rcu: Reduce overhead of cond_resched() checks for RCU
Commit ac1bea8578 (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@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Christoph Lameter <cl@gentwo.org>
Cc: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
[ paulmck: Made rcu_momentary_dyntick_idle() as suggested by the
ktest build robot. Also fixed smp_mb() comment as noted by
Oleg Nesterov. ]
Merge with e552592e (Reduce overhead of cond_resched() checks for RCU)
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney
Родитель
546a9d8519
Коммит
4a81e8328d
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@ -2785,6 +2785,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
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leaf rcu_node structure. Useful for very large
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systems.
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rcutree.jiffies_till_sched_qs= [KNL]
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Set required age in jiffies for a
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given grace period before RCU starts
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soliciting quiescent-state help from
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rcu_note_context_switch().
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rcutree.jiffies_till_first_fqs= [KNL]
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Set delay from grace-period initialization to
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first attempt to force quiescent states.
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@ -44,7 +44,6 @@
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#include <linux/debugobjects.h>
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#include <linux/bug.h>
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#include <linux/compiler.h>
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#include <linux/percpu.h>
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#include <asm/barrier.h>
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extern int rcu_expedited; /* for sysctl */
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@ -299,41 +298,6 @@ static inline void rcu_user_hooks_switch(struct task_struct *prev,
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bool __rcu_is_watching(void);
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#endif /* #if defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_RCU_TRACE) || defined(CONFIG_SMP) */
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/*
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* Hooks for cond_resched() and friends to avoid RCU CPU stall warnings.
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*/
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#define RCU_COND_RESCHED_LIM 256 /* ms vs. 100s of ms. */
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DECLARE_PER_CPU(int, rcu_cond_resched_count);
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void rcu_resched(void);
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/*
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* Is it time to report RCU quiescent states?
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*
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* Note unsynchronized access to rcu_cond_resched_count. Yes, we might
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* increment some random CPU's count, and possibly also load the result from
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* yet another CPU's count. We might even clobber some other CPU's attempt
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* to zero its counter. This is all OK because the goal is not precision,
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* but rather reasonable amortization of rcu_note_context_switch() overhead
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* and extremely high probability of avoiding RCU CPU stall warnings.
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* Note that this function has to be preempted in just the wrong place,
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* many thousands of times in a row, for anything bad to happen.
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*/
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static inline bool rcu_should_resched(void)
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{
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return raw_cpu_inc_return(rcu_cond_resched_count) >=
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RCU_COND_RESCHED_LIM;
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}
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/*
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* Report quiscent states to RCU if it is time to do so.
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*/
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static inline void rcu_cond_resched(void)
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{
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if (unlikely(rcu_should_resched()))
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rcu_resched();
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}
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/*
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* Infrastructure to implement the synchronize_() primitives in
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* TREE_RCU and rcu_barrier_() primitives in TINY_RCU.
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@ -206,6 +206,70 @@ void rcu_bh_qs(int cpu)
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rdp->passed_quiesce = 1;
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}
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static DEFINE_PER_CPU(int, rcu_sched_qs_mask);
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static DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
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.dynticks_nesting = DYNTICK_TASK_EXIT_IDLE,
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.dynticks = ATOMIC_INIT(1),
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#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
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.dynticks_idle_nesting = DYNTICK_TASK_NEST_VALUE,
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.dynticks_idle = ATOMIC_INIT(1),
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#endif /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
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};
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/*
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* Let the RCU core know that this CPU has gone through the scheduler,
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* which is a quiescent state. This is called when the need for a
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* quiescent state is urgent, so we burn an atomic operation and full
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* memory barriers to let the RCU core know about it, regardless of what
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* this CPU might (or might not) do in the near future.
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*
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* We inform the RCU core by emulating a zero-duration dyntick-idle
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* period, which we in turn do by incrementing the ->dynticks counter
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* by two.
