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

Vladimir Oltean
2023-11-09T11:51:01Z

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@linux.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
>