Re: [PATCH v2 07/35] sched: define *_tsk_need_resched_lazy() helpers

[Peter Zijlstra <peterz@infradead.org>]

On Mon, May 27, 2024 at 05:34:53PM -0700, Ankur Arora wrote:
> Define __{set,test}_tsk_need_resched() to test for the immediacy of the
> need-resched.
> 
> The current helpers, {set,test}_tsk_need_resched(...) stay the same.
> 
> In scheduler code, switch to the more explicit variants,
> __set_tsk_need_resched(...), __test_tsk_need_resched(...).
> 
> Note that clear_tsk_need_resched() is only used from __schedule()
> to clear the flags before switching context. Now it clears all the
> need-resched flags.
> 
> Cc: Peter Ziljstra <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> Cc: Paul E. McKenney <paulmck@kernel.org>
> Originally-by: Thomas Gleixner <tglx@linutronix.de>
> Link: https://lore.kernel.org/lkml/87jzshhexi.ffs@tglx/
> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
> ---
>  include/linux/sched.h   | 45 +++++++++++++++++++++++++++++++++++++----
>  kernel/sched/core.c     |  9 +++++----
>  kernel/sched/deadline.c |  4 ++--
>  kernel/sched/fair.c     |  2 +-
>  kernel/sched/rt.c       |  4 ++--
>  5 files changed, 51 insertions(+), 13 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 37a51115b691..804a76e6f3c5 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1952,19 +1952,56 @@ static inline bool test_tsk_thread_flag(struct task_struct *tsk, int flag)
>  	return test_ti_thread_flag(task_thread_info(tsk), flag);
>  }
>  
> -static inline void set_tsk_need_resched(struct task_struct *tsk)
> +/*
> + * With !CONFIG_PREEMPT_AUTO, tif_resched(RESCHED_LAZY) reduces to
> + * tif_resched(RESCHED_NOW). Add a check in the helpers below to ensure
> + * we don't touch the tif_reshed(RESCHED_NOW) bit unnecessarily.
> + */
> +static inline void __set_tsk_need_resched(struct task_struct *tsk, resched_t rs)
>  {
> -	set_tsk_thread_flag(tsk,TIF_NEED_RESCHED);
> +	if (IS_ENABLED(CONFIG_PREEMPT_AUTO) || rs == RESCHED_NOW)
> +		set_tsk_thread_flag(tsk, tif_resched(rs));
> +	else
> +		/*
> +		 * RESCHED_LAZY is only touched under CONFIG_PREEMPT_AUTO.
> +		 */
> +		BUG();
>  }

This straight up violates coding style and would require a dose of {}.

	if (!IS_ENABLED(CONFIG_PREEMPT_AUTO && rs == RESCHED_LAZY)
		BUG();

	set_tsk_thread_flag(tsk, tif_resched(rs));

seems much saner to me.

>  static inline void clear_tsk_need_resched(struct task_struct *tsk)
>  {
> -	clear_tsk_thread_flag(tsk,TIF_NEED_RESCHED);
> +	clear_tsk_thread_flag(tsk, tif_resched(RESCHED_NOW));
> +
> +	if (IS_ENABLED(CONFIG_PREEMPT_AUTO))
> +		clear_tsk_thread_flag(tsk, tif_resched(RESCHED_LAZY));
> +}
> +
> +static inline bool __test_tsk_need_resched(struct task_struct *tsk, resched_t rs)
> +{
> +	if (IS_ENABLED(CONFIG_PREEMPT_AUTO) || rs == RESCHED_NOW)
> +		return unlikely(test_tsk_thread_flag(tsk, tif_resched(rs)));
> +	else
> +		return false;
>  }

	if (!IS_ENABLED(CONFIG_PREEMPT_AUTO) && rs == RESCHED_LAZY)
		return false;

	return unlikely(test_tsk_thread_flag(tsk, tif_resched(rs)));

>  
>  static inline bool test_tsk_need_resched(struct task_struct *tsk)
>  {
> -	return unlikely(test_tsk_thread_flag(tsk,TIF_NEED_RESCHED));
> +	return __test_tsk_need_resched(tsk, RESCHED_NOW);
> +}
> +
> +static inline bool test_tsk_need_resched_lazy(struct task_struct *tsk)
> +{
> +	return __test_tsk_need_resched(tsk, RESCHED_LAZY);
> +}
> +
> +static inline void set_tsk_need_resched(struct task_struct *tsk)
> +{
> +	return __set_tsk_need_resched(tsk, RESCHED_NOW);
> +}
> +
> +static inline void set_tsk_need_resched_lazy(struct task_struct *tsk)
> +{
> +	return __set_tsk_need_resched(tsk, RESCHED_LAZY);
>  }
>  
>  /*

