Re: [PATCH v2 net 1/7] net/sched: taprio: fix too early schedules switching

["Abdul Rahim, Faizal" <faizal.abdul.rahim@linux.intel.com>]

On 9/11/2023 6:27 am, Vladimir Oltean wrote:
> On Tue, Nov 07, 2023 at 06:20:17AM -0500, Faizal Rahim wrote:
>> In the current taprio code for dynamic schedule change,
>> admin/oper schedule switching happens immediately when
>> should_change_schedules() is true. Then the last entry of
>> the old admin schedule stops being valid anymore from
>> taprio_dequeue_from_txq’s perspective.
> 
> Admittedly, I may have become a bit detached from this code base in the
> past months, but I don't understand the reasoning here.
> 
> Could you please explain what makes the last entry of the old admin
> schedule be invalid from taprio_dequeue_from_txq()'s perspective?
> 
> What I see is that when should_change_schedules() is true, we change
> q->oper_sched and q->admin_sched through the switch_schedules() call,
> but we don't change q->current_entry, so I fail to understand the
> connection you are implying.

My bad – I used part of the explanation from the original author without 
thoroughly checking it. I have some guesses about 
taprio_dequeue_from_txq(), but they're not solid without more testing.

So, I'll swap it with your suggestion below, which highlights the obvious 
issue.

> On the other hand (and I see I did mention this in the other thread),
> it seems that taprio_skb_exceeds_queue_max_sdu() - called from the
> enqueue() path - looks at q->oper_sched, and that's a valid reason why
> we'd want to delay the schedule switch until admin's actual base time,
> rather than the current oper's cycle_end_time.

Agree, observed this too.


> But please, let's spare no expense in providing a proper problem
> description, justification for the change and Fixes: tag. This is not
> optional.
> 
>> To solve this, we have to delay the switch_schedules() call via
>> the new cycle_time_correction variable. The variable serves 2
>> purposes:
>> 1. Upon entering advance_sched(), if the value is set to a
>> non-initialized value, it indicates that we need to change
>> schedule.
>> 2. Store the cycle time correction value which will be used for
>> negative or positive correction.
> 
> It needs to be stated much more clearly that only purpose 1 is relevant
> here (I would even go as far as to omit its secondary purpose here).
> The only reason we are using the correction variable is because it
> happens that we'll need that in later changes.

Got it.

> 
>>
>> Fixes: a3d43c0d56f1 ("taprio: Add support adding an admin schedule")
> 
> I believe that since the only observable problem has to do with
> taprio_skb_exceeds_queue_max_sdu(), the Fixes: tag should be the commit
> which added that logic. Which is:
> 
> Fixes: a878fd46fe43 ("net/sched: keep the max_frm_len information inside struct sched_gate_list")

Will replace the patch explanation with this. Thanks.

>> Signed-off-by: Faizal Rahim <faizal.abdul.rahim@linux.intel.com>
>> ---
>>   net/sched/sch_taprio.c | 9 +++++++--
>>   1 file changed, 7 insertions(+), 2 deletions(-)
>>
>> diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
>> index 2e1949de4171..dee103647823 100644
>> --- a/net/sched/sch_taprio.c
>> +++ b/net/sched/sch_taprio.c
>> @@ -41,6 +41,7 @@ static struct static_key_false taprio_have_working_mqprio;
>>   #define TXTIME_ASSIST_IS_ENABLED(flags) ((flags) & TCA_TAPRIO_ATTR_FLAG_TXTIME_ASSIST)
>>   #define FULL_OFFLOAD_IS_ENABLED(flags) ((flags) & TCA_TAPRIO_ATTR_FLAG_FULL_OFFLOAD)
>>   #define TAPRIO_FLAGS_INVALID U32_MAX
>> +#define INIT_CYCLE_TIME_CORRECTION S64_MIN
> 
> I would prefer naming it CYCLE_TIME_CORRECTION_INVALID or _UNSPEC.
> It is not just used as the "initial" value.
> 
>>   
>>   struct sched_entry {
>>   	/* Durations between this GCL entry and the GCL entry where the
>> @@ -75,6 +76,7 @@ struct sched_gate_list {
>>   	ktime_t cycle_end_time;
>>   	s64 cycle_time;
>>   	s64 cycle_time_extension;
>> +	s64 cycle_time_correction;
>>   	s64 base_time;
>>   };
>>   
>> @@ -940,8 +942,10 @@ static enum hrtimer_restart advance_sched(struct hrtimer *timer)
>>   	admin = rcu_dereference_protected(q->admin_sched,
>>   					  lockdep_is_held(&q->current_entry_lock));
>>   
>> -	if (!oper)
>> +	if (!oper || oper->cycle_time_correction != INIT_CYCLE_TIME_CORRECTION) {
> 
> You could introduce even as early as this change a "static bool
> sched_switch_pending(struct sched_gate_list *oper)" function, which
> incorporates the entire body of this "if" expression.
> 
>> +		oper->cycle_time_correction = INIT_CYCLE_TIME_CORRECTION;
>>   		switch_schedules(q, &admin, &oper);
>> +	}
>>   
>>   	/* This can happen in two cases: 1. this is the very first run
>>   	 * of this function (i.e. we weren't running any schedule
>> @@ -981,7 +985,7 @@ static enum hrtimer_restart advance_sched(struct hrtimer *timer)
>>   		 * schedule runs.
>>   		 */
>>   		end_time = sched_base_time(admin);
>> -		switch_schedules(q, &admin, &oper);
>> +		oper->cycle_time_correction = 0;
>>   	}
>>   
>>   	next->end_time = end_time;
>> @@ -1174,6 +1178,7 @@ static int parse_taprio_schedule(struct taprio_sched *q, struct nlattr **tb,
>>   	}
>>   
>>   	taprio_calculate_gate_durations(q, new);
>> +	new->cycle_time_correction = INIT_CYCLE_TIME_CORRECTION;
>>   
>>   	return 0;
>>   }
>> -- 
>> 2.25.1
>>