Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753470AbaFUEaR (ORCPT ); Sat, 21 Jun 2014 00:30:17 -0400 Received: from relay3-d.mail.gandi.net ([217.70.183.195]:39192 "EHLO relay3-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751294AbaFUEaP (ORCPT ); Sat, 21 Jun 2014 00:30:15 -0400 X-Originating-IP: 50.43.32.211 Date: Fri, 20 Jun 2014 21:29:58 -0700 From: Josh Triplett To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, dvhart@linux.intel.com, fweisbec@gmail.com, oleg@redhat.com, dave.hansen@intel.com, ak@linux.intel.com, cl@gentwo.org, umgwanakikbuti@gmail.com Subject: Re: [PATCH tip/core/rcu] Reduce overhead of cond_resched() checks for RCU Message-ID: <20140621042958.GA4515@thin> References: <20140621025958.GA7185@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140621025958.GA7185@linux.vnet.ibm.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > Signed-off-by: Paul E. McKenney > Cc: Josh Triplett > Cc: Andi Kleen > Cc: Christoph Lameter > Cc: Mike Galbraith > Cc: Eric Dumazet 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 > --- > > 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 > #include > #include > -#include > #include > > 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(); > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/