Re: [PATCH V2 1/1] Improved RTDS scheduler

[Tianyang Chen <tiche@seas.upenn.edu>]

On 12/31/2015 8:44 AM, Meng Xu wrote:
> On Thu, Dec 31, 2015 at 6:20 PM, Tianyang Chen <tiche@seas.upenn.edu> wrote:
>>
>> Budget replenishment is now handled by a dedicated timer which is
>> triggered at the most imminent release time of all runnable vcpus.
>>
>> Changes since V1:
>>      None
>>
>> Signed-off-by: Tianyang Chen <tiche@seas.upenn.edu>
>> Signed-off-by: Meng Xu <mengxu@cis.upenn.edu>
>> Signed-off-by: Dagaen Golomb <dgolomb@seas.upenn.edu>
>> ---
>>   xen/common/sched_rt.c |  159 +++++++++++++++++++++++++++++--------------------
>>   1 file changed, 95 insertions(+), 64 deletions(-)
>>
>> diff --git a/xen/common/sched_rt.c b/xen/common/sched_rt.c
>> index 4372486..d522272 100644
>> --- a/xen/common/sched_rt.c
>> +++ b/xen/common/sched_rt.c
>> @@ -16,6 +16,7 @@
>>   #include <xen/delay.h>
>>   #include <xen/event.h>
>>   #include <xen/time.h>
>> +#include <xen/timer.h>
>>   #include <xen/perfc.h>
>>   #include <xen/sched-if.h>
>>   #include <xen/softirq.h>
>> @@ -147,6 +148,16 @@ static unsigned int nr_rt_ops;
>>    * Global lock is referenced by schedule_data.schedule_lock from all
>>    * physical cpus. It can be grabbed via vcpu_schedule_lock_irq()
>>    */
>> +
>> +/* dedicated timer for replenishment */
>> +static struct timer repl_timer;
>> +
>> +/* controls when to first start the timer*/
>
>
> missing a space between * and / at the end of this line.
>
>
>>
>> +static int timer_started;
>> +
>> +/* handler for the replenishment timer */
>> +static void repl_handler(void *data);
>> +
>>   struct rt_private {
>>       spinlock_t lock;            /* the global coarse grand lock */
>>       struct list_head sdom;      /* list of availalbe domains, used for dump */
>> @@ -426,6 +437,7 @@ __runq_insert(const struct scheduler *ops, struct rt_vcpu *svc)
>>   static int
>>   rt_init(struct scheduler *ops)
>>   {
>> +    const int cpu = smp_processor_id();
>
>
> Did the "cpu" be used? If not, please remove it here.
>
>
>>
>>       struct rt_private *prv = xzalloc(struct rt_private);
>>
>>       printk("Initializing RTDS scheduler\n"
>> @@ -454,6 +466,8 @@ rt_init(struct scheduler *ops)
>>
>>       ops->sched_data = prv;
>>
>> +    init_timer(&repl_timer, repl_handler, ops, 0);
>> +
>>       return 0;
>>
>>    no_mem:
>> @@ -473,6 +487,9 @@ rt_deinit(const struct scheduler *ops)
>>           xfree(_cpumask_scratch);
>>           _cpumask_scratch = NULL;
>>       }
>> +
>> +    kill_timer(&repl_timer);
>> +
>>       xfree(prv);
>>   }
>>
>> @@ -635,6 +652,13 @@ rt_vcpu_insert(const struct scheduler *ops, struct vcpu *vc)
>>
>>       /* add rt_vcpu svc to scheduler-specific vcpu list of the dom */
>>       list_add_tail(&svc->sdom_elem, &svc->sdom->vcpu);
>> +
>> +    if(!timer_started)
>
>
> space should be added..

Thanks, Meng. I will fix that.

