Re: [PATCH tip/core/rcu] Reduce overhead of cond_resched() checks for RCU

[Josh Triplett]

On Fri, Jun 20, 2014 at 07:59:58PM -0700, Paul E. McKenney wrote:
> Commit ac1bea85781e (Make cond_resched() report RCU quiescent states)
> fixed a problem where a CPU looping in the kernel with but one runnable
> task would give RCU CPU stall warnings, even if the in-kernel loop
> contained cond_resched() calls.  Unfortunately, in so doing, it introduced
> performance regressions in Anton Blanchard's will-it-scale "open1" test.
> The problem appears to be not so much the increased cond_resched() path
> length as an increase in the rate at which grace periods complete, which
> increased per-update grace-period overhead.
> 
> This commit takes a different approach to fixing this bug, mainly by
> moving the RCU-visible quiescent state from cond_resched() to
> rcu_note_context_switch(), and by further reducing the check to a
> simple non-zero test of a single per-CPU variable.  However, this
> approach requires that the force-quiescent-state processing send
> resched IPIs to the offending CPUs.  These will be sent only once
> the grace period has reached an age specified by the boot/sysfs
> parameter rcutree.jiffies_till_sched_qs, or once the grace period
> reaches an age halfway to the point at which RCU CPU stall warnings
> will be emitted, whichever comes first.
> 
> Reported-by: Dave Hansen <dave.hansen@xxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxxx>
> Cc: Mike Galbraith <umgwanakikbuti@xxxxxxxxx>
> Cc: Eric Dumazet <eric.dumazet@xxxxxxxxx>

I like this approach *far* better.  This is the kind of thing I had in
mind when I suggested using the fqs machinery: remove the poll entirely
and just thwack a CPU if it takes too long without a quiescent state.
Reviewed-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>

> ---
> 
>  b/Documentation/kernel-parameters.txt |    6 +
>  b/include/linux/rcupdate.h            |   36 --------
>  b/kernel/rcu/tree.c                   |  140 +++++++++++++++++++++++++++-------
>  b/kernel/rcu/tree.h                   |    6 +
>  b/kernel/rcu/tree_plugin.h            |    2 
>  b/kernel/rcu/update.c                 |   18 ----
>  b/kernel/sched/core.c                 |    7 -
>  7 files changed, 125 insertions(+), 90 deletions(-)
> 
> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> index 6eaa9cdb7094..910c3829f81d 100644
> --- a/Documentation/kernel-parameters.txt
> +++ b/Documentation/kernel-parameters.txt
> @@ -2785,6 +2785,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>  			leaf rcu_node structure.  Useful for very large
>  			systems.
>  
> +	rcutree.jiffies_till_sched_qs= [KNL]
> +			Set required age in jiffies for a
> +			given grace period before RCU starts
> +			soliciting quiescent-state help from
> +			rcu_note_context_switch().
> +
>  	rcutree.jiffies_till_first_fqs= [KNL]
>  			Set delay from grace-period initialization to
>  			first attempt to force quiescent states.
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 5a75d19aa661..243aa4656cb7 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -44,7 +44,6 @@
>  #include <linux/debugobjects.h>
>  #include <linux/bug.h>
>  #include <linux/compiler.h>
> -#include <linux/percpu.h>
>  #include <asm/barrier.h>
>  
>  extern int rcu_expedited; /* for sysctl */
> @@ -300,41 +299,6 @@ bool __rcu_is_watching(void);
>  #endif /* #if defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_RCU_TRACE) || defined(CONFIG_SMP) */
>  
>  /*
> - * Hooks for cond_resched() and friends to avoid RCU CPU stall warnings.
> - */
> -
> -#define RCU_COND_RESCHED_LIM 256	/* ms vs. 100s of ms. */
> -DECLARE_PER_CPU(int, rcu_cond_resched_count);
> -void rcu_resched(void);
> -
> -/*
> - * Is it time to report RCU quiescent states?
> - *
> - * Note unsynchronized access to rcu_cond_resched_count.  Yes, we might
> - * increment some random CPU's count, and possibly also load the result from
> - * yet another CPU's count.  We might even clobber some other CPU's attempt
> - * to zero its counter.  This is all OK because the goal is not precision,
> - * but rather reasonable amortization of rcu_note_context_switch() overhead
> - * and extremely high probability of avoiding RCU CPU stall warnings.
> - * Note that this function has to be preempted in just the wrong place,
> - * many thousands of times in a row, for anything bad to happen.
> - */
> -static inline bool rcu_should_resched(void)
> -{
> -	return raw_cpu_inc_return(rcu_cond_resched_count) >=
> -	       RCU_COND_RESCHED_LIM;
> -}
> -
> -/*
> - * Report quiscent states to RCU if it is time to do so.
