diff options
Diffstat (limited to 'include/linux/preempt.h')
| -rw-r--r-- | include/linux/preempt.h | 113 |
1 files changed, 76 insertions, 37 deletions
diff --git a/include/linux/preempt.h b/include/linux/preempt.h index 7d9c1c0e149c..69cc8b64aa3a 100644 --- a/include/linux/preempt.h +++ b/include/linux/preempt.h @@ -77,31 +77,33 @@ /* preempt_count() and related functions, depends on PREEMPT_NEED_RESCHED */ #include <asm/preempt.h> +#define nmi_count() (preempt_count() & NMI_MASK) #define hardirq_count() (preempt_count() & HARDIRQ_MASK) #define softirq_count() (preempt_count() & SOFTIRQ_MASK) -#define irq_count() (preempt_count() & (HARDIRQ_MASK | SOFTIRQ_MASK \ - | NMI_MASK)) +#define irq_count() (nmi_count() | hardirq_count() | softirq_count()) /* - * Are we doing bottom half or hardware interrupt processing? + * Macros to retrieve the current execution context: * - * in_irq() - We're in (hard) IRQ context + * in_nmi() - We're in NMI context + * in_hardirq() - We're in hard IRQ context + * in_serving_softirq() - We're in softirq context + * in_task() - We're in task context + */ +#define in_nmi() (nmi_count()) +#define in_hardirq() (hardirq_count()) +#define in_serving_softirq() (softirq_count() & SOFTIRQ_OFFSET) +#define in_task() (!(in_nmi() | in_hardirq() | in_serving_softirq())) + +/* + * The following macros are deprecated and should not be used in new code: + * in_irq() - Obsolete version of in_hardirq() * in_softirq() - We have BH disabled, or are processing softirqs * in_interrupt() - We're in NMI,IRQ,SoftIRQ context or have BH disabled - * in_serving_softirq() - We're in softirq context - * in_nmi() - We're in NMI context - * in_task() - We're in task context - * - * Note: due to the BH disabled confusion: in_softirq(),in_interrupt() really - * should not be used in new code. */ #define in_irq() (hardirq_count()) #define in_softirq() (softirq_count()) #define in_interrupt() (irq_count()) -#define in_serving_softirq() (softirq_count() & SOFTIRQ_OFFSET) -#define in_nmi() (preempt_count() & NMI_MASK) -#define in_task() (!(preempt_count() & \ - (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET))) /* * The preempt_count offset after preempt_disable(); @@ -322,34 +324,71 @@ static inline void preempt_notifier_init(struct preempt_notifier *notifier, #endif -/** - * migrate_disable - Prevent migration of the current task +#ifdef CONFIG_SMP + +/* + * Migrate-Disable and why it is undesired. * - * Maps to preempt_disable() which also disables preemption. Use - * migrate_disable() to annotate that the intent is to prevent migration, - * but not necessarily preemption. + * When a preempted task becomes elegible to run under the ideal model (IOW it + * becomes one of the M highest priority tasks), it might still have to wait + * for the preemptee's migrate_disable() section to complete. Thereby suffering + * a reduction in bandwidth in the exact duration of the migrate_disable() + * section. * - * Can be invoked nested like preempt_disable() and needs the corresponding - * number of migrate_enable() invocations. - */ -static __always_inline void migrate_disable(void) -{ - preempt_disable(); -} - -/** - * migrate_enable - Allow migration of the current task + * Per this argument, the change from preempt_disable() to migrate_disable() + * gets us: + * + * - a higher priority tasks gains reduced wake-up latency; with preempt_disable() + * it would have had to wait for the lower priority task. + * + * - a lower priority tasks; which under preempt_disable() could've instantly + * migrated away when another CPU becomes available, is now constrained + * by the ability to push the higher priority task away, which might itself be + * in a migrate_disable() section, reducing it's available bandwidth. * - * Counterpart to migrate_disable(). + * IOW it trades latency / moves the interference term, but it stays in the + * system, and as long as it remains unbounded, the system is not fully + * deterministic. * - * As migrate_disable() can be invoked nested, only the outermost invocation - * reenables migration. * - * Currently mapped to preempt_enable(). + * The reason we have it anyway. + * + * PREEMPT_RT breaks a number of assumptions traditionally held. By forcing a + * number of primitives into becoming preemptible, they would also allow + * migration. This turns out to break a bunch of per-cpu usage. To this end, + * all these primitives employ migirate_disable() to restore this implicit + * assumption. + * + * This is a 'temporary' work-around at best. The correct solution is getting + * rid of the above assumptions and reworking the code to employ explicit + * per-cpu locking or short preempt-disable regions. + * + * The end goal must be to get rid of migrate_disable(), alternatively we need + * a schedulability theory that does not depend on abritrary migration. + * + * + * Notes on the implementation. + * + * The implementation is particularly tricky since existing code patterns + * dictate neither migrate_disable() nor migrate_enable() is allowed to block. + * This means that it cannot use cpus_read_lock() to serialize against hotplug, + * nor can it easily migrate itself into a pending affinity mask change on + * migrate_enable(). + * + * + * Note: even non-work-conserving schedulers like semi-partitioned depends on + * migration, so migrate_disable() is not only a problem for + * work-conserving schedulers. + * */ -static __always_inline void migrate_enable(void) -{ - preempt_enable(); -} +extern void migrate_disable(void); +extern void migrate_enable(void); + +#else + +static inline void migrate_disable(void) { } +static inline void migrate_enable(void) { } + +#endif /* CONFIG_SMP */ #endif /* __LINUX_PREEMPT_H */ |