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Diffstat (limited to '')
-rw-r--r-- | kernel/context_tracking.c | 223 |
1 files changed, 223 insertions, 0 deletions
diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c new file mode 100644 index 000000000..36a98c48a --- /dev/null +++ b/kernel/context_tracking.c @@ -0,0 +1,223 @@ +// SPDX-License-Identifier: GPL-2.0-only +/* + * Context tracking: Probe on high level context boundaries such as kernel + * and userspace. This includes syscalls and exceptions entry/exit. + * + * This is used by RCU to remove its dependency on the timer tick while a CPU + * runs in userspace. + * + * Started by Frederic Weisbecker: + * + * Copyright (C) 2012 Red Hat, Inc., Frederic Weisbecker <fweisbec@redhat.com> + * + * Many thanks to Gilad Ben-Yossef, Paul McKenney, Ingo Molnar, Andrew Morton, + * Steven Rostedt, Peter Zijlstra for suggestions and improvements. + * + */ + +#include <linux/context_tracking.h> +#include <linux/rcupdate.h> +#include <linux/sched.h> +#include <linux/hardirq.h> +#include <linux/export.h> +#include <linux/kprobes.h> + +#define CREATE_TRACE_POINTS +#include <trace/events/context_tracking.h> + +DEFINE_STATIC_KEY_FALSE(context_tracking_key); +EXPORT_SYMBOL_GPL(context_tracking_key); + +DEFINE_PER_CPU(struct context_tracking, context_tracking); +EXPORT_SYMBOL_GPL(context_tracking); + +static noinstr bool context_tracking_recursion_enter(void) +{ + int recursion; + + recursion = __this_cpu_inc_return(context_tracking.recursion); + if (recursion == 1) + return true; + + WARN_ONCE((recursion < 1), "Invalid context tracking recursion value %d\n", recursion); + __this_cpu_dec(context_tracking.recursion); + + return false; +} + +static __always_inline void context_tracking_recursion_exit(void) +{ + __this_cpu_dec(context_tracking.recursion); +} + +/** + * context_tracking_enter - Inform the context tracking that the CPU is going + * enter user or guest space mode. + * + * This function must be called right before we switch from the kernel + * to user or guest space, when it's guaranteed the remaining kernel + * instructions to execute won't use any RCU read side critical section + * because this function sets RCU in extended quiescent state. + */ +void noinstr __context_tracking_enter(enum ctx_state state) +{ + /* Kernel threads aren't supposed to go to userspace */ + WARN_ON_ONCE(!current->mm); + + if (!context_tracking_recursion_enter()) + return; + + if ( __this_cpu_read(context_tracking.state) != state) { + if (__this_cpu_read(context_tracking.active)) { + /* + * At this stage, only low level arch entry code remains and + * then we'll run in userspace. We can assume there won't be + * any RCU read-side critical section until the next call to + * user_exit() or rcu_irq_enter(). Let's remove RCU's dependency + * on the tick. + */ + if (state == CONTEXT_USER) { + instrumentation_begin(); + trace_user_enter(0); + vtime_user_enter(current); + instrumentation_end(); + } + rcu_user_enter(); + } + /* + * Even if context tracking is disabled on this CPU, because it's outside + * the full dynticks mask for example, we still have to keep track of the + * context transitions and states to prevent inconsistency on those of + * other CPUs. + * If a task triggers an exception in userspace, sleep on the exception + * handler and then migrate to another CPU, that new CPU must know where + * the exception returns by the time we call exception_exit(). + * This information can only be provided by the previous CPU when it called + * exception_enter(). + * OTOH we can spare the calls to vtime and RCU when context_tracking.active + * is false because we know that CPU is not tickless. + */ + __this_cpu_write(context_tracking.state, state); + } + context_tracking_recursion_exit(); +} +EXPORT_SYMBOL_GPL(__context_tracking_enter); + +void context_tracking_enter(enum ctx_state state) +{ + unsigned long flags; + + /* + * Some contexts may involve an exception occuring in an irq, + * leading to that nesting: + * rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit() + * This would mess up the dyntick_nesting count though. And rcu_irq_*() + * helpers are enough to protect RCU uses inside the exception. So + * just return immediately if we detect we are in an IRQ. + */ + if (in_interrupt()) + return; + + local_irq_save(flags); + __context_tracking_enter(state); + local_irq_restore(flags); +} +NOKPROBE_SYMBOL(context_tracking_enter); +EXPORT_SYMBOL_GPL(context_tracking_enter); + +void context_tracking_user_enter(void) +{ + user_enter(); +} +NOKPROBE_SYMBOL(context_tracking_user_enter); + +/** + * context_tracking_exit - Inform the context tracking that the CPU is + * exiting user or guest mode and entering the kernel. + * + * This function must be called after we entered the kernel from user or + * guest space before any use of RCU read side critical section. This + * potentially include any high level kernel code like syscalls, exceptions, + * signal handling, etc... + * + * This call supports re-entrancy. This way it can be called from any exception + * handler without needing to know if we came from userspace or not. + */ +void noinstr __context_tracking_exit(enum ctx_state state) +{ + if (!context_tracking_recursion_enter()) + return; + + if (__this_cpu_read(context_tracking.state) == state) { + if (__this_cpu_read(context_tracking.active)) { + /* + * We are going to run code that may use RCU. Inform + * RCU core about that (ie: we may need the tick again). + */ + rcu_user_exit(); + if (state == CONTEXT_USER) { + instrumentation_begin(); + vtime_user_exit(current); + trace_user_exit(0); + instrumentation_end(); + } + } + __this_cpu_write(context_tracking.state, CONTEXT_KERNEL); + } + context_tracking_recursion_exit(); +} +EXPORT_SYMBOL_GPL(__context_tracking_exit); + +void context_tracking_exit(enum ctx_state state) +{ + unsigned long flags; + + if (in_interrupt()) + return; + + local_irq_save(flags); + __context_tracking_exit(state); + local_irq_restore(flags); +} +NOKPROBE_SYMBOL(context_tracking_exit); +EXPORT_SYMBOL_GPL(context_tracking_exit); + +void context_tracking_user_exit(void) +{ + user_exit(); +} +NOKPROBE_SYMBOL(context_tracking_user_exit); + +void __init context_tracking_cpu_set(int cpu) +{ + static __initdata bool initialized = false; + + if (!per_cpu(context_tracking.active, cpu)) { + per_cpu(context_tracking.active, cpu) = true; + static_branch_inc(&context_tracking_key); + } + + if (initialized) + return; + +#ifdef CONFIG_HAVE_TIF_NOHZ + /* + * Set TIF_NOHZ to init/0 and let it propagate to all tasks through fork + * This assumes that init is the only task at this early boot stage. + */ + set_tsk_thread_flag(&init_task, TIF_NOHZ); +#endif + WARN_ON_ONCE(!tasklist_empty()); + + initialized = true; +} + +#ifdef CONFIG_CONTEXT_TRACKING_FORCE +void __init context_tracking_init(void) +{ + int cpu; + + for_each_possible_cpu(cpu) + context_tracking_cpu_set(cpu); +} +#endif |