> - */
> -static inline void rcu_cond_resched(void)
> -{
> -	if (unlikely(rcu_should_resched()))
> -		rcu_resched();
> -}
> -
> -/*
>   * Infrastructure to implement the synchronize_() primitives in
>   * TREE_RCU and rcu_barrier_() primitives in TINY_RCU.
>   */
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index f1ba77363fbb..7d711f9a2e86 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -206,6 +206,70 @@ void rcu_bh_qs(int cpu)
>  	rdp->passed_quiesce = 1;
>  }
>  
> +static DEFINE_PER_CPU(int, rcu_sched_qs_mask);
> +
> +static DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
> +	.dynticks_nesting = DYNTICK_TASK_EXIT_IDLE,
> +	.dynticks = ATOMIC_INIT(1),
> +#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 */
> +};
> +
> +/*
> + * Let the RCU core know that this CPU has gone through the scheduler,
> + * which is a quiescent state.  This is called when the need for a
> + * quiescent state is urgent, so we burn an atomic operation and full
> + * memory barriers to let the RCU core know about it, regardless of what
> + * this CPU might (or might not) do in the near future.
> + *
> + * We inform the RCU core by emulating a zero-duration dyntick-idle
> + * period, which we in turn do by incrementing the ->dynticks counter
> + * by two.
> + */
> +void rcu_momentary_dyntick_idle(void)
> +{
> +	unsigned long flags;
> +	struct rcu_data *rdp;
> +	struct rcu_dynticks *rdtp;
> +	int resched_mask;
> +	struct rcu_state *rsp;
> +
> +	local_irq_save(flags);
> +
> +	/*
> +	 * Yes, we can lose flag-setting operations.  This is OK, because
> +	 * the flag will be set again after some delay.
> +	 */
> +	resched_mask = raw_cpu_read(rcu_sched_qs_mask);
> +	raw_cpu_write(rcu_sched_qs_mask, 0);
> +
> +	/* Find the flavor that needs a quiescent state. */
> +	for_each_rcu_flavor(rsp) {
> +		rdp = raw_cpu_ptr(rsp->rda);
> +		if (!(resched_mask & rsp->flavor_mask))
> +			continue;
> +		smp_mb(); /* ->flavor_mask before ->cond_resched_completed. */
> +		if (ACCESS_ONCE(rdp->mynode->completed) !=
> +		    ACCESS_ONCE(rdp->cond_resched_completed))
> +			continue;
> +
> +		/*
> +		 * Pretend to be momentarily idle for the quiescent state.
> +		 * This allows the grace-period kthread to record the
> +		 * quiescent state, with no need for this CPU to do anything
> +		 * further.
> +		 */
> +		rdtp = this_cpu_ptr(&rcu_dynticks);
> +		smp_mb__before_atomic(); /* Earlier stuff before QS. */
> +		atomic_add(2, &rdtp->dynticks);  /* QS. */
> +		smp_mb__after_atomic(); /* Later stuff after QS. */
> +		break;
> +	}
> +	local_irq_restore(flags);
> +}
> +
>  /*
>   * Note a context switch.  This is a quiescent state for RCU-sched,
>   * and requires special handling for preemptible RCU.
> @@ -216,19 +280,12 @@ void rcu_note_context_switch(int cpu)
>  	trace_rcu_utilization(TPS("Start context switch"));
>  	rcu_sched_qs(cpu);
>  	rcu_preempt_note_context_switch(cpu);
> +	if (unlikely(raw_cpu_read(rcu_sched_qs_mask)))
> +		rcu_momentary_dyntick_idle();
>  	trace_rcu_utilization(TPS("End context switch"));
>  }
>  EXPORT_SYMBOL_GPL(rcu_note_context_switch);
>  
> -static DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
> -	.dynticks_nesting = DYNTICK_TASK_EXIT_IDLE,
> -	.dynticks = ATOMIC_INIT(1),
> -#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 */
> -};
> -
>  static long blimit = 10;	/* Maximum callbacks per rcu_do_batch. */
>  static long qhimark = 10000;	/* If this many pending, ignore blimit. */
>  static long qlowmark = 100;	/* Once only this many pending, use blimit. */
> @@ -243,6 +300,13 @@ static ulong jiffies_till_next_fqs = ULONG_MAX;
>  module_param(jiffies_till_first_fqs, ulong, 0644);
>  module_param(jiffies_till_next_fqs, ulong, 0644);
>  
> +/*
> + * How long the grace period must be before we start recruiting
> + * quiescent-state help from rcu_note_context_switch().
