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![Daniel Lezcano](/assets/img/avatar_default.png)
When the cpu enters idle, it stores the cpuidle state pointer in its struct rq instance which in turn could be used to make a better decision when balancing tasks. As soon as the cpu exits its idle state, the struct rq reference is cleared. There are a couple of situations where the idle state pointer could be changed while it is being consulted: 1. For x86/acpi with dynamic c-states, when a laptop switches from battery to AC that could result on removing the deeper idle state. The acpi driver triggers: 'acpi_processor_cst_has_changed' 'cpuidle_pause_and_lock' 'cpuidle_uninstall_idle_handler' 'kick_all_cpus_sync'. All cpus will exit their idle state and the pointed object will be set to NULL. 2. The cpuidle driver is unloaded. Logically that could happen but not in practice because the drivers are always compiled in and 95% of them are not coded to unregister themselves. In any case, the unloading code must call 'cpuidle_unregister_device', that calls 'cpuidle_pause_and_lock' leading to 'kick_all_cpus_sync' as mentioned above. A race can happen if we use the pointer and then one of these two scenarios occurs at the same moment. In order to be safe, the idle state pointer stored in the rq must be used inside a rcu_read_lock section where we are protected with the 'rcu_barrier' in the 'cpuidle_uninstall_idle_handler' function. The idle_get_state() and idle_put_state() accessors should be used to that effect. Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org> Signed-off-by: Nicolas Pitre <nico@linaro.org> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net> Cc: linux-pm@vger.kernel.org Cc: linaro-kernel@lists.linaro.org Cc: Daniel Lezcano <daniel.lezcano@linaro.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Link: http://lkml.kernel.org/n/tip-@git.kernel.org Signed-off-by: Ingo Molnar <mingo@kernel.org>
275 lines
6.6 KiB
C
275 lines
6.6 KiB
C
/*
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* Generic entry point for the idle threads
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*/
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#include <linux/sched.h>
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#include <linux/cpu.h>
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#include <linux/cpuidle.h>
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#include <linux/tick.h>
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#include <linux/mm.h>
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#include <linux/stackprotector.h>
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#include <asm/tlb.h>
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#include <trace/events/power.h>
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#include "sched.h"
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static int __read_mostly cpu_idle_force_poll;
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void cpu_idle_poll_ctrl(bool enable)
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{
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if (enable) {
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cpu_idle_force_poll++;
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} else {
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cpu_idle_force_poll--;
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WARN_ON_ONCE(cpu_idle_force_poll < 0);
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}
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}
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#ifdef CONFIG_GENERIC_IDLE_POLL_SETUP
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static int __init cpu_idle_poll_setup(char *__unused)
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{
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cpu_idle_force_poll = 1;
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return 1;
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}
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__setup("nohlt", cpu_idle_poll_setup);
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static int __init cpu_idle_nopoll_setup(char *__unused)
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{
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cpu_idle_force_poll = 0;
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return 1;
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}
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__setup("hlt", cpu_idle_nopoll_setup);
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#endif
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static inline int cpu_idle_poll(void)
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{
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rcu_idle_enter();
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trace_cpu_idle_rcuidle(0, smp_processor_id());
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local_irq_enable();
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while (!tif_need_resched())
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cpu_relax();
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trace_cpu_idle_rcuidle(PWR_EVENT_EXIT, smp_processor_id());
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rcu_idle_exit();
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return 1;
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}
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/* Weak implementations for optional arch specific functions */
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void __weak arch_cpu_idle_prepare(void) { }
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void __weak arch_cpu_idle_enter(void) { }
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void __weak arch_cpu_idle_exit(void) { }
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void __weak arch_cpu_idle_dead(void) { }
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void __weak arch_cpu_idle(void)
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{
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cpu_idle_force_poll = 1;
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local_irq_enable();
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}
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/**
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* cpuidle_idle_call - the main idle function
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*
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* NOTE: no locks or semaphores should be used here
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*
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* On archs that support TIF_POLLING_NRFLAG, is called with polling
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* set, and it returns with polling set. If it ever stops polling, it
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* must clear the polling bit.
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*/
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static void cpuidle_idle_call(void)
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{
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struct cpuidle_device *dev = __this_cpu_read(cpuidle_devices);
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struct cpuidle_driver *drv = cpuidle_get_cpu_driver(dev);
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int next_state, entered_state;
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unsigned int broadcast;
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/*
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* Check if the idle task must be rescheduled. If it is the
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* case, exit the function after re-enabling the local irq.
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*/
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if (need_resched()) {
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local_irq_enable();
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return;
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}
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/*
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* During the idle period, stop measuring the disabled irqs
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* critical sections latencies
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*/
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stop_critical_timings();
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/*
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* Tell the RCU framework we are entering an idle section,
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* so no more rcu read side critical sections and one more
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* step to the grace period
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*/
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rcu_idle_enter();
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/*
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* Ask the cpuidle framework to choose a convenient idle state.