>
>
>>
>> +    {
>> +        /* the first vcpu starts the timer for the first time*/
>> +        timer_started = 1;
>> +        set_timer(&repl_timer,svc->cur_deadline);
>> +    }
>>   }
>>
>>   /*
>> @@ -792,44 +816,6 @@ __runq_pick(const struct scheduler *ops, const cpumask_t *mask)
>>   }
>>
>>   /*
>> - * Update vcpu's budget and
>> - * sort runq by insert the modifed vcpu back to runq
>> - * lock is grabbed before calling this function
>> - */
>> -static void
>> -__repl_update(const struct scheduler *ops, s_time_t now)
>> -{
>> -    struct list_head *runq = rt_runq(ops);
>> -    struct list_head *depletedq = rt_depletedq(ops);
>> -    struct list_head *iter;
>> -    struct list_head *tmp;
>> -    struct rt_vcpu *svc = NULL;
>> -
>> -    list_for_each_safe(iter, tmp, runq)
>> -    {
>> -        svc = __q_elem(iter);
>> -        if ( now < svc->cur_deadline )
>> -            break;
>> -
>> -        rt_update_deadline(now, svc);
>> -        /* reinsert the vcpu if its deadline is updated */
>> -        __q_remove(svc);
>> -        __runq_insert(ops, svc);
>> -    }
>> -
>> -    list_for_each_safe(iter, tmp, depletedq)
>> -    {
>> -        svc = __q_elem(iter);
>> -        if ( now >= svc->cur_deadline )
>> -        {
>> -            rt_update_deadline(now, svc);
>> -            __q_remove(svc); /* remove from depleted queue */
>> -            __runq_insert(ops, svc); /* add to runq */
>> -        }
>> -    }
>> -}
>> -
>> -/*
>>    * schedule function for rt scheduler.
>>    * The lock is already grabbed in schedule.c, no need to lock here
>>    */
>> @@ -848,7 +834,6 @@ rt_schedule(const struct scheduler *ops, s_time_t now, bool_t tasklet_work_sched
>>       /* burn_budget would return for IDLE VCPU */
>>       burn_budget(ops, scurr, now);
>>
>> -    __repl_update(ops, now);
>>
>>       if ( tasklet_work_scheduled )
>>       {
>> @@ -889,7 +874,7 @@ rt_schedule(const struct scheduler *ops, s_time_t now, bool_t tasklet_work_sched
>>           }
>>       }
>>
>> -    ret.time = MIN(snext->budget, MAX_SCHEDULE); /* sched quantum */
>> +    ret.time = snext->budget; /* invoke the scheduler next time */
>>       ret.task = snext->vcpu;
>>
>>       /* TRACE */
>> @@ -1033,10 +1018,6 @@ rt_vcpu_wake(const struct scheduler *ops, struct vcpu *vc)
>>   {
>>       struct rt_vcpu * const svc = rt_vcpu(vc);
>>       s_time_t now = NOW();
>> -    struct rt_private *prv = rt_priv(ops);
>> -    struct rt_vcpu *snext = NULL; /* highest priority on RunQ */
>> -    struct rt_dom *sdom = NULL;
