Donate to e Foundation | Murena handsets with /e/OS | Own a part of Murena! Learn more

Commit 80d02085 authored by Paul E. McKenney's avatar Paul E. McKenney Committed by Ingo Molnar
Browse files

Revert "rcu: Decrease memory-barrier usage based on semi-formal proof" 



This reverts commit e59fb312.

This reversion was due to (extreme) boot-time slowdowns on SPARC seen by
Yinghai Lu and on x86 by Ingo
.
This is a non-trivial reversion due to intervening commits.

Conflicts:

	Documentation/RCU/trace.txt
	kernel/rcutree.c

Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 11c476f3

Documentation/RCU/trace.txt

+12 −5
Original line number Diff line number Diff line
@@ -99,11 +99,18 @@ o "qp" indicates that RCU still expects a quiescent state from 


o	"dt" is the current value of the dyntick counter that is incremented

	when entering or leaving dynticks idle state, either by the

	scheduler or by irq.  This number is even if the CPU is in

	dyntick idle mode and odd otherwise.  The number after the first

	"/" is the interrupt nesting depth when in dyntick-idle state,

	or one greater than the interrupt-nesting depth otherwise.

	The number after the second "/" is the NMI nesting depth.

	scheduler or by irq.  The number after the "/" is the interrupt

	nesting depth when in dyntick-idle state, or one greater than

	the interrupt-nesting depth otherwise.



	This field is displayed only for CONFIG_NO_HZ kernels.



o	"dn" is the current value of the dyntick counter that is incremented

	when entering or leaving dynticks idle state via NMI.  If both

	the "dt" and "dn" values are even, then this CPU is in dynticks

	idle mode and may be ignored by RCU.  If either of these two

	counters is odd, then RCU must be alert to the possibility of

	an RCU read-side critical section running on this CPU.



	This field is displayed only for CONFIG_NO_HZ kernels.

kernel/rcutree.c

+74 −56
Original line number Diff line number Diff line
@@ -162,7 +162,7 @@ EXPORT_SYMBOL_GPL(rcu_note_context_switch); 
#ifdef CONFIG_NO_HZ

DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {

	.dynticks_nesting = 1,

	.dynticks = ATOMIC_INIT(1),

	.dynticks = 1,

};

#endif /* #ifdef CONFIG_NO_HZ */
@@ -321,25 +321,13 @@ void rcu_enter_nohz(void) 
	unsigned long flags;

	struct rcu_dynticks *rdtp;



	smp_mb(); /* CPUs seeing ++ must see prior RCU read-side crit sects */

	local_irq_save(flags);

	rdtp = &__get_cpu_var(rcu_dynticks);

	if (--rdtp->dynticks_nesting) {

	rdtp->dynticks++;

	rdtp->dynticks_nesting--;

	WARN_ON_ONCE(rdtp->dynticks & 0x1);

	local_irq_restore(flags);

		return;

	}

	/* CPUs seeing atomic_inc() must see prior RCU read-side crit sects */

	smp_mb__before_atomic_inc();  /* See above. */

	atomic_inc(&rdtp->dynticks);

	smp_mb__after_atomic_inc();  /* Force ordering with next sojourn. */

	WARN_ON_ONCE(atomic_read(&rdtp->dynticks) & 0x1);

	local_irq_restore(flags);



	/* If the interrupt queued a callback, get out of dyntick mode. */

	if (in_irq() &&

	    (__get_cpu_var(rcu_sched_data).nxtlist ||

	     __get_cpu_var(rcu_bh_data).nxtlist ||

	     rcu_preempt_needs_cpu(smp_processor_id())))

		set_need_resched();

}



/*
@@ -355,16 +343,11 @@ void rcu_exit_nohz(void) 


	local_irq_save(flags);

	rdtp = &__get_cpu_var(rcu_dynticks);

	if (rdtp->dynticks_nesting++) {

		local_irq_restore(flags);

		return;

	}

	smp_mb__before_atomic_inc();  /* Force ordering w/previous sojourn. */

	atomic_inc(&rdtp->dynticks);