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* Fall back to the default arch idle method on errors.
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*/
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next_state = cpuidle_select(drv, dev);
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if (next_state < 0) {
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use_default:
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/*
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* We can't use the cpuidle framework, let's use the default
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* idle routine.
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*/
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if (current_clr_polling_and_test())
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local_irq_enable();
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else
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arch_cpu_idle();
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goto exit_idle;
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}
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/*
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* The idle task must be scheduled, it is pointless to
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* go to idle, just update no idle residency and get
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* out of this function
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*/
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if (current_clr_polling_and_test()) {
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dev->last_residency = 0;
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entered_state = next_state;
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local_irq_enable();
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goto exit_idle;
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}
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broadcast = drv->states[next_state].flags & CPUIDLE_FLAG_TIMER_STOP;
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/*
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* Tell the time framework to switch to a broadcast timer
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* because our local timer will be shutdown. If a local timer
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* is used from another cpu as a broadcast timer, this call may
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* fail if it is not available
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*/
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if (broadcast &&
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clockevents_notify(CLOCK_EVT_NOTIFY_BROADCAST_ENTER, &dev->cpu))
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goto use_default;
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/* Take note of the planned idle state. */
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idle_set_state(this_rq(), &drv->states[next_state]);
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/*
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* Enter the idle state previously returned by the governor decision.
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* This function will block until an interrupt occurs and will take
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* care of re-enabling the local interrupts
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*/
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entered_state = cpuidle_enter(drv, dev, next_state);
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/* The cpu is no longer idle or about to enter idle. */
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idle_set_state(this_rq(), NULL);
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if (broadcast)
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clockevents_notify(CLOCK_EVT_NOTIFY_BROADCAST_EXIT, &dev->cpu);
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/*
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* Give the governor an opportunity to reflect on the outcome
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*/
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cpuidle_reflect(dev, entered_state);
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exit_idle:
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__current_set_polling();
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/*
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* It is up to the idle functions to reenable local interrupts
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*/
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if (WARN_ON_ONCE(irqs_disabled()))
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local_irq_enable();
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rcu_idle_exit();
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start_critical_timings();
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}
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/*
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* Generic idle loop implementation
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*
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* Called with polling cleared.
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*/
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static void cpu_idle_loop(void)
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{
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while (1) {
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/*
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* If the arch has a polling bit, we maintain an invariant:
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*
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* Our polling bit is clear if we're not scheduled (i.e. if
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* rq->curr != rq->idle). This means that, if rq->idle has
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* the polling bit set, then setting need_resched is
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* guaranteed to cause the cpu to reschedule.
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*/
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__current_set_polling();
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tick_nohz_idle_enter();
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while (!need_resched()) {
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check_pgt_cache();
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rmb();
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if (cpu_is_offline(smp_processor_id()))
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arch_cpu_idle_dead();
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local_irq_disable();
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arch_cpu_idle_enter();
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/*
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* In poll mode we reenable interrupts and spin.
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*
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* Also if we detected in the wakeup from idle
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* path that the tick broadcast device expired
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* for us, we don't want to go deep idle as we
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* know that the IPI is going to arrive right
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* away
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*/
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if (cpu_idle_force_poll || tick_check_broadcast_expired())
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cpu_idle_poll();
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else
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cpuidle_idle_call();
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arch_cpu_idle_exit();
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}
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/*
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* Since we fell out of the loop above, we know
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* TIF_NEED_RESCHED must be set, propagate it into
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* PREEMPT_NEED_RESCHED.
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*
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* This is required because for polling idle loops we will
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* not have had an IPI to fold the state for us.
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*/
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preempt_set_need_resched();
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tick_nohz_idle_exit();
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__current_clr_polling();
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/*
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* We promise to call sched_ttwu_pending and reschedule
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* if need_resched is set while polling is set. That
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* means that clearing polling needs to be visible
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* before doing these things.
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*/
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smp_mb__after_atomic();
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sched_ttwu_pending();
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schedule_preempt_disabled();
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}
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}
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void cpu_startup_entry(enum cpuhp_state state)
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{
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/*
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* This #ifdef needs to die, but it's too late in the cycle to
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* make this generic (arm and sh have never invoked the canary
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* init for the non boot cpus!). Will be fixed in 3.11
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*/
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#ifdef CONFIG_X86
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/*
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* If we're the non-boot CPU, nothing set the stack canary up
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* for us. The boot CPU already has it initialized but no harm
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* in doing it again. This is a good place for updating it, as
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* we wont ever return from this function (so the invalid
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* canaries already on the stack wont ever trigger).
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*/
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boot_init_stack_canary();
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#endif
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arch_cpu_idle_prepare();
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cpu_idle_loop();
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}
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