rcu: Remove nohz_full full-system-idle state machine

The NO_HZ_FULL_SYSIDLE full-system-idle capability was added in 2013
by commit 0edd1b17 ("nohz_full: Add full-system-idle state machine"),
but has not been used.  This commit therefore removes it.

If it turns out to be needed later, this commit can always be reverted.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/RCU/Design/Requirements/Requirements.html

@@ -2520,11 +2520,7 @@ It is similarly socially unacceptable to interrupt an
 <tt>nohz_full</tt> CPU running in userspace.
 RCU must therefore track <tt>nohz_full</tt> userspace
 execution.
-And in
-<a href="https://lwn.net/Articles/558284/"><tt>CONFIG_NO_HZ_FULL_SYSIDLE=y</tt></a>
-kernels, RCU must separately track idle CPUs on the one hand and
-CPUs that are either idle or executing in userspace on the other.
-In both cases, RCU must be able to sample state at two points in
+RCU must therefore be able to sample state at two points in
 time, and be able to determine whether or not some other CPU spent
 any time idle and/or executing in userspace.
 

include/linux/rcupdate.h

@@ -854,15 +854,6 @@ static inline notrace void rcu_read_unlock_sched_notrace(void)
 #define kfree_rcu(ptr, rcu_head)					\
 	__kfree_rcu(&((ptr)->rcu_head), offsetof(typeof(*(ptr)), rcu_head))
 
-/* Only for use by adaptive-ticks code. */
-#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
-bool rcu_sys_is_idle(void);
-void rcu_sysidle_force_exit(void);
-#else /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
-static inline bool rcu_sys_is_idle(void) { return false; }
-static inline void rcu_sysidle_force_exit(void) { }
-#endif /* #else #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
-
 
 /*
  * Place this after a lock-acquisition primitive to guarantee that

kernel/rcu/tree.c

@@ -270,10 +270,6 @@ void rcu_bh_qs(void)
 static DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
 	.dynticks_nesting = DYNTICK_TASK_EXIT_IDLE,
 	.dynticks = ATOMIC_INIT(RCU_DYNTICK_CTRL_CTR),
-#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
-	.dynticks_idle_nesting = DYNTICK_TASK_NEST_VALUE,
-	.dynticks_idle = ATOMIC_INIT(1),
-#endif /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
 };
 
 /*
@@ -546,10 +542,7 @@ module_param(jiffies_till_sched_qs, ulong, 0644);
 
 static bool rcu_start_gp_advanced(struct rcu_state *rsp, struct rcu_node *rnp,
 				  struct rcu_data *rdp);
-static void force_qs_rnp(struct rcu_state *rsp,
-			 int (*f)(struct rcu_data *rsp, bool *isidle,
-				  unsigned long *maxj),
-			 bool *isidle, unsigned long *maxj);
+static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *rsp));
 static void force_quiescent_state(struct rcu_state *rsp);
 static int rcu_pending(void);
 
@@ -854,7 +847,6 @@ void rcu_idle_enter(void)
 
 	local_irq_save(flags);
 	rcu_eqs_enter(false);
-	rcu_sysidle_enter(0);
 	local_irq_restore(flags);
 }
 EXPORT_SYMBOL_GPL(rcu_idle_enter);
@@ -904,7 +896,6 @@ void rcu_irq_exit(void)
 		trace_rcu_dyntick(TPS("--="), rdtp->dynticks_nesting, rdtp->dynticks_nesting - 1);
 		rdtp->dynticks_nesting--;
 	}
-	rcu_sysidle_enter(1);
 }
 
 /*
@@ -986,7 +977,6 @@ void rcu_idle_exit(void)
 
 	local_irq_save(flags);
 	rcu_eqs_exit(false);
-	rcu_sysidle_exit(0);
 	local_irq_restore(flags);
 }
 EXPORT_SYMBOL_GPL(rcu_idle_exit);
@@ -1038,7 +1028,6 @@ void rcu_irq_enter(void)
 		trace_rcu_dyntick(TPS("++="), oldval, rdtp->dynticks_nesting);
 	else
 		rcu_eqs_exit_common(oldval, true);
-	rcu_sysidle_exit(1);
 }
 