> + */
> +static ulong jiffies_till_sched_qs = HZ / 20;
> +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,
> @@ -853,6 +917,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
>  				    bool *isidle, unsigned long *maxj)
>  {
>  	unsigned int curr;
> +	int *rcrmp;
>  	unsigned int snap;
>  
>  	curr = (unsigned int)atomic_add_return(0, &rdp->dynticks->dynticks);
> @@ -893,27 +958,43 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
>  	}
>  
>  	/*
> -	 * There is a possibility that a CPU in adaptive-ticks state
> -	 * might run in the kernel with the scheduling-clock tick disabled
> -	 * for an extended time period.  Invoke rcu_kick_nohz_cpu() to
> -	 * force the CPU to restart the scheduling-clock tick in this
> -	 * CPU is in this state.
> -	 */
> -	rcu_kick_nohz_cpu(rdp->cpu);
> -
> -	/*
> -	 * Alternatively, the CPU might be running in the kernel
> -	 * for an extended period of time without a quiescent state.
> -	 * Attempt to force the CPU through the scheduler to gain the
> -	 * needed quiescent state, but only if the grace period has gone
> -	 * on for an uncommonly long time.  If there are many stuck CPUs,
> -	 * we will beat on the first one until it gets unstuck, then move
> -	 * to the next.  Only do this for the primary flavor of RCU.
> +	 * A CPU running for an extended time within the kernel can
> +	 * delay RCU grace periods.  When the CPU is in NO_HZ_FULL mode,
> +	 * even context-switching back and forth between a pair of
> +	 * in-kernel CPU-bound tasks cannot advance grace periods.
> +	 * So if the grace period is old enough, make the CPU pay attention.
> +	 * Note that the unsynchronized assignments to the per-CPU
> +	 * rcu_sched_qs_mask variable are safe.  Yes, setting of
> +	 * bits can be lost, but they will be set again on the next
> +	 * force-quiescent-state pass.  So lost bit sets do not result
> +	 * in incorrect behavior, merely in a grace period lasting
> +	 * a few jiffies longer than it might otherwise.  Because
> +	 * there are at most four threads involved, and because the
> +	 * updates are only once every few jiffies, the probability of
> +	 * lossage (and thus of slight grace-period extension) is
> +	 * quite low.
> +	 *
> +	 * Note that if the jiffies_till_sched_qs boot/sysfs parameter
> +	 * is set too high, we override with half of the RCU CPU stall
> +	 * warning delay.
>  	 */
> -	if (rdp->rsp == rcu_state_p &&
> +	rcrmp = &per_cpu(rcu_sched_qs_mask, rdp->cpu);
> +	if (ULONG_CMP_GE(jiffies,
> +			 rdp->rsp->gp_start + jiffies_till_sched_qs) ||
>  	    ULONG_CMP_GE(jiffies, rdp->rsp->jiffies_resched)) {
> -		rdp->rsp->jiffies_resched += 5;
> -		resched_cpu(rdp->cpu);
> +		if (!(ACCESS_ONCE(*rcrmp) & rdp->rsp->flavor_mask)) {
> +			ACCESS_ONCE(rdp->cond_resched_completed) =
> +				ACCESS_ONCE(rdp->mynode->completed);
> +			smp_mb(); /* ->cond_resched_completed before *rcrmp. */
> +			ACCESS_ONCE(*rcrmp) =
> +				ACCESS_ONCE(*rcrmp) + rdp->rsp->flavor_mask;
> +			resched_cpu(rdp->cpu);  /* Force CPU into scheduler. */
> +			rdp->rsp->jiffies_resched += 5; /* Enable beating. */
> +		} else if (ULONG_CMP_GE(jiffies, rdp->rsp->jiffies_resched)) {
> +			/* Time to beat on that CPU again! */
> +			resched_cpu(rdp->cpu);  /* Force CPU into scheduler. */
> +			rdp->rsp->jiffies_resched += 5; /* Re-enable beating. */
> +		}
>  	}
>  
>  	return 0;
> @@ -3491,6 +3572,7 @@ static void __init rcu_init_one(struct rcu_state *rsp,
>  			       "rcu_node_fqs_1",
>  			       "rcu_node_fqs_2",
>  			       "rcu_node_fqs_3" };  /* Match MAX_RCU_LVLS */
> +	static u8 fl_mask = 0x1;
>  	int cpustride = 1;
>  	int i;
>  	int j;
> @@ -3509,6 +3591,8 @@ static void __init rcu_init_one(struct rcu_state *rsp,
>  	for (i = 1; i < rcu_num_lvls; i++)
>  		rsp->level[i] = rsp->level[i - 1] + rsp->levelcnt[i - 1];
>  	rcu_init_levelspread(rsp);
> +	rsp->flavor_mask = fl_mask;
> +	fl_mask <<= 1;
>  
>  	/* Initialize the elements themselves, starting from the leaves. */
>  
> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> index bf2c1e669691..0f69a79c5b7d 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -307,6 +307,9 @@ struct rcu_data {
>  	/* 4) reasons this CPU needed to be kicked by force_quiescent_state */
>  	unsigned long dynticks_fqs;	/* Kicked due to dynticks idle. */
>  	unsigned long offline_fqs;	/* Kicked due to being offline. */
> +	unsigned long cond_resched_completed;
> +					/* Grace period that needs help */
> +					/*  from cond_resched(). */
>  
>  	/* 5) __rcu_pending() statistics. */
>  	unsigned long n_rcu_pending;	/* rcu_pending() calls since boot. */
> @@ -392,6 +395,7 @@ struct rcu_state {
>  	struct rcu_node *level[RCU_NUM_LVLS];	/* Hierarchy levels. */
>  	u32 levelcnt[MAX_RCU_LVLS + 1];		/* # nodes in each level. */
>  	u8 levelspread[RCU_NUM_LVLS];		/* kids/node in each level. */
> +	u8 flavor_mask;				/* bit in flavor mask. */
>  	struct rcu_data __percpu *rda;		/* pointer of percu rcu_data. */
>  	void (*call)(struct rcu_head *head,	/* call_rcu() flavor. */
>  		     void (*func)(struct rcu_head *head));
> @@ -563,7 +567,7 @@ static bool rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp);
>  static void do_nocb_deferred_wakeup(struct rcu_data *rdp);
>  static void rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp);
>  static void rcu_spawn_nocb_kthreads(struct rcu_state *rsp);
> -static void rcu_kick_nohz_cpu(int 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(struct rcu_dynticks *rdtp, int irq);
>  static void rcu_sysidle_exit(struct rcu_dynticks *rdtp, int irq);
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index cbc2c45265e2..02ac0fb186b8 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -2404,7 +2404,7 @@ static bool init_nocb_callback_list(struct rcu_data *rdp)
>   * if an adaptive-ticks CPU is failing to respond to the current grace
>   * period and has not be idle from an RCU perspective, kick it.
>   */
> -static void rcu_kick_nohz_cpu(int cpu)
> +static void __maybe_unused rcu_kick_nohz_cpu(int cpu)
>  {
>  #ifdef CONFIG_NO_HZ_FULL
>  	if (tick_nohz_full_cpu(cpu))
> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> index a2aeb4df0f60..d22309cae9f5 100644
> --- a/kernel/rcu/update.c
> +++ b/kernel/rcu/update.c
> @@ -350,21 +350,3 @@ static int __init check_cpu_stall_init(void)
>  early_initcall(check_cpu_stall_init);
>  
>  #endif /* #ifdef CONFIG_RCU_STALL_COMMON */
> -
> -/*
> - * Hooks for cond_resched() and friends to avoid RCU CPU stall warnings.
> - */
> -
> -DEFINE_PER_CPU(int, rcu_cond_resched_count);
> -
> -/*
> - * Report a set of RCU quiescent states, for use by cond_resched()
> - * and friends.  Out of line due to being called infrequently.
> - */
> -void rcu_resched(void)
> -{
> -	preempt_disable();
> -	__this_cpu_write(rcu_cond_resched_count, 0);
> -	rcu_note_context_switch(smp_processor_id());
> -	preempt_enable();
> -}
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 3bdf01b494fe..bc1638b33449 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4147,7 +4147,6 @@ static void __cond_resched(void)
>  
>  int __sched _cond_resched(void)
>  {
> -	rcu_cond_resched();
>  	if (should_resched()) {
>  		__cond_resched();
>  		return 1;
> @@ -4166,18 +4165,15 @@ EXPORT_SYMBOL(_cond_resched);
>   */
>  int __cond_resched_lock(spinlock_t *lock)
>  {
> -	bool need_rcu_resched = rcu_should_resched();
>  	int resched = should_resched();
>  	int ret = 0;
>  
>  	lockdep_assert_held(lock);
>  
> -	if (spin_needbreak(lock) || resched || need_rcu_resched) {
> +	if (spin_needbreak(lock) || resched) {
>  		spin_unlock(lock);
>  		if (resched)
>  			__cond_resched();
> -		else if (unlikely(need_rcu_resched))
> -			rcu_resched();
>  		else
>  			cpu_relax();
>  		ret = 1;
> @@ -4191,7 +4187,6 @@ int __sched __cond_resched_softirq(void)
>  {
>  	BUG_ON(!in_softirq());
>  
> -	rcu_cond_resched();  /* BH disabled OK, just recording QSes. */
>  	if (should_resched()) {
>  		local_bh_enable();
>  		__cond_resched();
> 
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