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Date: Tue, 19 Dec 2017 12:23:57 -0200
From: Rodrigo Siqueira <rodrigosiqueiramelo@...il.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...nel.org>, kernel-janitors@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] Adjustments: lock/unlock task in context_switch
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@...il.com>
> 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@...il.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Link: http://lkml.kernel.org/r/20171215140603.gxe5i2y6fg5ojfpp@smtp.gmail.com
> ---
> 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
> */
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