 /*
@@ -1217,11 +1206,9 @@ static int rcu_is_cpu_rrupt_from_idle(void)
  * credit them with an implicit quiescent state.  Return 1 if this CPU
  * is in dynticks idle mode, which is an extended quiescent state.
  */
-static int dyntick_save_progress_counter(struct rcu_data *rdp,
-					 bool *isidle, unsigned long *maxj)
+static int dyntick_save_progress_counter(struct rcu_data *rdp)
 {
 	rdp->dynticks_snap = rcu_dynticks_snap(rdp->dynticks);
-	rcu_sysidle_check_cpu(rdp, isidle, maxj);
 	if (rcu_dynticks_in_eqs(rdp->dynticks_snap)) {
 		trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, TPS("dti"));
 		if (ULONG_CMP_LT(READ_ONCE(rdp->gpnum) + ULONG_MAX / 4,
@@ -1238,8 +1225,7 @@ static int dyntick_save_progress_counter(struct rcu_data *rdp,
  * idle state since the last call to dyntick_save_progress_counter()
  * for this same CPU, or by virtue of having been offline.
  */
-static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
-				    bool *isidle, unsigned long *maxj)
+static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
 {
 	unsigned long jtsq;
 	bool *rnhqp;
@@ -2105,25 +2091,16 @@ static bool rcu_gp_fqs_check_wake(struct rcu_state *rsp, int *gfp)
  */
 static void rcu_gp_fqs(struct rcu_state *rsp, bool first_time)
 {
-	bool isidle = false;
-	unsigned long maxj;
 	struct rcu_node *rnp = rcu_get_root(rsp);
 
 	WRITE_ONCE(rsp->gp_activity, jiffies);
 	rsp->n_force_qs++;
 	if (first_time) {
 		/* Collect dyntick-idle snapshots. */
-		if (is_sysidle_rcu_state(rsp)) {
-			isidle = true;
-			maxj = jiffies - ULONG_MAX / 4;
-		}
-		force_qs_rnp(rsp, dyntick_save_progress_counter,
-			     &isidle, &maxj);
-		rcu_sysidle_report_gp(rsp, isidle, maxj);
+		force_qs_rnp(rsp, dyntick_save_progress_counter);
 	} else {
 		/* Handle dyntick-idle and offline CPUs. */
-		isidle = true;
-		force_qs_rnp(rsp, rcu_implicit_dynticks_qs, &isidle, &maxj);
+		force_qs_rnp(rsp, rcu_implicit_dynticks_qs);
 	}
 	/* Clear flag to prevent immediate re-entry. */
 	if (READ_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS) {
@@ -2895,10 +2872,7 @@ void rcu_check_callbacks(int user)
  *
  * The caller must have suppressed start of new grace periods.
  */
-static void force_qs_rnp(struct rcu_state *rsp,
-			 int (*f)(struct rcu_data *rsp, bool *isidle,
-				  unsigned long *maxj),
-			 bool *isidle, unsigned long *maxj)
+static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *rsp))
 {
 	int cpu;
 	unsigned long flags;
@@ -2937,7 +2911,7 @@ static void force_qs_rnp(struct rcu_state *rsp,
 		for_each_leaf_node_possible_cpu(rnp, cpu) {
 			unsigned long bit = leaf_node_cpu_bit(rnp, cpu);
 			if ((rnp->qsmask & bit) != 0) {
-				if (f(per_cpu_ptr(rsp->rda, cpu), isidle, maxj))
+				if (f(per_cpu_ptr(rsp->rda, cpu)))
 					mask |= bit;
 			}
 		}
@@ -3793,7 +3767,6 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp)
 	    !init_nocb_callback_list(rdp))
 		rcu_segcblist_init(&rdp->cblist);  /* Re-enable callbacks. */
 	rdp->dynticks->dynticks_nesting = DYNTICK_TASK_EXIT_IDLE;
-	rcu_sysidle_init_percpu_data(rdp->dynticks);
 	rcu_dynticks_eqs_online();
 	raw_spin_unlock_rcu_node(rnp);		/* irqs remain disabled. */
 