	/* CPUs seeing atomic_inc() must see later RCU read-side crit sects */

	smp_mb__after_atomic_inc();  /* See above. */

	WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1));

	rdtp->dynticks++;

	rdtp->dynticks_nesting++;

	WARN_ON_ONCE(!(rdtp->dynticks & 0x1));

	local_irq_restore(flags);

	smp_mb(); /* CPUs seeing ++ must see later RCU read-side crit sects */

}



/**
@@ -378,15 +361,11 @@ void rcu_nmi_enter(void) 
{

	struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);



	if (rdtp->dynticks_nmi_nesting == 0 &&

	    (atomic_read(&rdtp->dynticks) & 0x1))

	if (rdtp->dynticks & 0x1)

		return;

	rdtp->dynticks_nmi_nesting++;

	smp_mb__before_atomic_inc();  /* Force delay from prior write. */

	atomic_inc(&rdtp->dynticks);

	/* CPUs seeing atomic_inc() must see later RCU read-side crit sects */

	smp_mb__after_atomic_inc();  /* See above. */

	WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1));

	rdtp->dynticks_nmi++;

	WARN_ON_ONCE(!(rdtp->dynticks_nmi & 0x1));

	smp_mb(); /* CPUs seeing ++ must see later RCU read-side crit sects */

}



/**
@@ -400,14 +379,11 @@ void rcu_nmi_exit(void) 
{

	struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);



	if (rdtp->dynticks_nmi_nesting == 0 ||

	    --rdtp->dynticks_nmi_nesting != 0)

	if (rdtp->dynticks & 0x1)

		return;

	/* CPUs seeing atomic_inc() must see prior RCU read-side crit sects */

	smp_mb__before_atomic_inc();  /* See above. */

	atomic_inc(&rdtp->dynticks);

	smp_mb__after_atomic_inc();  /* Force delay to next write. */

	WARN_ON_ONCE(atomic_read(&rdtp->dynticks) & 0x1);

	smp_mb(); /* CPUs seeing ++ must see prior RCU read-side crit sects */

	rdtp->dynticks_nmi++;

	WARN_ON_ONCE(rdtp->dynticks_nmi & 0x1);

}



/**
@@ -418,7 +394,13 @@ void rcu_nmi_exit(void) 
 */

void rcu_irq_enter(void)

{

	rcu_exit_nohz();

	struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);



	if (rdtp->dynticks_nesting++)

		return;

	rdtp->dynticks++;

	WARN_ON_ONCE(!(rdtp->dynticks & 0x1));

	smp_mb(); /* CPUs seeing ++ must see later RCU read-side crit sects */

}



/**
@@ -430,7 +412,18 @@ void rcu_irq_enter(void) 
 */

void rcu_irq_exit(void)

{

	rcu_enter_nohz();

	struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);



	if (--rdtp->dynticks_nesting)

		return;

	smp_mb(); /* CPUs seeing ++ must see prior RCU read-side crit sects */

	rdtp->dynticks++;

	WARN_ON_ONCE(rdtp->dynticks & 0x1);



	/* If the interrupt queued a callback, get out of dyntick mode. */

	if (__this_cpu_read(rcu_sched_data.nxtlist) ||

	    __this_cpu_read(rcu_bh_data.nxtlist))

		set_need_resched();

}



#ifdef CONFIG_SMP
@@ -442,8 +435,19 @@ void rcu_irq_exit(void) 
 */

static int dyntick_save_progress_counter(struct rcu_data *rdp)

{

	rdp->dynticks_snap = atomic_add_return(0, &rdp->dynticks->dynticks);

	return 0;

	int ret;

	int snap;

	int snap_nmi;



	snap = rdp->dynticks->dynticks;

	snap_nmi = rdp->dynticks->dynticks_nmi;

	smp_mb();	/* Order sampling of snap with end of grace period. */

	rdp->dynticks_snap = snap;

	rdp->dynticks_nmi_snap = snap_nmi;

	ret = ((snap & 0x1) == 0) && ((snap_nmi & 0x1) == 0);

	if (ret)

		rdp->dynticks_fqs++;

	return ret;

