linux-kernel - Re: [PATCH v3 1/2] sched/core: Avoid obvious double update_rq

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Message-ID: <0058249a-5f9f-9f67-db5b-f7c3c10c3729@arm.com>
Date:   Fri, 29 Apr 2022 14:59:16 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Hao Jia <jiahao.os@...edance.com>, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, rostedt@...dmis.org,
        bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] sched/core: Avoid obvious double update_rq_clock
 warning

On 27/04/2022 10:00, Hao Jia wrote:

[...]

LGTM, comments are only nit-picks.

> Since we directly use raw_spin_rq_lock() to acquire rq lock instead of
> rq_lock(), there is no corresponding change to rq->clock_update_flags.
> In particular, we have obtained the rq lock of other cores,

s/cores/CPUs, with SMT, a core can have multiple (logical) CPUs.

[...]

> So we need to clear RQCF_UPDATED of rq->clock_update_flags synchronously
> to avoid the WARN_DOUBLE_CLOCK warning.

Why you mention `synchronously` here? Isn't this obvious? (1)

[...]

> @@ -1833,6 +1833,7 @@ select_task_rq_dl(struct task_struct *p, int cpu, int flags)
>  static void migrate_task_rq_dl(struct task_struct *p, int new_cpu __maybe_unused)
>  {
>  	struct rq *rq;
> +	struct rq_flags rf;

-       struct rq *rq;
        struct rq_flags rf;
+       struct rq *rq;

Use reverse fir tree order for variable declarations.
(./Documentation/process/maintainer-tip.rst)

[...]

> +#ifdef CONFIG_SCHED_DEBUG
> +/*
> + * In double_lock_balance()/double_rq_lock(), we use raw_spin_rq_lock() to acquire
> + * rq lock instead of rq_lock(). So at the end of these two functions we need to
> + * call double_rq_clock_clear_update() synchronously to clear RQCF_UPDATED of
                                          ^^^^^^^^^^^^^
                                          (1)

> + * rq->clock_update_flags to avoid the WARN_DOUBLE_CLOCK warning.
> + */
> +static inline void double_rq_clock_clear_update(struct rq *rq1, struct rq *rq2)
> +{
> +	rq1->clock_update_flags &= (RQCF_REQ_SKIP|RQCF_ACT_SKIP);
> +	/*
> +	 * If CONFIG_SMP is not defined, rq1 and rq2 are the same,
> +	 * so we just clear RQCF_UPDATED one of them.
> +	 */

Maybe shorter:

      /* rq1 == rq2 for !CONFIG_SMP, so just clear RQCF_UPDATED once. */

[...]

> @@ -2543,14 +2564,15 @@ static inline int _double_lock_balance(struct rq *this_rq, struct rq *busiest)
>  	__acquires(busiest->lock)
>  	__acquires(this_rq->lock)
>  {
> -	if (__rq_lockp(this_rq) == __rq_lockp(busiest))
> -		return 0;
> -
> -	if (likely(raw_spin_rq_trylock(busiest)))
> +	if (__rq_lockp(this_rq) == __rq_lockp(busiest) ||
> +		likely(raw_spin_rq_trylock(busiest))) {

indention:

        if (__rq_lockp(this_rq) == __rq_lockp(busiest) ||
-               likely(raw_spin_rq_trylock(busiest))) {
+           likely(raw_spin_rq_trylock(busiest))) {

[...]

Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>

Tested on Arm64 Juno with mainline & preempt-rt (linux-5.17.y-rt).