kernel/rcu/tree.h

@@ -45,14 +45,6 @@ struct rcu_dynticks {
 	bool rcu_need_heavy_qs;     /* GP old, need heavy quiescent state. */
 	unsigned long rcu_qs_ctr;   /* Light universal quiescent state ctr. */
 	bool rcu_urgent_qs;	    /* GP old need light quiescent state. */
-#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
-	long long dynticks_idle_nesting;
-				    /* irq/process nesting level from idle. */
-	atomic_t dynticks_idle;	    /* Even value for idle, else odd. */
-				    /*  "Idle" excludes userspace execution. */
-	unsigned long dynticks_idle_jiffies;
-				    /* End of last non-NMI non-idle period. */
-#endif /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
 #ifdef CONFIG_RCU_FAST_NO_HZ
 	bool all_lazy;		    /* Are all CPU's CBs lazy? */
 	unsigned long nonlazy_posted;
@@ -529,15 +521,7 @@ static void __init rcu_organize_nocb_kthreads(struct rcu_state *rsp);
 #endif /* #ifdef CONFIG_RCU_NOCB_CPU */
 static void __maybe_unused rcu_kick_nohz_cpu(int cpu);
 static bool init_nocb_callback_list(struct rcu_data *rdp);
-static void rcu_sysidle_enter(int irq);
-static void rcu_sysidle_exit(int irq);
-static void rcu_sysidle_check_cpu(struct rcu_data *rdp, bool *isidle,
-				  unsigned long *maxj);
-static bool is_sysidle_rcu_state(struct rcu_state *rsp);
-static void rcu_sysidle_report_gp(struct rcu_state *rsp, int isidle,
-				  unsigned long maxj);
 static void rcu_bind_gp_kthread(void);
-static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp);
 static bool rcu_nohz_full_cpu(struct rcu_state *rsp);
 static void rcu_dynticks_task_enter(void);
 static void rcu_dynticks_task_exit(void);

kernel/rcu/tree_plugin.h

@@ -2563,429 +2563,6 @@ static void __maybe_unused rcu_kick_nohz_cpu(int cpu)
 #endif /* #ifdef CONFIG_NO_HZ_FULL */
 }
 
