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

[Abdul Rahim, Faizal]

On 9/11/2023 7:50 pm, Vladimir Oltean wrote:
> 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?

Ohh actually this issue happens even without my whole patch set.

> > Signed-off-by: Faizal Rahim <faizal.abdul.rahim@xxxxxxxxxxxxxxx>
> > ---
> >   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 ..."?

Sure

> > +		 * 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?

Negative correction and its calculation is a separate problem handled in 
different patch.

My intention is to highlight a specific issue and address it with a single 
patch. The core problem stemmed from the new admin schedule not making an 
immediate transition in advance_sched().

I'll rework this patch to focus specifically on resolving this problem 
while gradually aligning with the overall series. Importantly, I won't be 
removing anything from this patch in the process.

Is that okay ?

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