rcu: Make RCU_FAST_NO_HZ account for pauses out of idle
Both Steven Rostedt's new idle-capable trace macros and the RCU_NONIDLE() macro can cause RCU to momentarily pause out of idle without the rest of the system being involved. This can cause rcu_prepare_for_idle() to run through its state machine too quickly, which can in turn result in needless scheduling-clock interrupts. This commit therefore adds code to enable rcu_prepare_for_idle() to distinguish between an initial entry to idle on the one hand (which needs to advance the rcu_prepare_for_idle() state machine) and an idle reentry due to idle-capable trace macros and RCU_NONIDLE() on the other hand (which should avoid advancing the rcu_prepare_for_idle() state machine). Additional state is maintained to allow the timer to be correctly reposted when returning after a momentary pause out of idle, and even more state is maintained to detect when new non-lazy callbacks have been enqueued (which may require re-evaluation of the approach to idleness). Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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@ -1829,6 +1829,8 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
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rdp->qlen++;
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if (lazy)
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rdp->qlen_lazy++;
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else
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rcu_idle_count_callbacks_posted();
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if (__is_kfree_rcu_offset((unsigned long)func))
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trace_rcu_kfree_callback(rsp->name, head, (unsigned long)func,
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@ -471,6 +471,7 @@ static void __cpuinit rcu_prepare_kthreads(int cpu);
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static void rcu_prepare_for_idle_init(int cpu);
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static void rcu_cleanup_after_idle(int cpu);
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static void rcu_prepare_for_idle(int cpu);
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static void rcu_idle_count_callbacks_posted(void);
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static void print_cpu_stall_info_begin(void);
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static void print_cpu_stall_info(struct rcu_state *rsp, int cpu);
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static void print_cpu_stall_info_end(void);
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@ -1938,6 +1938,14 @@ static void rcu_prepare_for_idle(int cpu)
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{
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}
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/*
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* Don't bother keeping a running count of the number of RCU callbacks
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* posted because CONFIG_RCU_FAST_NO_HZ=n.
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*/
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static void rcu_idle_count_callbacks_posted(void)
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{
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}
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#else /* #if !defined(CONFIG_RCU_FAST_NO_HZ) */
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/*
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@ -1981,6 +1989,10 @@ static void rcu_prepare_for_idle(int cpu)
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static DEFINE_PER_CPU(int, rcu_dyntick_drain);
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static DEFINE_PER_CPU(unsigned long, rcu_dyntick_holdoff);
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static DEFINE_PER_CPU(struct timer_list, rcu_idle_gp_timer);
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static DEFINE_PER_CPU(unsigned long, rcu_idle_gp_timer_expires);
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static DEFINE_PER_CPU(bool, rcu_idle_first_pass);
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static DEFINE_PER_CPU(unsigned long, rcu_nonlazy_posted);
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static DEFINE_PER_CPU(unsigned long, rcu_nonlazy_posted_snap);
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/*
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* Allow the CPU to enter dyntick-idle mode if either: (1) There are no
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@ -1993,6 +2005,8 @@ static DEFINE_PER_CPU(struct timer_list, rcu_idle_gp_timer);
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*/
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int rcu_needs_cpu(int cpu)
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{
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/* Flag a new idle sojourn to the idle-entry state machine. */
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per_cpu(rcu_idle_first_pass, cpu) = 1;
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/* If no callbacks, RCU doesn't need the CPU. */
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if (!rcu_cpu_has_callbacks(cpu))
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return 0;
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@ -2095,6 +2109,26 @@ static void rcu_cleanup_after_idle(int cpu)
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*/
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static void rcu_prepare_for_idle(int cpu)
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{
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/*
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* If this is an idle re-entry, for example, due to use of
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* RCU_NONIDLE() or the new idle-loop tracing API within the idle
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* loop, then don't take any state-machine actions, unless the
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* momentary exit from idle queued additional non-lazy callbacks.
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* Instead, repost the rcu_idle_gp_timer if this CPU has callbacks
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* pending.
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*/
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if (!per_cpu(rcu_idle_first_pass, cpu) &&
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(per_cpu(rcu_nonlazy_posted, cpu) ==
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per_cpu(rcu_nonlazy_posted_snap, cpu))) {
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if (rcu_cpu_has_callbacks(cpu))
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mod_timer(&per_cpu(rcu_idle_gp_timer, cpu),
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per_cpu(rcu_idle_gp_timer_expires, cpu));
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return;
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}
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per_cpu(rcu_idle_first_pass, cpu) = 0;
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per_cpu(rcu_nonlazy_posted_snap, cpu) =
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per_cpu(rcu_nonlazy_posted, cpu) - 1;
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/*
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* If there are no callbacks on this CPU, enter dyntick-idle mode.
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* Also reset state to avoid prejudicing later attempts.
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@ -2127,11 +2161,15 @@ static void rcu_prepare_for_idle(int cpu)
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per_cpu(rcu_dyntick_drain, cpu) = 0;
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per_cpu(rcu_dyntick_holdoff, cpu) = jiffies;
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if (rcu_cpu_has_nonlazy_callbacks(cpu))
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mod_timer(&per_cpu(rcu_idle_gp_timer, cpu),
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jiffies + RCU_IDLE_GP_DELAY);
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per_cpu(rcu_idle_gp_timer_expires, cpu) =
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jiffies + RCU_IDLE_GP_DELAY;
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else
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per_cpu(rcu_idle_gp_timer_expires, cpu) =
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jiffies + RCU_IDLE_LAZY_GP_DELAY;
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mod_timer(&per_cpu(rcu_idle_gp_timer, cpu),
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jiffies + RCU_IDLE_LAZY_GP_DELAY);
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per_cpu(rcu_idle_gp_timer_expires, cpu));
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per_cpu(rcu_nonlazy_posted_snap, cpu) =
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per_cpu(rcu_nonlazy_posted, cpu);
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return; /* Nothing more to do immediately. */
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} else if (--per_cpu(rcu_dyntick_drain, cpu) <= 0) {
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/* We have hit the limit, so time to give up. */
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@ -2171,6 +2209,17 @@ static void rcu_prepare_for_idle(int cpu)
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trace_rcu_prep_idle("Callbacks drained");
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}
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/*
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* Keep a running count of callbacks posted so that rcu_prepare_for_idle()
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* can detect when something out of the idle loop posts a callback.
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* Of course, it had better do so either from a trace event designed to
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* be called from idle or from within RCU_NONIDLE().
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*/
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static void rcu_idle_count_callbacks_posted(void)
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{
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__this_cpu_add(rcu_nonlazy_posted, 1);
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}
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#endif /* #else #if !defined(CONFIG_RCU_FAST_NO_HZ) */
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#ifdef CONFIG_RCU_CPU_STALL_INFO
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