-
-#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
-
-static int full_sysidle_state;		/* Current system-idle state. */
-#define RCU_SYSIDLE_NOT		0	/* Some CPU is not idle. */
-#define RCU_SYSIDLE_SHORT	1	/* All CPUs idle for brief period. */
-#define RCU_SYSIDLE_LONG	2	/* All CPUs idle for long enough. */
-#define RCU_SYSIDLE_FULL	3	/* All CPUs idle, ready for sysidle. */
-#define RCU_SYSIDLE_FULL_NOTED	4	/* Actually entered sysidle state. */
-
-/*
- * Invoked to note exit from irq or task transition to idle.  Note that
- * usermode execution does -not- count as idle here!  After all, we want
- * to detect full-system idle states, not RCU quiescent states and grace
- * periods.  The caller must have disabled interrupts.
- */
-static void rcu_sysidle_enter(int irq)
-{
-	unsigned long j;
-	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
-
-	RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_sysidle_enter() invoked with irqs enabled!!!");
-
-	/* If there are no nohz_full= CPUs, no need to track this. */
-	if (!tick_nohz_full_enabled())
-		return;
-
-	/* Adjust nesting, check for fully idle. */
-	if (irq) {
-		rdtp->dynticks_idle_nesting--;
-		WARN_ON_ONCE(rdtp->dynticks_idle_nesting < 0);
-		if (rdtp->dynticks_idle_nesting != 0)
-			return;  /* Still not fully idle. */
-	} else {
-		if ((rdtp->dynticks_idle_nesting & DYNTICK_TASK_NEST_MASK) ==
-		    DYNTICK_TASK_NEST_VALUE) {
-			rdtp->dynticks_idle_nesting = 0;
-		} else {
-			rdtp->dynticks_idle_nesting -= DYNTICK_TASK_NEST_VALUE;
-			WARN_ON_ONCE(rdtp->dynticks_idle_nesting < 0);
-			return;  /* Still not fully idle. */
-		}
-	}
-
-	/* Record start of fully idle period. */
-	j = jiffies;
-	WRITE_ONCE(rdtp->dynticks_idle_jiffies, j);
-	smp_mb__before_atomic();
-	atomic_inc(&rdtp->dynticks_idle);
-	smp_mb__after_atomic();
-	WARN_ON_ONCE(atomic_read(&rdtp->dynticks_idle) & 0x1);
-}
-
-/*
- * Unconditionally force exit from full system-idle state.  This is
- * invoked when a normal CPU exits idle, but must be called separately
- * for the timekeeping CPU (tick_do_timer_cpu).  The reason for this
- * is that the timekeeping CPU is permitted to take scheduling-clock
- * interrupts while the system is in system-idle state, and of course
- * rcu_sysidle_exit() has no way of distinguishing a scheduling-clock
- * interrupt from any other type of interrupt.
- */
-void rcu_sysidle_force_exit(void)
-{
-	int oldstate = READ_ONCE(full_sysidle_state);
-	int newoldstate;
-
-	/*
-	 * Each pass through the following loop attempts to exit full
-	 * system-idle state.  If contention proves to be a problem,
-	 * a trylock-based contention tree could be used here.
-	 */
-	while (oldstate > RCU_SYSIDLE_SHORT) {
-		newoldstate = cmpxchg(&full_sysidle_state,
-				      oldstate, RCU_SYSIDLE_NOT);
-		if (oldstate == newoldstate &&
-		    oldstate == RCU_SYSIDLE_FULL_NOTED) {
-			rcu_kick_nohz_cpu(tick_do_timer_cpu);
-			return; /* We cleared it, done! */
-		}
-		oldstate = newoldstate;
-	}
-	smp_mb(); /* Order initial oldstate fetch vs. later non-idle work. */
-}
-
-/*
- * Invoked to note entry to irq or task transition from idle.  Note that
- * usermode execution does -not- count as idle here!  The caller must
- * have disabled interrupts.
- */
-static void rcu_sysidle_exit(int irq)
-{
-	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
-
