 
 extern struct rt_bandwidth def_rt_bandwidth;
Index: linux-6.8.2-rt10/kernel/trace/trace.c
===================================================================
--- linux-6.8.2-rt10.orig/kernel/trace/trace.c
+++ linux-6.8.2-rt10/kernel/trace/trace.c
@@ -2717,6 +2717,8 @@ unsigned int tracing_gen_ctx_irq_test(un
 
 	if (tif_need_resched())
 		trace_flags |= TRACE_FLAG_NEED_RESCHED;
+	if (tif_need_resched_lazy())
+		trace_flags |= TRACE_FLAG_NEED_RESCHED_LAZY;
 	if (test_preempt_need_resched())
 		trace_flags |= TRACE_FLAG_PREEMPT_RESCHED;
 	return (trace_flags << 16) | (min_t(unsigned int, pc & 0xff, 0xf)) |
Index: linux-6.8.2-rt10/kernel/trace/trace_output.c
===================================================================
--- linux-6.8.2-rt10.orig/kernel/trace/trace_output.c
+++ linux-6.8.2-rt10/kernel/trace/trace_output.c
@@ -460,17 +460,29 @@ int trace_print_lat_fmt(struct trace_seq
 		(entry->flags & TRACE_FLAG_IRQS_OFF && bh_off) ? 'D' :
 		(entry->flags & TRACE_FLAG_IRQS_OFF) ? 'd' :
 		bh_off ? 'b' :
-		(entry->flags & TRACE_FLAG_IRQS_NOSUPPORT) ? 'X' :
+		!IS_ENABLED(CONFIG_TRACE_IRQFLAGS_SUPPORT) ? 'X' :
 		'.';
 
-	switch (entry->flags & (TRACE_FLAG_NEED_RESCHED |
+	switch (entry->flags & (TRACE_FLAG_NEED_RESCHED | TRACE_FLAG_NEED_RESCHED_LAZY |
 				TRACE_FLAG_PREEMPT_RESCHED)) {
+	case TRACE_FLAG_NEED_RESCHED | TRACE_FLAG_NEED_RESCHED_LAZY | TRACE_FLAG_PREEMPT_RESCHED:
+		need_resched = 'B';
+		break;
 	case TRACE_FLAG_NEED_RESCHED | TRACE_FLAG_PREEMPT_RESCHED:
 		need_resched = 'N';
 		break;
+	case TRACE_FLAG_NEED_RESCHED_LAZY | TRACE_FLAG_PREEMPT_RESCHED:
+		need_resched = 'L';
+		break;
+	case TRACE_FLAG_NEED_RESCHED | TRACE_FLAG_NEED_RESCHED_LAZY:
+		need_resched = 'b';
+		break;
 	case TRACE_FLAG_NEED_RESCHED:
 		need_resched = 'n';
 		break;
+	case TRACE_FLAG_NEED_RESCHED_LAZY:
+		need_resched = 'l';
+		break;
 	case TRACE_FLAG_PREEMPT_RESCHED:
 		need_resched = 'p';
 		break;