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

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

On 12/11/2023 6:31 pm, Simon Horman 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.
>>
>> 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.
>>
>> Fixes: a3d43c0d56f1 ("taprio: Add support adding an admin schedule")
>> 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
>>   
>>   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) {
> 
> Hi Faizal,
> 
> The first condition above expects that oper may be NULL, but the line below
> dereferences it unconditionally. This doesn't seem correct, one way or the
> other.
> 

Thanks for pointing this out, sorry for this blunder.
Will update.

> As flagged by Smatch and Coccinelle.
> 
>> +		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
> 
> ...
>