-	RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_sysidle_exit() invoked with irqs enabled!!!");
-
-	/* If there are no nohz_full= CPUs, no need to track this. */
-	if (!tick_nohz_full_enabled())
-		return;
-
-	/* Adjust nesting, check for already non-idle. */
-	if (irq) {
-		rdtp->dynticks_idle_nesting++;
-		WARN_ON_ONCE(rdtp->dynticks_idle_nesting <= 0);
-		if (rdtp->dynticks_idle_nesting != 1)
-			return; /* Already non-idle. */
-	} else {
-		/*
-		 * Allow for irq misnesting.  Yes, it really is possible
-		 * to enter an irq handler then never leave it, and maybe
-		 * also vice versa.  Handle both possibilities.
-		 */
-		if (rdtp->dynticks_idle_nesting & DYNTICK_TASK_NEST_MASK) {
-			rdtp->dynticks_idle_nesting += DYNTICK_TASK_NEST_VALUE;
-			WARN_ON_ONCE(rdtp->dynticks_idle_nesting <= 0);
-			return; /* Already non-idle. */
-		} else {
-			rdtp->dynticks_idle_nesting = DYNTICK_TASK_EXIT_IDLE;
-		}
-	}
-
-	/* Record end of idle period. */
-	smp_mb__before_atomic();
-	atomic_inc(&rdtp->dynticks_idle);
-	smp_mb__after_atomic();
-	WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks_idle) & 0x1));
-
-	/*
-	 * If we are the timekeeping CPU, we are permitted to be non-idle
-	 * during a system-idle state.  This must be the case, because
-	 * the timekeeping CPU has to take scheduling-clock interrupts
-	 * during the time that the system is transitioning to full
-	 * system-idle state.  This means that the timekeeping CPU must
-	 * invoke rcu_sysidle_force_exit() directly if it does anything
-	 * more than take a scheduling-clock interrupt.
-	 */
-	if (smp_processor_id() == tick_do_timer_cpu)
-		return;
-
-	/* Update system-idle state: We are clearly no longer fully idle! */
-	rcu_sysidle_force_exit();
-}
-
-/*
- * Check to see if the current CPU is idle.  Note that usermode execution
- * does not count as idle.  The caller must have disabled interrupts,
- * and must be running on tick_do_timer_cpu.
- */
-static void rcu_sysidle_check_cpu(struct rcu_data *rdp, bool *isidle,
-				  unsigned long *maxj)
-{
-	int cur;
-	unsigned long j;
-	struct rcu_dynticks *rdtp = rdp->dynticks;
-
-	RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_sysidle_check_cpu() invoked with irqs enabled!!!");
-
-	/* If there are no nohz_full= CPUs, don't check system-wide idleness. */
-	if (!tick_nohz_full_enabled())
-		return;
-
-	/*
-	 * If some other CPU has already reported non-idle, if this is
-	 * not the flavor of RCU that tracks sysidle state, or if this
-	 * is an offline or the timekeeping CPU, nothing to do.
-	 */
-	if (!*isidle || rdp->rsp != rcu_state_p ||
-	    cpu_is_offline(rdp->cpu) || rdp->cpu == tick_do_timer_cpu)
-		return;
-	/* Verify affinity of current kthread. */
-	WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu);
-
-	/* Pick up current idle and NMI-nesting counter and check. */
-	cur = atomic_read(&rdtp->dynticks_idle);
-	if (cur & 0x1) {
-		*isidle = false; /* We are not idle! */
-		return;
-	}
-	smp_mb(); /* Read counters before timestamps. */
-
-	/* Pick up timestamps. */
-	j = READ_ONCE(rdtp->dynticks_idle_jiffies);
-	/* If this CPU entered idle more recently, update maxj timestamp. */
-	if (ULONG_CMP_LT(*maxj, j))
-		*maxj = j;
-}
-
-/*
- * Is this the flavor of RCU that is handling full-system idle?
- */
-static bool is_sysidle_rcu_state(struct rcu_state *rsp)
