netdev - Re: [PATCH v2 net 5/7] net/sched: taprio: fix delayed switching to new schedule after timer expiry

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Date: Thu, 9 Nov 2023 13:50:52 +0200
From: Vladimir Oltean <vladimir.oltean@....com>
To: Faizal Rahim <faizal.abdul.rahim@...ux.intel.com>
Cc: Vinicius Costa Gomes <vinicius.gomes@...el.com>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	Cong Wang <xiyou.wangcong@...il.com>, Jiri Pirko <jiri@...nulli.us>,
	"David S . Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 net 5/7] net/sched: taprio: fix delayed switching to
 new schedule after timer expiry

On Tue, Nov 07, 2023 at 06:20:21AM -0500, Faizal Rahim wrote:
> If a new GCL is triggered and the new admin base time falls before the
> expiry of advance_timer (current running entry from oper),
> taprio_start_sched() resets the current advance_timer expiry to the
> new admin base time. However, upon expiry, advance_sched() doesn't
> immediately switch to the admin schedule. It continues running entries
> from the old oper schedule, and only switches to the new admin schedule
> much later. Ideally, if the advance_timer is shorten to align with the
> new admin base time, when the timer expires, advance_sched() should
> trigger switch_schedules() at the beginning.
> 
> To resolve this issue, set the cycle_time_correction to a non-initialized
> value in taprio_start_sched(). advance_sched() will use it to initiate
> switch_schedules() at the beginning.
> 
> Fixes: a3d43c0d56f1 ("taprio: Add support adding an admin schedule")

Did the commit you blame really introduce this issue, or was it your
rework to trigger switch_schedules() based on the correction?

> Signed-off-by: Faizal Rahim <faizal.abdul.rahim@...ux.intel.com>
> ---
>  net/sched/sch_taprio.c | 21 ++++++++++++++++++---
>  1 file changed, 18 insertions(+), 3 deletions(-)
> 
> diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
> index f18a5fe12f0c..01b114edec30 100644
> --- a/net/sched/sch_taprio.c
> +++ b/net/sched/sch_taprio.c
> @@ -1379,14 +1379,19 @@ static void setup_first_end_time(struct taprio_sched *q,
>  }
>  
>  static void taprio_start_sched(struct Qdisc *sch,
> -			       ktime_t start, struct sched_gate_list *new)
> +			       ktime_t new_base_time,
> +			       struct sched_gate_list *new)
>  {
>  	struct taprio_sched *q = qdisc_priv(sch);
> -	ktime_t expires;
> +	struct sched_gate_list *oper = NULL;
> +	ktime_t expires, start;
>  
>  	if (FULL_OFFLOAD_IS_ENABLED(q->flags))
>  		return;
>  
> +	oper = rcu_dereference_protected(q->oper_sched,
> +					 lockdep_is_held(&q->current_entry_lock));
> +
>  	expires = hrtimer_get_expires(&q->advance_timer);
>  	if (expires == 0)
>  		expires = KTIME_MAX;
> @@ -1395,7 +1400,17 @@ static void taprio_start_sched(struct Qdisc *sch,
>  	 * reprogram it to the earliest one, so we change the admin
>  	 * schedule to the operational one at the right time.
>  	 */
> -	start = min_t(ktime_t, start, expires);
> +	start = min_t(ktime_t, new_base_time, expires);
> +
> +	if (expires != KTIME_MAX &&
> +	    ktime_compare(start, new_base_time) == 0) {
> +		/* Since timer was changed to align to the new admin schedule,
> +		 * setting the variable below to a non-initialized value will

I find the wording "setting the variable below to a non-initialized value"
confusing. 0 is non-initialized? You're talking about a value different
than INIT_CYCLE_TIME_CORRECTION. What about "setting a specific cycle
correction will indicate ..."?

> +		 * indicate to advance_sched() to call switch_schedules() after
> +		 * this timer expires.
> +		 */
> +		oper->cycle_time_correction = 0;

Why 0 and not ktime_sub(new_base_time, oper->cycle_end_time)? Doesn't
the precise correction value make a difference?

> +	}
>  
>  	hrtimer_start(&q->advance_timer, start, HRTIMER_MODE_ABS);
>  }
> -- 
> 2.25.1
>