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*/
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static void rcu_momentary_dyntick_idle(void)
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{
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unsigned long flags;
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struct rcu_data *rdp;
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struct rcu_dynticks *rdtp;
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int resched_mask;
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struct rcu_state *rsp;
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local_irq_save(flags);
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/*
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* Yes, we can lose flag-setting operations. This is OK, because
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* the flag will be set again after some delay.
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*/
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resched_mask = raw_cpu_read(rcu_sched_qs_mask);
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raw_cpu_write(rcu_sched_qs_mask, 0);
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/* Find the flavor that needs a quiescent state. */
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for_each_rcu_flavor(rsp) {
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rdp = raw_cpu_ptr(rsp->rda);
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if (!(resched_mask & rsp->flavor_mask))
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continue;
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smp_mb(); /* rcu_sched_qs_mask before cond_resched_completed. */
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if (ACCESS_ONCE(rdp->mynode->completed) !=
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ACCESS_ONCE(rdp->cond_resched_completed))
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continue;
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/*
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* Pretend to be momentarily idle for the quiescent state.
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* This allows the grace-period kthread to record the
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* quiescent state, with no need for this CPU to do anything
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* further.
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*/
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rdtp = this_cpu_ptr(&rcu_dynticks);
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smp_mb__before_atomic(); /* Earlier stuff before QS. */
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atomic_add(2, &rdtp->dynticks); /* QS. */
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smp_mb__after_atomic(); /* Later stuff after QS. */
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break;
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}
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local_irq_restore(flags);
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}
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/*
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* Note a context switch. This is a quiescent state for RCU-sched,
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* and requires special handling for preemptible RCU.
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@ -216,19 +280,12 @@ void rcu_note_context_switch(int cpu)
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trace_rcu_utilization(TPS("Start context switch"));
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rcu_sched_qs(cpu);
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rcu_preempt_note_context_switch(cpu);
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if (unlikely(raw_cpu_read(rcu_sched_qs_mask)))
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rcu_momentary_dyntick_idle();
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trace_rcu_utilization(TPS("End context switch"));
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}
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EXPORT_SYMBOL_GPL(rcu_note_context_switch);
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static DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
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.dynticks_nesting = DYNTICK_TASK_EXIT_IDLE,
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.dynticks = ATOMIC_INIT(1),
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#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
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.dynticks_idle_nesting = DYNTICK_TASK_NEST_VALUE,
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.dynticks_idle = ATOMIC_INIT(1),
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#endif /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
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};
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static long blimit = 10; /* Maximum callbacks per rcu_do_batch. */
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static long qhimark = 10000; /* If this many pending, ignore blimit. */
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static long qlowmark = 100; /* Once only this many pending, use blimit. */
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@ -243,6 +300,13 @@ static ulong jiffies_till_next_fqs = ULONG_MAX;
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module_param(jiffies_till_first_fqs, ulong, 0644);
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module_param(jiffies_till_next_fqs, ulong, 0644);
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/*
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* How long the grace period must be before we start recruiting
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* quiescent-state help from rcu_note_context_switch().
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*/
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static ulong jiffies_till_sched_qs = HZ / 20;
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module_param(jiffies_till_sched_qs, ulong, 0644);
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static bool rcu_start_gp_advanced(struct rcu_state *rsp, struct rcu_node *rnp,
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struct rcu_data *rdp);
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static void force_qs_rnp(struct rcu_state *rsp,
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@ -853,6 +917,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
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bool *isidle, unsigned long *maxj)
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{
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unsigned int curr;
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int *rcrmp;
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unsigned int snap;
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curr = (unsigned int)atomic_add_return(0, &rdp->dynticks->dynticks);
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@ -893,27 +958,43 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
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}
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/*
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* There is a possibility that a CPU in adaptive-ticks state
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* might run in the kernel with the scheduling-clock tick disabled
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* for an extended time period. Invoke rcu_kick_nohz_cpu() to
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* force the CPU to restart the scheduling-clock tick in this
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* CPU is in this state.