-{
-	return rsp == rcu_state_p;
-}
-
-/*
- * Return a delay in jiffies based on the number of CPUs, rcu_node
- * leaf fanout, and jiffies tick rate.  The idea is to allow larger
- * systems more time to transition to full-idle state in order to
- * avoid the cache thrashing that otherwise occur on the state variable.
- * Really small systems (less than a couple of tens of CPUs) should
- * instead use a single global atomically incremented counter, and later
- * versions of this will automatically reconfigure themselves accordingly.
- */
-static unsigned long rcu_sysidle_delay(void)
-{
-	if (nr_cpu_ids <= CONFIG_NO_HZ_FULL_SYSIDLE_SMALL)
-		return 0;
-	return DIV_ROUND_UP(nr_cpu_ids * HZ, rcu_fanout_leaf * 1000);
-}
-
-/*
- * Advance the full-system-idle state.  This is invoked when all of
- * the non-timekeeping CPUs are idle.
- */
-static void rcu_sysidle(unsigned long j)
-{
-	/* Check the current state. */
-	switch (READ_ONCE(full_sysidle_state)) {
-	case RCU_SYSIDLE_NOT:
-
-		/* First time all are idle, so note a short idle period. */
-		WRITE_ONCE(full_sysidle_state, RCU_SYSIDLE_SHORT);
-		break;
-
-	case RCU_SYSIDLE_SHORT:
-
-		/*
-		 * Idle for a bit, time to advance to next state?
-		 * cmpxchg failure means race with non-idle, let them win.
-		 */
-		if (ULONG_CMP_GE(jiffies, j + rcu_sysidle_delay()))
-			(void)cmpxchg(&full_sysidle_state,
-				      RCU_SYSIDLE_SHORT, RCU_SYSIDLE_LONG);
-		break;
-
-	case RCU_SYSIDLE_LONG:
-
-		/*
-		 * Do an additional check pass before advancing to full.
-		 * cmpxchg failure means race with non-idle, let them win.
-		 */
-		if (ULONG_CMP_GE(jiffies, j + rcu_sysidle_delay()))
-			(void)cmpxchg(&full_sysidle_state,
-				      RCU_SYSIDLE_LONG, RCU_SYSIDLE_FULL);
-		break;
-
-	default:
-		break;
-	}
-}
-
-/*
- * Found a non-idle non-timekeeping CPU, so kick the system-idle state
- * back to the beginning.
- */
-static void rcu_sysidle_cancel(void)
-{
-	smp_mb();
-	if (full_sysidle_state > RCU_SYSIDLE_SHORT)
-		WRITE_ONCE(full_sysidle_state, RCU_SYSIDLE_NOT);
-}
-
-/*
- * Update the sysidle state based on the results of a force-quiescent-state
- * scan of the CPUs' dyntick-idle state.
- */
-static void rcu_sysidle_report(struct rcu_state *rsp, int isidle,
-			       unsigned long maxj, bool gpkt)
-{
-	if (rsp != rcu_state_p)
-		return;  /* Wrong flavor, ignore. */
-	if (gpkt && nr_cpu_ids <= CONFIG_NO_HZ_FULL_SYSIDLE_SMALL)
-		return;  /* Running state machine from timekeeping CPU. */
-	if (isidle)
-		rcu_sysidle(maxj);    /* More idle! */
-	else
-		rcu_sysidle_cancel(); /* Idle is over. */
-}
-
-/*
- * Wrapper for rcu_sysidle_report() when called from the grace-period
- * kthread's context.
- */
-static void rcu_sysidle_report_gp(struct rcu_state *rsp, int isidle,
-				  unsigned long maxj)
-{
-	/* If there are no nohz_full= CPUs, no need to track this. */
-	if (!tick_nohz_full_enabled())
-		return;
-
-	rcu_sysidle_report(rsp, isidle, maxj, true);
-}
-
-/* Callback and function for forcing an RCU grace period. */
-struct rcu_sysidle_head {
-	struct rcu_head rh;
-	int inuse;
-};
-
-static void rcu_sysidle_cb(struct rcu_head *rhp)
-{
-	struct rcu_sysidle_head *rshp;
-
-	/*
-	 * The following memory barrier is needed to replace the
-	 * memory barriers that would normally be in the memory
-	 * allocator.
-	 */
-	smp_mb();  /* grace period precedes setting inuse. */
