Re: [PATCH v2] Adjustments: lock/unlock task in context_switch

[Rodrigo Siqueira]

Thanks for the review :)

Below I just have a small comment in the changed version of the patch

> Thanks; I've slightly changed it, find below. I'll queue it for the next
> merge window.
> 
> ---
> Subject: sched: Rework / clarify prepare_lock_switch()
> From: rodrigosiqueira <rodrigosiqueiramelo@xxxxxxxxx>
> Date: Fri, 15 Dec 2017 12:06:03 -0200
> 
> The function prepare_lock_switch has an unused parameter, and also the
> function name was not descriptive. To improve the readability and remove
> the extra parameter, the following changes were made:
> 
> * Moved prepare_lock_switch from kernel/sched/sched.h to
>   kernel/sched/core.c, renamed it to acquire_task, and removed the
>   unused parameter.
> 
> * Split the smp_store_release() out from finish_lock_switch() to a
>   function named release_task.
> 
> * Comments ajdustments.
> 
> Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@xxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Link: http://lkml.kernel.org/r/20171215140603.gxe5i2y6fg5ojfpp@xxxxxxxxxxxxxx
> ---
>  kernel/sched/core.c  |   53 +++++++++++++++++++++++++++++++++++++++++++++++----
>  kernel/sched/sched.h |   41 ---------------------------------------
>  2 files changed, 49 insertions(+), 45 deletions(-)
> 
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2045,7 +2045,7 @@ try_to_wake_up(struct task_struct *p, un
>  	 * If the owning (remote) CPU is still in the middle of schedule() with
>  	 * this task as prev, wait until its done referencing the task.
>  	 *
> -	 * Pairs with the smp_store_release() in finish_lock_switch().
> +	 * Pairs with the smp_store_release() in release_task().
>  	 *
>  	 * This ensures that tasks getting woken will be fully ordered against
>  	 * their previous state and preserve Program Order.
> @@ -2571,6 +2571,50 @@ fire_sched_out_preempt_notifiers(struct
>  
>  #endif /* CONFIG_PREEMPT_NOTIFIERS */
>  
> +static inline void acquire_task(struct task_struct *next)

In the original patch, I called this function as release_lock_task,
because the release_task was already declared as extern in
include/linux/sched/task.h. I believe there is a function name conflict
here, is that correct?

> +{
> +#ifdef CONFIG_SMP
> +	/*
> +	 * Claim the task as running, we do this before switching to it
> +	 * such that any running task will have this set.
> +	 */
> +	next->on_cpu = 1;
> +#endif
> +}
> +
> +static inline void release_task(struct task_struct *prev)
> +{
> +#ifdef CONFIG_SMP
> +	/*
> +	 * After ->on_cpu is cleared, the task can be moved to a different CPU.
> +	 * We must ensure this doesn't happen until the switch is completely
> +	 * finished.
> +	 *
> +	 * In particular, the load of prev->state in finish_task_switch() must
> +	 * happen before this.
> +	 *
> +	 * Pairs with the smp_cond_load_acquire() in try_to_wake_up().
> +	 */
> +	smp_store_release(&prev->on_cpu, 0);
> +#endif
> +}
> +
> +static inline void finish_lock_switch(struct rq *rq)
> +{
> +#ifdef CONFIG_DEBUG_SPINLOCK
> +	/* this is a valid case when another task releases the spinlock */
> +	rq->lock.owner = current;
> +#endif
> +	/*
> +	 * If we are tracking spinlock dependencies then we have to
> +	 * fix up the runqueue lock - which gets 'carried over' from
> +	 * prev into current:
> +	 */
> +	spin_acquire(&rq->lock.dep_map, 0, 0, _THIS_IP_);
> +
> +	raw_spin_unlock_irq(&rq->lock);
> +}
> +
>  /**
>   * prepare_task_switch - prepare to switch tasks
>   * @rq: the runqueue preparing to switch
> @@ -2591,7 +2635,7 @@ prepare_task_switch(struct rq *rq, struc
>  	sched_info_switch(rq, prev, next);
>  	perf_event_task_sched_out(prev, next);
>  	fire_sched_out_preempt_notifiers(prev, next);
> -	prepare_lock_switch(rq, next);
> +	acquire_task(next);
>  	prepare_arch_switch(next);
>  }
>  
> @@ -2646,7 +2690,7 @@ static struct rq *finish_task_switch(str
>  	 * the scheduled task must drop that reference.
>  	 *
>  	 * We must observe prev->state before clearing prev->on_cpu (in
> -	 * finish_lock_switch), otherwise a concurrent wakeup can get prev
> +	 * release_task), otherwise a concurrent wakeup can get prev
>  	 * running on another CPU and we could rave with its RUNNING -> DEAD
>  	 * transition, resulting in a double drop.
>  	 */
> @@ -2663,7 +2707,8 @@ static struct rq *finish_task_switch(str
>  	 * to use.
>  	 */
>  	smp_mb__after_unlock_lock();
> -	finish_lock_switch(rq, prev);
> +	release_task(prev);
> +	finish_lock_switch(rq);
>  	finish_arch_post_lock_switch();
>  
>  	fire_sched_in_preempt_notifiers(current);
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1328,47 +1328,6 @@ static inline int task_on_rq_migrating(s
>  # define finish_arch_post_lock_switch()	do { } while (0)
>  #endif
>  
> -static inline void prepare_lock_switch(struct rq *rq, struct task_struct *next)
> -{
> -#ifdef CONFIG_SMP
> -	/*
> -	 * We can optimise this out completely for !SMP, because the
> -	 * SMP rebalancing from interrupt is the only thing that cares
> -	 * here.
> -	 */
> -	next->on_cpu = 1;
> -#endif
> -}
> -
> -static inline void finish_lock_switch(struct rq *rq, struct task_struct *prev)
> -{
> -#ifdef CONFIG_SMP
> -	/*
> -	 * After ->on_cpu is cleared, the task can be moved to a different CPU.
> -	 * We must ensure this doesn't happen until the switch is completely
> -	 * finished.
> -	 *
> -	 * In particular, the load of prev->state in finish_task_switch() must
> -	 * happen before this.
> -	 *
> -	 * Pairs with the smp_cond_load_acquire() in try_to_wake_up().
> -	 */
> -	smp_store_release(&prev->on_cpu, 0);
> -#endif
> -#ifdef CONFIG_DEBUG_SPINLOCK
> -	/* this is a valid case when another task releases the spinlock */
> -	rq->lock.owner = current;
> -#endif
> -	/*
> -	 * If we are tracking spinlock dependencies then we have to
> -	 * fix up the runqueue lock - which gets 'carried over' from
> -	 * prev into current:
> -	 */
> -	spin_acquire(&rq->lock.dep_map, 0, 0, _THIS_IP_);
> -
> -	raw_spin_unlock_irq(&rq->lock);
> -}
> -
>  /*
>   * wake flags
>   */