diff options
Diffstat (limited to 'debian/patches-rt/timers-Introduce-expiry-spin-lock.patch')
| -rw-r--r-- | debian/patches-rt/timers-Introduce-expiry-spin-lock.patch | 153 |
1 files changed, 153 insertions, 0 deletions
diff --git a/debian/patches-rt/timers-Introduce-expiry-spin-lock.patch b/debian/patches-rt/timers-Introduce-expiry-spin-lock.patch new file mode 100644 index 000000000..69dc03c5f --- /dev/null +++ b/debian/patches-rt/timers-Introduce-expiry-spin-lock.patch @@ -0,0 +1,153 @@ +From: Anna-Maria Gleixner <anna-maria@linutronix.de> +Date: Thu, 10 Jan 2019 13:00:06 +0100 +Subject: [PATCH] timers: Introduce expiry spin lock +Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/5.2/older/patches-5.2-rt1.tar.xz + +When del_timer_sync() is called, it is possible, that the CPU has to +spin, because the timer is marked as running. The function will +repeatedly try to delete the timer until the timer callback completes +and the function succeeds. +On a virtual machine this spinning can waste CPU cycles if the vCPU +invoking the timer callback is not scheduled by the host (and making no +progress). + +The spinning and time wasting, could be prevented by using PARAVIRT_SPINLOCKS +and introducing a per timer base spin lock for expiry. The lock is hold during +expiring the timers of a base. When the deletion of a timer wasn't successful, +because the timer is running at the moment, the expiry lock is trying to +accessed instead of cpu_realax(). The lock is already held by the CPU expiring +the timers, so the CPU could be scheduled out instead of spinning until the lock +is released, because of the PARAVIRT_SPINLOCKS code. Thereby wasting time +spinning around is prevented. + +The code isn't done conditionally on PARAVIRT_SPINLOCKS. The lock is taken only +at two places. In one of them the lock is directly dropped after accessing +it. So the probability for a slowpath when taking the lock is very low. But this +keeps the code cleaner than introducing several CONFIG_PARAVIRT_SPINLOCKS +dependend code paths and struct members. + +Signed-off-by: Anna-Maria Gleixner <anna-maria@linutronix.de> +[bigeasy: Patch description reworded] +Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> +--- + kernel/time/timer.c | 57 ++++++++++++++++++++++++++++++++++++++-------------- + 1 file changed, 42 insertions(+), 15 deletions(-) + +--- a/kernel/time/timer.c ++++ b/kernel/time/timer.c +@@ -196,6 +196,7 @@ EXPORT_SYMBOL(jiffies_64); + struct timer_base { + raw_spinlock_t lock; + struct timer_list *running_timer; ++ spinlock_t expiry_lock; + unsigned long clk; + unsigned long next_expiry; + unsigned int cpu; +@@ -1201,14 +1202,8 @@ int del_timer(struct timer_list *timer) + } + EXPORT_SYMBOL(del_timer); + +-/** +- * try_to_del_timer_sync - Try to deactivate a timer +- * @timer: timer to delete +- * +- * This function tries to deactivate a timer. Upon successful (ret >= 0) +- * exit the timer is not queued and the handler is not running on any CPU. +- */ +-int try_to_del_timer_sync(struct timer_list *timer) ++static int __try_to_del_timer_sync(struct timer_list *timer, ++ struct timer_base **basep) + { + struct timer_base *base; + unsigned long flags; +@@ -1216,7 +1211,7 @@ int try_to_del_timer_sync(struct timer_l + + debug_assert_init(timer); + +- base = lock_timer_base(timer, &flags); ++ *basep = base = lock_timer_base(timer, &flags); + + if (base->running_timer != timer) + ret = detach_if_pending(timer, base, true); +@@ -1225,9 +1220,42 @@ int try_to_del_timer_sync(struct timer_l + + return ret; + } ++ ++/** ++ * try_to_del_timer_sync - Try to deactivate a timer ++ * @timer: timer to delete ++ * ++ * This function tries to deactivate a timer. Upon successful (ret >= 0) ++ * exit the timer is not queued and the handler is not running on any CPU. ++ */ ++int try_to_del_timer_sync(struct timer_list *timer) ++{ ++ struct timer_base *base; ++ ++ return __try_to_del_timer_sync(timer, &base); ++} + EXPORT_SYMBOL(try_to_del_timer_sync); + + #ifdef CONFIG_SMP ++static int __del_timer_sync(struct timer_list *timer) ++{ ++ struct timer_base *base; ++ int ret; ++ ++ for (;;) { ++ ret = __try_to_del_timer_sync(timer, &base); ++ if (ret >= 0) ++ return ret; ++ ++ /* ++ * When accessing the lock, timers of base are no longer expired ++ * and so timer is no longer running. ++ */ ++ spin_lock(&base->expiry_lock); ++ spin_unlock(&base->expiry_lock); ++ } ++} ++ + /** + * del_timer_sync - deactivate a timer and wait for the handler to finish. + * @timer: the timer to be deactivated +@@ -1283,12 +1311,8 @@ int del_timer_sync(struct timer_list *ti + * could lead to deadlock. + */ + WARN_ON(in_irq() && !(timer->flags & TIMER_IRQSAFE)); +- for (;;) { +- int ret = try_to_del_timer_sync(timer); +- if (ret >= 0) +- return ret; +- cpu_relax(); +- } ++ ++ return __del_timer_sync(timer); + } + EXPORT_SYMBOL(del_timer_sync); + #endif +@@ -1658,6 +1682,7 @@ static inline void __run_timers(struct t + if (!time_after_eq(jiffies, base->clk)) + return; + ++ spin_lock(&base->expiry_lock); + raw_spin_lock_irq(&base->lock); + + /* +@@ -1686,6 +1711,7 @@ static inline void __run_timers(struct t + } + base->running_timer = NULL; + raw_spin_unlock_irq(&base->lock); ++ spin_unlock(&base->expiry_lock); + } + + /* +@@ -1930,6 +1956,7 @@ static void __init init_timer_cpu(int cp + base->cpu = cpu; + raw_spin_lock_init(&base->lock); + base->clk = jiffies; ++ spin_lock_init(&base->expiry_lock); + } + } + |