-
-	rshp = container_of(rhp, struct rcu_sysidle_head, rh);
-	WRITE_ONCE(rshp->inuse, 0);
-}
-
-/*
- * Check to see if the system is fully idle, other than the timekeeping CPU.
- * The caller must have disabled interrupts.  This is not intended to be
- * called unless tick_nohz_full_enabled().
- */
-bool rcu_sys_is_idle(void)
-{
-	static struct rcu_sysidle_head rsh;
-	int rss = READ_ONCE(full_sysidle_state);
-
-	RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_sys_is_idle() invoked with irqs enabled!!!");
-
-	if (WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu))
-		return false;
-
-	/* Handle small-system case by doing a full scan of CPUs. */
-	if (nr_cpu_ids <= CONFIG_NO_HZ_FULL_SYSIDLE_SMALL) {
-		int oldrss = rss - 1;
-
-		/*
-		 * One pass to advance to each state up to _FULL.
-		 * Give up if any pass fails to advance the state.
-		 */
-		while (rss < RCU_SYSIDLE_FULL && oldrss < rss) {
-			int cpu;
-			bool isidle = true;
-			unsigned long maxj = jiffies - ULONG_MAX / 4;
-			struct rcu_data *rdp;
-
-			/* Scan all the CPUs looking for nonidle CPUs. */
-			for_each_possible_cpu(cpu) {
-				rdp = per_cpu_ptr(rcu_state_p->rda, cpu);
-				rcu_sysidle_check_cpu(rdp, &isidle, &maxj);
-				if (!isidle)
-					break;
-			}
-			rcu_sysidle_report(rcu_state_p, isidle, maxj, false);
-			oldrss = rss;
-			rss = READ_ONCE(full_sysidle_state);
-		}
-	}
-
-	/* If this is the first observation of an idle period, record it. */
-	if (rss == RCU_SYSIDLE_FULL) {
-		rss = cmpxchg(&full_sysidle_state,
-			      RCU_SYSIDLE_FULL, RCU_SYSIDLE_FULL_NOTED);
-		return rss == RCU_SYSIDLE_FULL;
-	}
-
-	smp_mb(); /* ensure rss load happens before later caller actions. */
-
-	/* If already fully idle, tell the caller (in case of races). */
-	if (rss == RCU_SYSIDLE_FULL_NOTED)
-		return true;
-
-	/*
-	 * If we aren't there yet, and a grace period is not in flight,
-	 * initiate a grace period.  Either way, tell the caller that
-	 * we are not there yet.  We use an xchg() rather than an assignment
-	 * to make up for the memory barriers that would otherwise be
-	 * provided by the memory allocator.
-	 */
-	if (nr_cpu_ids > CONFIG_NO_HZ_FULL_SYSIDLE_SMALL &&
-	    !rcu_gp_in_progress(rcu_state_p) &&
-	    !rsh.inuse && xchg(&rsh.inuse, 1) == 0)
-		call_rcu(&rsh.rh, rcu_sysidle_cb);
-	return false;
-}
-
-/*
- * Initialize dynticks sysidle state for CPUs coming online.
- */
-static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp)
-{
-	rdtp->dynticks_idle_nesting = DYNTICK_TASK_NEST_VALUE;
-}
-
-#else /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
-
-static void rcu_sysidle_enter(int irq)
-{
-}
-
-static void rcu_sysidle_exit(int irq)
-{
-}
-
-static void rcu_sysidle_check_cpu(struct rcu_data *rdp, bool *isidle,
-				  unsigned long *maxj)
-{
-}
-
-static bool is_sysidle_rcu_state(struct rcu_state *rsp)
-{
-	return false;
-}
-
-static void rcu_sysidle_report_gp(struct rcu_state *rsp, int isidle,
-				  unsigned long maxj)
-{
-}
-
-static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp)
-{
-}
-
-#endif /* #else #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
-
 /*
  * Is this CPU a NO_HZ_FULL CPU that should ignore RCU so that the
  * grace-period kthread will do force_quiescent_state() processing?
@@ -3016,13 +2593,7 @@ static void rcu_bind_gp_kthread(void)
 
