Re: [External] Re: [PATCH v3 1/2] sched/core: Avoid obvious double update_rq_clock warning

[Hao Jia]

On 2022/4/29 Dietmar Eggemann wrote:
> 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.

Thanks for your review comments.
I will do it in patch v4.
Thanks.

> [...]
>
> > 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)

I will do it in patch v4.
Thanks.

> [...]
>
> > @@ -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)

I will do it in patch v4.
Thanks.

> [...]
>
> > +#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. */

I will do it in patch v4.
Thanks.
> [...]
>
> > @@ -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))) {

Thanks again for your review and suggestion.
I will do it in patch v4.
Thanks.

> [...]
>
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
>
> Tested on Arm64 Juno with mainline & preempt-rt (linux-5.17.y-rt).