>> -    cpumask_t *online;
>>
>>       BUG_ON( is_idle_vcpu(vc) );
>>
>> @@ -1074,14 +1055,7 @@ rt_vcpu_wake(const struct scheduler *ops, struct vcpu *vc)
>>       /* insert svc to runq/depletedq because svc is not in queue now */
>>       __runq_insert(ops, svc);
>>
>> -    __repl_update(ops, now);
>> -
>> -    ASSERT(!list_empty(&prv->sdom));
>> -    sdom = list_entry(prv->sdom.next, struct rt_dom, sdom_elem);
>> -    online = cpupool_scheduler_cpumask(sdom->dom->cpupool);
>> -    snext = __runq_pick(ops, online); /* pick snext from ALL valid cpus */
>> -
>> -    runq_tickle(ops, snext);
>> +    runq_tickle(ops, svc);
>>
>>       return;
>>   }
>> @@ -1094,10 +1068,6 @@ static void
>>   rt_context_saved(const struct scheduler *ops, struct vcpu *vc)
>>   {
>>       struct rt_vcpu *svc = rt_vcpu(vc);
>> -    struct rt_vcpu *snext = NULL;
>> -    struct rt_dom *sdom = NULL;
>> -    struct rt_private *prv = rt_priv(ops);
>> -    cpumask_t *online;
>>       spinlock_t *lock = vcpu_schedule_lock_irq(vc);
>>
>>       clear_bit(__RTDS_scheduled, &svc->flags);
>> @@ -1108,15 +1078,8 @@ rt_context_saved(const struct scheduler *ops, struct vcpu *vc)
>>       if ( test_and_clear_bit(__RTDS_delayed_runq_add, &svc->flags) &&
>>            likely(vcpu_runnable(vc)) )
>>       {
>> +        /* only insert the pre-empted vcpu back*/
>>           __runq_insert(ops, svc);
>> -        __repl_update(ops, NOW());
>> -
>> -        ASSERT(!list_empty(&prv->sdom));
>> -        sdom = list_entry(prv->sdom.next, struct rt_dom, sdom_elem);
>> -        online = cpupool_scheduler_cpumask(sdom->dom->cpupool);
>> -        snext = __runq_pick(ops, online); /* pick snext from ALL cpus */
>> -
>> -        runq_tickle(ops, snext);
>>       }
>>   out:
>>       vcpu_schedule_unlock_irq(lock, vc);
>> @@ -1167,6 +1130,74 @@ rt_dom_cntl(
>>       return rc;
>>   }
>>
>> +static void repl_handler(void *data){
>> +    unsigned long flags;
>> +    s_time_t now = NOW();
>> +    s_time_t min_repl = LONG_MAX; /* max time used in comparison*/
>> +    struct scheduler *ops = data;
>> +    struct rt_private *prv = rt_priv(ops);
>> +    struct list_head *runq = rt_runq(ops);
>> +    struct list_head *depletedq = rt_depletedq(ops);
>> +    struct list_head *iter;
>> +    struct list_head *tmp;
>> +    struct rt_vcpu *svc = NULL;
>> +
>> +    spin_lock_irqsave(&prv->lock,flags);
>> +
>> +    stop_timer(&repl_timer);
>
>
> Move the stop_timer before spin_lock_irqsave() since no more than one
> core will run this handler in the current design.

