Re: [Xen-devel] [PATCH V2 1/1] Improved RTDS scheduler

[Meng Xu <xumengpanda@xxxxxxxxx>]

On Thu, Dec 31, 2015 at 6:20 PM, Tianyang Chen <tiche@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx>
> Signed-off-by: Meng Xu <mengxu@xxxxxxxxxxxxx>
> Signed-off-by: Dagaen Golomb <dgolomb@xxxxxxxxxxxxxx>
> ---
>  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..


>
> +    {
> +        /* 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.

>
>
>
> +
> +    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 ).

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.

> +    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


Best,

Meng

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

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