So far so good, however:

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 7019a40457a6..d00d7b45303e 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -933,7 +933,7 @@ static bool set_nr_if_polling(struct task_struct *p)
>  #else
>  static inline bool set_nr_and_not_polling(struct task_struct *p)
>  {
> -	set_tsk_need_resched(p);
> +	__set_tsk_need_resched(p, RESCHED_NOW);
>  	return true;
>  }
>  
> @@ -1045,13 +1045,13 @@ void resched_curr(struct rq *rq)
>  
>  	lockdep_assert_rq_held(rq);
>  
> -	if (test_tsk_need_resched(curr))
> +	if (__test_tsk_need_resched(curr, RESCHED_NOW))
>  		return;
>  
>  	cpu = cpu_of(rq);
>  
>  	if (cpu == smp_processor_id()) {
> -		set_tsk_need_resched(curr);
> +		__set_tsk_need_resched(curr, RESCHED_NOW);
>  		set_preempt_need_resched();
>  		return;
>  	}
> @@ -2245,7 +2245,8 @@ void wakeup_preempt(struct rq *rq, struct task_struct *p, int flags)
>  	 * A queue event has occurred, and we're going to schedule.  In
>  	 * this case, we can save a useless back to back clock update.
>  	 */
> -	if (task_on_rq_queued(rq->curr) && test_tsk_need_resched(rq->curr))
> +	if (task_on_rq_queued(rq->curr) &&
> +	    __test_tsk_need_resched(rq->curr, RESCHED_NOW))
>  		rq_clock_skip_update(rq);
>  }
>  
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index a04a436af8cc..d24d6bfee293 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -2035,7 +2035,7 @@ static void wakeup_preempt_dl(struct rq *rq, struct task_struct *p,
>  	 * let us try to decide what's the best thing to do...
>  	 */
>  	if ((p->dl.deadline == rq->curr->dl.deadline) &&
> -	    !test_tsk_need_resched(rq->curr))
> +	    !__test_tsk_need_resched(rq->curr, RESCHED_NOW))
>  		check_preempt_equal_dl(rq, p);
>  #endif /* CONFIG_SMP */
>  }
> @@ -2564,7 +2564,7 @@ static void pull_dl_task(struct rq *this_rq)
>  static void task_woken_dl(struct rq *rq, struct task_struct *p)
>  {
>  	if (!task_on_cpu(rq, p) &&
> -	    !test_tsk_need_resched(rq->curr) &&
> +	    !__test_tsk_need_resched(rq->curr, RESCHED_NOW) &&
>  	    p->nr_cpus_allowed > 1 &&
>  	    dl_task(rq->curr) &&
>  	    (rq->curr->nr_cpus_allowed < 2 ||
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index c62805dbd608..c5171c247466 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8316,7 +8316,7 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int
>  	 * prevents us from potentially nominating it as a false LAST_BUDDY
>  	 * below.
>  	 */
> -	if (test_tsk_need_resched(curr))
> +	if (__test_tsk_need_resched(curr, RESCHED_NOW))
>  		return;
>  
>  	/* Idle tasks are by definition preempted by non-idle tasks. */
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 3261b067b67e..f0a6c9bb890b 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1680,7 +1680,7 @@ static void wakeup_preempt_rt(struct rq *rq, struct task_struct *p, int flags)
>  	 * to move current somewhere else, making room for our non-migratable
>  	 * task.
>  	 */
> -	if (p->prio == rq->curr->prio && !test_tsk_need_resched(rq->curr))
> +	if (p->prio == rq->curr->prio && !__test_tsk_need_resched(rq->curr, RESCHED_NOW))
>  		check_preempt_equal_prio(rq, p);
>  #endif
>  }
> @@ -2415,7 +2415,7 @@ static void pull_rt_task(struct rq *this_rq)
>  static void task_woken_rt(struct rq *rq, struct task_struct *p)
>  {
>  	bool need_to_push = !task_on_cpu(rq, p) &&
> -			    !test_tsk_need_resched(rq->curr) &&
> +			    !__test_tsk_need_resched(rq->curr, RESCHED_NOW) &&
>  			    p->nr_cpus_allowed > 1 &&
>  			    (dl_task(rq->curr) || rt_task(rq->curr)) &&
>  			    (rq->curr->nr_cpus_allowed < 2 ||

These are all NO-OPs.... Changelog says:
 
> In scheduler code, switch to the more explicit variants,
> __set_tsk_need_resched(...), __test_tsk_need_resched(...).

But leaves me wondering *WHY* ?!?

I can't help but feel this patch attempts to do 2 things and fails to
justify at least one of them.