Right. We want the critical section as short as possible.

>
>>
>>
>>
>> +
>> +    list_for_each_safe(iter, tmp, runq)
>> +    {
>> +        svc = __q_elem(iter);
>> +        if ( now < svc->cur_deadline )
>> +            break;
>> +        rt_update_deadline(now, svc);
>> +        /* scan the runq to find the min release time
>> +         * this happens when vcpus on runq miss deadline
>> +         */
>> +        if( min_repl> svc->cur_deadline )
>> +        {
>> +            min_repl = svc->cur_deadline;
>> +        }
>
> The logic here is incorrect. The comment you put is correct, but the
> code didn't reflect the comment.
>
> When the timer's handler is called, now should be equal to min_repl,
> shouldn't it?
> If now = min_repl, this if statement will never be called because the
> loop stops if ( now < svc->cur_deadline ).
>

Yes the handler will be invoked when it was programmed to but the value 
of min_repl is set to be LONG_MAX initially. It doesn't preserve values. 
Therefore, the IF statement is feasible.

> Only after you have replenished the vcpus on runq and depletedq,
> should you pick the earliest deadline of the front vcpus at the runq
> and depletedq as the min_repl.
>
> Please correct me  if I missed anything. :-)
>
>
>>
>> +        /* reinsert the vcpu if its deadline is updated */
>> +        __q_remove(svc);
>> +        __runq_insert(ops, svc);
>> +    }
>> +
>> +    list_for_each_safe(iter, tmp, depletedq)
>> +    {
>> +        svc = __q_elem(iter);
>> +        if ( now >= svc->cur_deadline )
>> +        {
>> +            rt_update_deadline(now, svc);
>> +
>> +            /* scan the delp_q to find the minimal release time */
>> +            if(min_repl > svc->cur_deadline)
>> +            {
>> +                min_repl = svc->cur_deadline;
>> +            }
>
>
> Please see the above comment. It may have the same issue here.
>
> In addition, the current running VCPUs on cores also need to replenish
> their budgets at the beginning of their next periods. Those running
> VCPUs are not in runq or depletedq in the current RTDS scheduler. So
> this patch won't replenish the current running VCPUs' budget until
> they are put back to runq or depletedq, which is incorrect. Think
> about the following  scenario: a VCPU with very small period  has a
> task always running on it to keep this vcpu always runnable; This VCPU
> has its period equal to its budget. Suppose this VCPU is the only VCPU
> on a 4 core machine. This VCPU should keep running on one core and
> never be put back to runq. In the current code, this VCPU won't have
> its budget replenished. Right? :-)
>
> You can choose one of the following three approaches to fix it:
> a) add the running VCPUs into the runq
> b) use another queue to keep track of the current running VCPUs
> c) just scan each core (in the cpupool) to get the current running VCPU on it.
> I personally prefer c) but I'm looking forward to others' opinion. :-)
>
>> +            __q_remove(svc);
>> +            __runq_insert(ops, svc);
>> +            runq_tickle(ops, svc);
>> +        }
>> +    }
>> +
>> +    /* if timer was triggered but none of the vcpus
>> +     * need to be refilled, set the timer to be the
>> +     * default period + now
>> +     */
>
>
> Hmm, when could this happen? If it could happen, please specify the situation.
> Otherwise, it may be better to print a warning (or cause crash?) when
> you set the timer to be default period + now?
>
> I'm not quite sure which is better, warning or crash?
>
>
>>
>> +    if(min_repl == LONG_MAX)
>> +    {
>>
>> +        set_timer(&repl_timer, now + RTDS_DEFAULT_PERIOD);
>> +    }
>
>
> Hmm,
>
> when could this happen? The only situation in my mind is that there is
> no vcpus in the cpupool. But if that is the case, shouldn't the timer
> never be triggered? Then this handler will never be called.

This is actually still a question for me. When I was testing the code in 
a virtual machine, it happens when the system boots up. I am guessing 
it's related to the same problem in nested-virtualization (just 
guessing). When I tested it on a real machine, it locks up and won't 
even respond to any key-board events. After some debugging, it appeared 
that the timer was only triggered once and in the timer handler, none of 
the vcpus was replenished. After the the system boots up, I have not 
seen any debug prints indicating that the timer is not programmed based 
on the release time. Besides looking for why, I think we should set some 
maximum interval for the timer to do the replenishment.


>
>> +    else
>> +    {
>> +        /* reprogram the timer using the most imminent release time*/
>> +        set_timer(&repl_timer, min_repl);
>> +    }
>>
>> +    spin_unlock_irqrestore(&prv->lock,flags);
>
>
> Same here, need to move spin_unlock_ before the if statement.
>
>> +}
>> +
>>   static struct rt_private _rt_priv;
>>
>>   const struct scheduler sched_rtds_def = {
>> --
>> 1.7.9.5
>>
>
> Thank you very much, Tianyang, for your hard working on this! The
> cover letter is much much better and is very clear now.
> Let's waiting for others' opinions and keep refining it until it gets
> merged and further improve the efficiency of the current RTDS
> scheduler. :-P

Thanks Meng, I will try my best.

Tianyang Chen

>
>
> Best,
>
> Meng
>
> -----------
> Meng Xu
> PhD Student in Computer and Information Science
> University of Pennsylvania
> http://www.cis.upenn.edu/~mengxu/
>