}



/*
@@ -454,11 +458,16 @@ static int dyntick_save_progress_counter(struct rcu_data *rdp) 
 */

static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)

{

	unsigned long curr;

	unsigned long snap;

	long curr;

	long curr_nmi;

	long snap;

	long snap_nmi;



	curr = (unsigned long)atomic_add_return(0, &rdp->dynticks->dynticks);

	snap = (unsigned long)rdp->dynticks_snap;

	curr = rdp->dynticks->dynticks;

	snap = rdp->dynticks_snap;

	curr_nmi = rdp->dynticks->dynticks_nmi;

	snap_nmi = rdp->dynticks_nmi_snap;

	smp_mb(); /* force ordering with cpu entering/leaving dynticks. */



	/*

	 * If the CPU passed through or entered a dynticks idle phase with
@@ -468,7 +477,8 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp) 
	 * read-side critical section that started before the beginning

	 * of the current RCU grace period.

	 */

	if ((curr & 0x1) == 0 || ULONG_CMP_GE(curr, snap + 2)) {

	if ((curr != snap || (curr & 0x1) == 0) &&

	    (curr_nmi != snap_nmi || (curr_nmi & 0x1) == 0)) {

		rdp->dynticks_fqs++;

		return 1;

	}
@@ -897,12 +907,6 @@ static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags) 
	unsigned long gp_duration;



	WARN_ON_ONCE(!rcu_gp_in_progress(rsp));



	/*

	 * Ensure that all grace-period and pre-grace-period activity

	 * is seen before the assignment to rsp->completed.

	 */

	smp_mb(); /* See above block comment. */

	gp_duration = jiffies - rsp->gp_start;

	if (gp_duration > rsp->gp_max)

		rsp->gp_max = gp_duration;
@@ -1450,11 +1454,25 @@ __rcu_process_callbacks(struct rcu_state *rsp, struct rcu_data *rdp) 
 */

static void rcu_process_callbacks(void)

{

	/*

	 * Memory references from any prior RCU read-side critical sections

	 * executed by the interrupted code must be seen before any RCU

	 * grace-period manipulations below.

	 */

	smp_mb(); /* See above block comment. */



	__rcu_process_callbacks(&rcu_sched_state,

				&__get_cpu_var(rcu_sched_data));

	__rcu_process_callbacks(&rcu_bh_state, &__get_cpu_var(rcu_bh_data));

	rcu_preempt_process_callbacks();



	/*

	 * Memory references from any later RCU read-side critical sections

	 * executed by the interrupted code must be seen after any RCU

	 * grace-period manipulations above.

	 */

	smp_mb(); /* See above block comment. */



	/* If we are last CPU on way to dyntick-idle mode, accelerate it. */

	rcu_needs_cpu_flush();

}

kernel/rcutree.h

+6 −3
Original line number Diff line number Diff line
@@ -84,9 +84,11 @@ 
 * Dynticks per-CPU state.

 */

struct rcu_dynticks {

	int dynticks_nesting;	/* Track irq/process nesting level. */

	int dynticks_nmi_nesting; /* Track NMI nesting level. */

	atomic_t dynticks;	/* Even value for dynticks-idle, else odd. */

	int dynticks_nesting;	/* Track nesting level, sort of. */

	int dynticks;		/* Even value for dynticks-idle, else odd. */

	int dynticks_nmi;	/* Even value for either dynticks-idle or */

				/*  not in nmi handler, else odd.  So this */

				/*  remains even for nmi from irq handler. */

};



