From cca26e8009d1939a6a5bf0200d276fa26f03e536 Mon Sep 17 00:00:00 2001 From: Kirill Tkhai Date: Wed, 20 Aug 2014 13:47:42 +0400 Subject: [PATCH] sched: Teach scheduler to understand TASK_ON_RQ_MIGRATING state This is a new p->on_rq state which will be used to indicate that a task is in a process of migrating between two RQs. It allows to get rid of double_rq_lock(), which we used to use to change a rq of a queued task before. Let's consider an example. To move a task between src_rq and dst_rq we will do the following: raw_spin_lock(&src_rq->lock); /* p is a task which is queued on src_rq */ p = ...; dequeue_task(src_rq, p, 0); p->on_rq = TASK_ON_RQ_MIGRATING; set_task_cpu(p, dst_cpu); raw_spin_unlock(&src_rq->lock); /* * Both RQs are unlocked here. * Task p is dequeued from src_rq * but its on_rq value is not zero. */ raw_spin_lock(&dst_rq->lock); p->on_rq = TASK_ON_RQ_QUEUED; enqueue_task(dst_rq, p, 0); raw_spin_unlock(&dst_rq->lock); While p->on_rq is TASK_ON_RQ_MIGRATING, task is considered as "migrating", and other parallel scheduler actions with it are not available to parallel callers. The parallel caller is spining till migration is completed. The unavailable actions are changing of cpu affinity, changing of priority etc, in other words all the functionality which used to require task_rq(p)->lock before (and related to the task). To implement TASK_ON_RQ_MIGRATING support we primarily are using the following fact. Most of scheduler users (from which we are protecting a migrating task) use task_rq_lock() and __task_rq_lock() to get the lock of task_rq(p). These primitives know that task's cpu may change, and they are spining while the lock of the right RQ is not held. We add one more condition into them, so they will be also spinning until the migration is finished. Signed-off-by: Kirill Tkhai Cc: Peter Zijlstra Cc: Paul Turner Cc: Oleg Nesterov Cc: Steven Rostedt Cc: Mike Galbraith Cc: Kirill Tkhai Cc: Tim Chen Cc: Nicolas Pitre Cc: Linus Torvalds Link: http://lkml.kernel.org/r/1408528062.23412.88.camel@tkhai Signed-off-by: Ingo Molnar --- kernel/sched/core.c | 12 +++++++++--- kernel/sched/sched.h | 6 ++++++ 2 files changed, 15 insertions(+), 3 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index a02b624fee6c..71b836034912 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -333,9 +333,12 @@ static inline struct rq *__task_rq_lock(struct task_struct *p) for (;;) { rq = task_rq(p); raw_spin_lock(&rq->lock); - if (likely(rq == task_rq(p))) + if (likely(rq == task_rq(p) && !task_on_rq_migrating(p))) return rq; raw_spin_unlock(&rq->lock); + + while (unlikely(task_on_rq_migrating(p))) + cpu_relax(); } } @@ -352,10 +355,13 @@ static struct rq *task_rq_lock(struct task_struct *p, unsigned long *flags) raw_spin_lock_irqsave(&p->pi_lock, *flags); rq = task_rq(p); raw_spin_lock(&rq->lock); - if (likely(rq == task_rq(p))) + if (likely(rq == task_rq(p) && !task_on_rq_migrating(p))) return rq; raw_spin_unlock(&rq->lock); raw_spin_unlock_irqrestore(&p->pi_lock, *flags); + + while (unlikely(task_on_rq_migrating(p))) + cpu_relax(); } } @@ -1678,7 +1684,7 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags) success = 1; /* we're going to change ->state */ cpu = task_cpu(p); - if (task_on_rq_queued(p) && ttwu_remote(p, wake_flags)) + if (p->on_rq && ttwu_remote(p, wake_flags)) goto stat; #ifdef CONFIG_SMP diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index 26566d0c67ac..aa0f73ba3777 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -17,6 +17,7 @@ struct rq; /* task_struct::on_rq states: */ #define TASK_ON_RQ_QUEUED 1 +#define TASK_ON_RQ_MIGRATING 2 extern __read_mostly int scheduler_running; @@ -950,6 +951,11 @@ static inline int task_on_rq_queued(struct task_struct *p) return p->on_rq == TASK_ON_RQ_QUEUED; } +static inline int task_on_rq_migrating(struct task_struct *p) +{ + return p->on_rq == TASK_ON_RQ_MIGRATING; +} + #ifndef prepare_arch_switch # define prepare_arch_switch(next) do { } while (0) #endif -- 2.39.5