 	if (!tick_nohz_full_enabled())
 		return;
-#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
-	cpu = tick_do_timer_cpu;
-	if (cpu >= 0 && cpu < nr_cpu_ids)
-		set_cpus_allowed_ptr(current, cpumask_of(cpu));
-#else /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
 	housekeeping_affine(current);
-#endif /* #else #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
 }
 
 /* Record the current task on dyntick-idle entry. */

kernel/time/Kconfig

@@ -126,56 +126,6 @@ config NO_HZ_FULL_ALL
 	 Note the boot CPU will still be kept outside the range to
 	 handle the timekeeping duty.
 
-config NO_HZ_FULL_SYSIDLE
-	bool "Detect full-system idle state for full dynticks system"
-	depends on NO_HZ_FULL
-	default n
-	help
-	 At least one CPU must keep the scheduling-clock tick running for
-	 timekeeping purposes whenever there is a non-idle CPU, where
-	 "non-idle" also includes dynticks CPUs as long as they are
-	 running non-idle tasks.  Because the underlying adaptive-tick
-	 support cannot distinguish between all CPUs being idle and
-	 all CPUs each running a single task in dynticks mode, the
-	 underlying support simply ensures that there is always a CPU
-	 handling the scheduling-clock tick, whether or not all CPUs
-	 are idle.  This Kconfig option enables scalable detection of
-	 the all-CPUs-idle state, thus allowing the scheduling-clock
-	 tick to be disabled when all CPUs are idle.  Note that scalable
-	 detection of the all-CPUs-idle state means that larger systems
-	 will be slower to declare the all-CPUs-idle state.
-
-	 Say Y if you would like to help debug all-CPUs-idle detection.
-
-	 Say N if you are unsure.
-
-config NO_HZ_FULL_SYSIDLE_SMALL
-	int "Number of CPUs above which large-system approach is used"
-	depends on NO_HZ_FULL_SYSIDLE
-	range 1 NR_CPUS
-	default 8
-	help
-	 The full-system idle detection mechanism takes a lazy approach
-	 on large systems, as is required to attain decent scalability.
-	 However, on smaller systems, scalability is not anywhere near as
-	 large a concern as is energy efficiency.  The sysidle subsystem
-	 therefore uses a fast but non-scalable algorithm for small
-	 systems and a lazier but scalable algorithm for large systems.
-	 This Kconfig parameter defines the number of CPUs in the largest
-	 system that will be considered to be "small".
-
-	 The default value will be fine in most cases.	Battery-powered
-	 systems that (1) enable NO_HZ_FULL_SYSIDLE, (2) have larger
-	 numbers of CPUs, and (3) are suffering from battery-lifetime
-	 problems due to long sysidle latencies might wish to experiment
-	 with larger values for this Kconfig parameter.  On the other
-	 hand, they might be even better served by disabling NO_HZ_FULL
-	 entirely, given that NO_HZ_FULL is intended for HPC and
-	 real-time workloads that at present do not tend to be run on
-	 battery-powered systems.
-
-	 Take the default if you are unsure.
-
 config NO_HZ
 	bool "Old Idle dynticks config"
 	depends on !ARCH_USES_GETTIMEOFFSET && GENERIC_CLOCKEVENTS

tools/testing/selftests/rcutorture/configs/rcu/TREE07

@@ -8,7 +8,6 @@ CONFIG_HZ_PERIODIC=n
 CONFIG_NO_HZ_IDLE=n
 CONFIG_NO_HZ_FULL=y
 CONFIG_NO_HZ_FULL_ALL=n
-CONFIG_NO_HZ_FULL_SYSIDLE=y
 CONFIG_RCU_FAST_NO_HZ=n
 CONFIG_RCU_TRACE=y
 CONFIG_HOTPLUG_CPU=y

tools/testing/selftests/rcutorture/doc/TINY_RCU.txt

@@ -18,7 +18,6 @@ CONFIG_PROVE_RCU
 
 	In common code tested by TREE_RCU test cases.
 
-CONFIG_NO_HZ_FULL_SYSIDLE
 CONFIG_RCU_NOCB_CPU
 
 	Meaningless for TINY_RCU.

tools/testing/selftests/rcutorture/doc/TREE_RCU-kconfig.txt

@@ -9,8 +9,7 @@ CONFIG_DEBUG_OBJECTS_RCU_HEAD -- Do one.
 CONFIG_HOTPLUG_CPU -- Do half.  (Every second.)
 CONFIG_HZ_PERIODIC -- Do one.
 CONFIG_NO_HZ_IDLE -- Do those not otherwise specified. (Groups of two.)
-CONFIG_NO_HZ_FULL -- Do two, one with CONFIG_NO_HZ_FULL_SYSIDLE.
-CONFIG_NO_HZ_FULL_SYSIDLE -- Do one.
+CONFIG_NO_HZ_FULL -- Do two, one with partial CPU enablement.
 CONFIG_PREEMPT -- Do half.  (First three and #8.)
 CONFIG_PROVE_LOCKING -- Do several, covering CONFIG_DEBUG_LOCK_ALLOC=y and not.
 CONFIG_PROVE_RCU -- Hardwired to CONFIG_PROVE_LOCKING.
@@ -48,10 +47,6 @@ CONFIG_64BIT
 
 	Used only to check CONFIG_RCU_FANOUT value, inspection suffices.
 
-CONFIG_NO_HZ_FULL_SYSIDLE_SMALL
-
-	Defer until Frederic uses this.
-
 CONFIG_PREEMPT_COUNT
 CONFIG_PREEMPT_RCU