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* A CPU running for an extended time within the kernel can
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* delay RCU grace periods. When the CPU is in NO_HZ_FULL mode,
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* even context-switching back and forth between a pair of
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* in-kernel CPU-bound tasks cannot advance grace periods.
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* So if the grace period is old enough, make the CPU pay attention.
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* Note that the unsynchronized assignments to the per-CPU
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* rcu_sched_qs_mask variable are safe. Yes, setting of
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* bits can be lost, but they will be set again on the next
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* force-quiescent-state pass. So lost bit sets do not result
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* in incorrect behavior, merely in a grace period lasting
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* a few jiffies longer than it might otherwise. Because
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* there are at most four threads involved, and because the
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* updates are only once every few jiffies, the probability of
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* lossage (and thus of slight grace-period extension) is
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* quite low.
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*
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* Note that if the jiffies_till_sched_qs boot/sysfs parameter
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* is set too high, we override with half of the RCU CPU stall
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* warning delay.
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*/
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rcu_kick_nohz_cpu(rdp->cpu);
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/*
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* Alternatively, the CPU might be running in the kernel
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* for an extended period of time without a quiescent state.
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* Attempt to force the CPU through the scheduler to gain the
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* needed quiescent state, but only if the grace period has gone
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* on for an uncommonly long time. If there are many stuck CPUs,
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* we will beat on the first one until it gets unstuck, then move
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* to the next. Only do this for the primary flavor of RCU.
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*/
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if (rdp->rsp == rcu_state_p &&
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rcrmp = &per_cpu(rcu_sched_qs_mask, rdp->cpu);
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if (ULONG_CMP_GE(jiffies,
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rdp->rsp->gp_start + jiffies_till_sched_qs) ||
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ULONG_CMP_GE(jiffies, rdp->rsp->jiffies_resched)) {
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rdp->rsp->jiffies_resched += 5;
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resched_cpu(rdp->cpu);
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if (!(ACCESS_ONCE(*rcrmp) & rdp->rsp->flavor_mask)) {
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ACCESS_ONCE(rdp->cond_resched_completed) =
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ACCESS_ONCE(rdp->mynode->completed);
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smp_mb(); /* ->cond_resched_completed before *rcrmp. */
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ACCESS_ONCE(*rcrmp) =
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ACCESS_ONCE(*rcrmp) + rdp->rsp->flavor_mask;
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resched_cpu(rdp->cpu); /* Force CPU into scheduler. */
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rdp->rsp->jiffies_resched += 5; /* Enable beating. */
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} else if (ULONG_CMP_GE(jiffies, rdp->rsp->jiffies_resched)) {
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/* Time to beat on that CPU again! */
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resched_cpu(rdp->cpu); /* Force CPU into scheduler. */
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rdp->rsp->jiffies_resched += 5; /* Re-enable beating. */
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}
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}
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return 0;
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@ -3491,6 +3572,7 @@ static void __init rcu_init_one(struct rcu_state *rsp,
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"rcu_node_fqs_1",
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"rcu_node_fqs_2",
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"rcu_node_fqs_3" }; /* Match MAX_RCU_LVLS */
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static u8 fl_mask = 0x1;
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int cpustride = 1;
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int i;
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int j;
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@ -3509,6 +3591,8 @@ static void __init rcu_init_one(struct rcu_state *rsp,
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for (i = 1; i < rcu_num_lvls; i++)
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rsp->level[i] = rsp->level[i - 1] + rsp->levelcnt[i - 1];
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rcu_init_levelspread(rsp);
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rsp->flavor_mask = fl_mask;
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fl_mask <<= 1;
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/* Initialize the elements themselves, starting from the leaves. */
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@ -307,6 +307,9 @@ struct rcu_data {
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/* 4) reasons this CPU needed to be kicked by force_quiescent_state */
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unsigned long dynticks_fqs; /* Kicked due to dynticks idle. */
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unsigned long offline_fqs; /* Kicked due to being offline. */
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unsigned long cond_resched_completed;
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/* Grace period that needs help */
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/* from cond_resched(). */
