linux-kernel - Re: [RFC PATCH 39/86] sched: handle lazy resched in set_nr_*

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Message-ID: <20231108091508.GJ8262@noisy.programming.kicks-ass.net>
Date:   Wed, 8 Nov 2023 10:15:08 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ankur Arora <ankur.a.arora@...cle.com>
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de,
        torvalds@...ux-foundation.org, paulmck@...nel.org,
        linux-mm@...ck.org, x86@...nel.org, akpm@...ux-foundation.org,
        luto@...nel.org, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com, mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, willy@...radead.org, mgorman@...e.de,
        jon.grimm@....com, bharata@....com, raghavendra.kt@....com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com,
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        krypton@...ich-teichert.org, rostedt@...dmis.org,
        David.Laight@...lab.com, richard@....at, mjguzik@...il.com
Subject: Re: [RFC PATCH 39/86] sched: handle lazy resched in
 set_nr_*_polling()

On Tue, Nov 07, 2023 at 01:57:25PM -0800, Ankur Arora wrote:
> To trigger a reschedule on a target runqueue a few things need
> to happen first:
> 
>   1. set_tsk_need_resched(target_rq->curr, RESCHED_eager)
>   2. ensure that the target CPU sees the need-resched bit
>   3. preempt_fold_need_resched()
> 
> Most of this is done via some combination of: resched_curr(),
> set_nr_if_polling(), and set_nr_and_not_polling().
> 
> Update the last two to also handle TIF_NEED_RESCHED_LAZY.
> 
> One thing to note is that TIF_NEED_RESCHED_LAZY has run to completion
> semantics, so unlike TIF_NEED_RESCHED, we don't need to ensure that
> the caller sees it, and of course there is no preempt folding.
> 
> Originally-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
> ---
>  kernel/sched/core.c | 17 +++++++++--------
>  1 file changed, 9 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index e2215c417323..01df5ac2982c 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -892,14 +892,15 @@ static inline void hrtick_rq_init(struct rq *rq)
>  
>  #if defined(CONFIG_SMP) && defined(TIF_POLLING_NRFLAG)
>  /*
> - * Atomically set TIF_NEED_RESCHED and test for TIF_POLLING_NRFLAG,
> + * Atomically set TIF_NEED_RESCHED[_LAZY] and test for TIF_POLLING_NRFLAG,
>   * this avoids any races wrt polling state changes and thereby avoids
>   * spurious IPIs.
>   */
> -static inline bool set_nr_and_not_polling(struct task_struct *p)
> +static inline bool set_nr_and_not_polling(struct task_struct *p, resched_t rs)
>  {
>  	struct thread_info *ti = task_thread_info(p);
> -	return !(fetch_or(&ti->flags, _TIF_NEED_RESCHED) & _TIF_POLLING_NRFLAG);
> +
> +	return !(fetch_or(&ti->flags, _tif_resched(rs)) & _TIF_POLLING_NRFLAG);
>  }

Argh, this it making the whole thing even worse, because now you're
using that eager naming for setting which has the exact opposite meaning
from testing.

> @@ -916,7 +917,7 @@ static bool set_nr_if_polling(struct task_struct *p)
>  	for (;;) {
>  		if (!(val & _TIF_POLLING_NRFLAG))
>  			return false;
> -		if (val & _TIF_NEED_RESCHED)
> +		if (val & (_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY))
>  			return true;
>  		if (try_cmpxchg(&ti->flags, &val, val | _TIF_NEED_RESCHED))
>  			break;

Depending on the exact semantics of LAZY this could be wrong, the
Changeog doesn't clarify.

Changing this in a different patch from resched_curr() makes it
impossible to review :/