/* RCU's kthread states for tracing. */
@@ -282,6 +284,7 @@ struct rcu_data { 
	/* 3) dynticks interface. */

	struct rcu_dynticks *dynticks;	/* Shared per-CPU dynticks state. */

	int dynticks_snap;		/* Per-GP tracking for dynticks. */

	int dynticks_nmi_snap;		/* Per-GP tracking for dynticks_nmi. */

#endif /* #ifdef CONFIG_NO_HZ */



	/* 4) reasons this CPU needed to be kicked by force_quiescent_state */

kernel/rcutree_plugin.h

+4 −3
Original line number Diff line number Diff line
@@ -1520,6 +1520,7 @@ int rcu_needs_cpu(int cpu) 
{

	int c = 0;

	int snap;

	int snap_nmi;

	int thatcpu;



	/* Check for being in the holdoff period. */
@@ -1530,10 +1531,10 @@ int rcu_needs_cpu(int cpu) 
	for_each_online_cpu(thatcpu) {

		if (thatcpu == cpu)

			continue;

		snap = atomic_add_return(0, &per_cpu(rcu_dynticks,

						     thatcpu).dynticks);

		snap = per_cpu(rcu_dynticks, thatcpu).dynticks;

		snap_nmi = per_cpu(rcu_dynticks, thatcpu).dynticks_nmi;

		smp_mb(); /* Order sampling of snap with end of grace period. */

		if ((snap & 0x1) != 0) {

		if (((snap & 0x1) != 0) || ((snap_nmi & 0x1) != 0)) {

			per_cpu(rcu_dyntick_drain, cpu) = 0;

			per_cpu(rcu_dyntick_holdoff, cpu) = jiffies - 1;

			return rcu_needs_cpu_quick_check(cpu);

kernel/rcutree_trace.c

+6 −6
Original line number Diff line number Diff line
@@ -69,10 +69,10 @@ static void print_one_rcu_data(struct seq_file *m, struct rcu_data *rdp) 
		   rdp->passed_quiesc, rdp->passed_quiesc_completed,

		   rdp->qs_pending);

#ifdef CONFIG_NO_HZ

	seq_printf(m, " dt=%d/%d/%d df=%lu",

		   atomic_read(&rdp->dynticks->dynticks),

	seq_printf(m, " dt=%d/%d dn=%d df=%lu",

		   rdp->dynticks->dynticks,

		   rdp->dynticks->dynticks_nesting,

		   rdp->dynticks->dynticks_nmi_nesting,

		   rdp->dynticks->dynticks_nmi,

		   rdp->dynticks_fqs);

#endif /* #ifdef CONFIG_NO_HZ */

	seq_printf(m, " of=%lu ri=%lu", rdp->offline_fqs, rdp->resched_ipi);
@@ -141,9 +141,9 @@ static void print_one_rcu_data_csv(struct seq_file *m, struct rcu_data *rdp) 
		   rdp->qs_pending);

#ifdef CONFIG_NO_HZ

	seq_printf(m, ",%d,%d,%d,%lu",

		   atomic_read(&rdp->dynticks->dynticks),

		   rdp->dynticks->dynticks,

		   rdp->dynticks->dynticks_nesting,

		   rdp->dynticks->dynticks_nmi_nesting,

		   rdp->dynticks->dynticks_nmi,

		   rdp->dynticks_fqs);

#endif /* #ifdef CONFIG_NO_HZ */

	seq_printf(m, ",%lu,%lu", rdp->offline_fqs, rdp->resched_ipi);
@@ -167,7 +167,7 @@ static int show_rcudata_csv(struct seq_file *m, void *unused) 
{

	seq_puts(m, "\"CPU\",\"Online?\",\"c\",\"g\",\"pq\",\"pqc\",\"pq\",");

#ifdef CONFIG_NO_HZ

	seq_puts(m, "\"dt\",\"dt nesting\",\"dt NMI nesting\",\"df\",");

	seq_puts(m, "\"dt\",\"dt nesting\",\"dn\",\"df\",");

#endif /* #ifdef CONFIG_NO_HZ */

	seq_puts(m, "\"of\",\"ri\",\"ql\",\"b\",\"ci\",\"co\",\"ca\"\n");

#ifdef CONFIG_TREE_PREEMPT_RCU