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/* 5) __rcu_pending() statistics. */
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unsigned long n_rcu_pending; /* rcu_pending() calls since boot. */
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@ -392,6 +395,7 @@ struct rcu_state {
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struct rcu_node *level[RCU_NUM_LVLS]; /* Hierarchy levels. */
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u32 levelcnt[MAX_RCU_LVLS + 1]; /* # nodes in each level. */
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u8 levelspread[RCU_NUM_LVLS]; /* kids/node in each level. */
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u8 flavor_mask; /* bit in flavor mask. */
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struct rcu_data __percpu *rda; /* pointer of percu rcu_data. */
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void (*call)(struct rcu_head *head, /* call_rcu() flavor. */
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void (*func)(struct rcu_head *head));
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@ -563,7 +567,7 @@ static bool rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp);
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static void do_nocb_deferred_wakeup(struct rcu_data *rdp);
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static void rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp);
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static void rcu_spawn_nocb_kthreads(struct rcu_state *rsp);
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static void rcu_kick_nohz_cpu(int cpu);
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static void __maybe_unused rcu_kick_nohz_cpu(int cpu);
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static bool init_nocb_callback_list(struct rcu_data *rdp);
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static void rcu_sysidle_enter(struct rcu_dynticks *rdtp, int irq);
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static void rcu_sysidle_exit(struct rcu_dynticks *rdtp, int irq);
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@ -2404,7 +2404,7 @@ static bool init_nocb_callback_list(struct rcu_data *rdp)
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* if an adaptive-ticks CPU is failing to respond to the current grace
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* period and has not be idle from an RCU perspective, kick it.
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*/
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static void rcu_kick_nohz_cpu(int cpu)
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static void __maybe_unused rcu_kick_nohz_cpu(int cpu)
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{
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#ifdef CONFIG_NO_HZ_FULL
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if (tick_nohz_full_cpu(cpu))
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@ -350,21 +350,3 @@ static int __init check_cpu_stall_init(void)
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early_initcall(check_cpu_stall_init);
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#endif /* #ifdef CONFIG_RCU_STALL_COMMON */
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/*
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* Hooks for cond_resched() and friends to avoid RCU CPU stall warnings.
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*/
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DEFINE_PER_CPU(int, rcu_cond_resched_count);
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/*
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* Report a set of RCU quiescent states, for use by cond_resched()
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* and friends. Out of line due to being called infrequently.
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*/
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void rcu_resched(void)
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{
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preempt_disable();
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__this_cpu_write(rcu_cond_resched_count, 0);
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rcu_note_context_switch(smp_processor_id());
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preempt_enable();
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}
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@ -4147,7 +4147,6 @@ static void __cond_resched(void)
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int __sched _cond_resched(void)
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{
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rcu_cond_resched();
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if (should_resched()) {
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__cond_resched();
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return 1;
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@ -4166,18 +4165,15 @@ EXPORT_SYMBOL(_cond_resched);
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*/
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int __cond_resched_lock(spinlock_t *lock)
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{
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bool need_rcu_resched = rcu_should_resched();
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int resched = should_resched();
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int ret = 0;
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lockdep_assert_held(lock);
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if (spin_needbreak(lock) || resched || need_rcu_resched) {
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if (spin_needbreak(lock) || resched) {
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spin_unlock(lock);
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if (resched)
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__cond_resched();
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else if (unlikely(need_rcu_resched))
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rcu_resched();
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else
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cpu_relax();
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ret = 1;
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@ -4191,7 +4187,6 @@ int __sched __cond_resched_softirq(void)
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{
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BUG_ON(!in_softirq());
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rcu_cond_resched(); /* BH disabled OK, just recording QSes. */
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if (should_resched()) {
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local_bh_enable();
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__cond_resched();
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