Re: [Xen-devel] [PATCH v2 2/4] xen: sched: deal with vCPUs being or becoming online or offline

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 7/26/19 7:25 AM, Dario Faggioli wrote:
> If a vCPU is, or is going, offline we want it to be neither
> assigned to a pCPU, nor in the wait list, so:
> - if an offline vcpu is inserted (or migrated) it must not
>   go on a pCPU, nor in the wait list;
> - if an offline vcpu is removed, we are sure that it is
>   neither on a pCPU nor in the wait list already, so we
>   should just bail, avoiding doing any further action;
> - if a vCPU goes offline we need to remove it either from
>   its pCPU or from the wait list.
> 
> Signed-off-by: Dario Faggioli <dfaggioli@xxxxxxxx>

Reviewed-by: George Dunlap <george.dunlap@xxxxxxxxxx>

> ---
> Cc: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
> Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> Cc: Roger Pau Monne <roger.pau@xxxxxxxxxx>
> ---
> Changes from v1:
> * improved wording in changelog and comments
> * this patch is the result of the merge of patches 2 and 3 from v1
> ---
>  xen/common/sched_null.c |   80 
> +++++++++++++++++++++++++++++++++++++++--------
>  1 file changed, 66 insertions(+), 14 deletions(-)
> 
> diff --git a/xen/common/sched_null.c b/xen/common/sched_null.c
> index c47c1b5aae..10e96f21dd 100644
> --- a/xen/common/sched_null.c
> +++ b/xen/common/sched_null.c
> @@ -339,6 +339,8 @@ static unsigned int pick_cpu(struct null_private *prv, 
> struct vcpu *v)
>  static void vcpu_assign(struct null_private *prv, struct vcpu *v,
>                          unsigned int cpu)
>  {
> +    ASSERT(is_vcpu_online(v));
> +
>      per_cpu(npc, cpu).vcpu = v;
>      v->processor = cpu;
>      cpumask_clear_cpu(cpu, &prv->cpus_free);
> @@ -358,7 +360,8 @@ static void vcpu_assign(struct null_private *prv, struct 
> vcpu *v,
>      }
>  }
>  
> -static void vcpu_deassign(struct null_private *prv, struct vcpu *v)
> +/* Returns true if a cpu was tickled */
> +static bool vcpu_deassign(struct null_private *prv, struct vcpu *v)
>  {
>      unsigned int bs;
>      unsigned int cpu = v->processor;
> @@ -403,11 +406,13 @@ static void vcpu_deassign(struct null_private *prv, 
> struct vcpu *v)
>                  vcpu_assign(prv, wvc->vcpu, cpu);
>                  cpu_raise_softirq(cpu, SCHEDULE_SOFTIRQ);
>                  spin_unlock(&prv->waitq_lock);
> -                return;
> +                return true;
>              }
>          }
>      }
>      spin_unlock(&prv->waitq_lock);
> +
> +    return false;
>  }
>  
>  /* Change the scheduler of cpu to us (null). */
> @@ -445,8 +450,14 @@ static void null_vcpu_insert(const struct scheduler 
> *ops, struct vcpu *v)
>      ASSERT(!is_idle_vcpu(v));
>  
>      lock = vcpu_schedule_lock_irq(v);
> - retry:
>  
> +    if ( unlikely(!is_vcpu_online(v)) )
> +    {
> +        vcpu_schedule_unlock_irq(lock, v);
> +        return;
> +    }
> +
> + retry:
>      cpu = v->processor = pick_cpu(prv, v);
>  
>      spin_unlock(lock);
> @@ -500,6 +511,14 @@ static void null_vcpu_remove(const struct scheduler 
> *ops, struct vcpu *v)
>  
>      lock = vcpu_schedule_lock_irq(v);
>  
> +    /* If offline, the vcpu shouldn't be assigned, nor in the waitqueue */
> +    if ( unlikely(!is_vcpu_online(v)) )
> +    {
> +        ASSERT(per_cpu(npc, v->processor).vcpu != v);
> +        ASSERT(list_empty(&nvc->waitq_elem));
> +        goto out;
> +    }
> +
>      /* If v is in waitqueue, just get it out of there and bail */
>      if ( unlikely(!list_empty(&nvc->waitq_elem)) )
>      {
> @@ -549,11 +568,33 @@ static void null_vcpu_wake(const struct scheduler *ops, 
> struct vcpu *v)
>  
>  static void null_vcpu_sleep(const struct scheduler *ops, struct vcpu *v)
>  {
> +    struct null_private *prv = null_priv(ops);
> +    unsigned int cpu = v->processor;
> +    bool tickled = false;
> +
>      ASSERT(!is_idle_vcpu(v));
>  
> +    /* 
> +     * Check if the vcpu is in the process of being offlined. if yes,
> +     * we need to remove it from either its pCPU or the waitqueue.
> +     */
> +    if ( unlikely(!is_vcpu_online(v)) )
> +    {
> +        struct null_vcpu *nvc = null_vcpu(v);
> +
> +        if ( unlikely(!list_empty(&nvc->waitq_elem)) )
> +        {
> +            spin_lock(&prv->waitq_lock);
> +            list_del_init(&nvc->waitq_elem);
> +            spin_unlock(&prv->waitq_lock);
> +        }
> +        else if ( per_cpu(npc, cpu).vcpu == v )
> +            tickled = vcpu_deassign(prv, v);
> +    }
> +
>      /* If v is not assigned to a pCPU, or is not running, no need to bother 
> */
> -    if ( curr_on_cpu(v->processor) == v )
> -        cpu_raise_softirq(v->processor, SCHEDULE_SOFTIRQ);
> +    if ( likely(!tickled && curr_on_cpu(cpu) == v) )
> +        cpu_raise_softirq(cpu, SCHEDULE_SOFTIRQ);
>  
>      SCHED_STAT_CRANK(vcpu_sleep);
>  }
> @@ -589,24 +630,34 @@ static void null_vcpu_migrate(const struct scheduler 
> *ops, struct vcpu *v,
>      }
>  
>      /*
> -     * v is either assigned to a pCPU, or in the waitqueue.
> -     *
> -     * In the former case, the pCPU to which it was assigned would
> -     * become free, and we, therefore, should check whether there is
> -     * anyone in the waitqueue that can be assigned to it.
> -     *
> -     * In the latter, there is just nothing to do.
> +     * If v is assigned to a pCPU, then such pCPU becomes free, and we
> +     * should look in the waitqueue if anyone else can be assigned to it.
>       */
> -    if ( likely(list_empty(&nvc->waitq_elem)) )
> +    if ( likely(per_cpu(npc, v->processor).vcpu == v) )
>      {
>          vcpu_deassign(prv, v);
>          SCHED_STAT_CRANK(migrate_running);
>      }
> -    else
> +    else if ( !list_empty(&nvc->waitq_elem) )
>          SCHED_STAT_CRANK(migrate_on_runq);
>  
>      SCHED_STAT_CRANK(migrated);
>  
> +    /*
> +     * If a vcpu is (going) offline, we want it to be neither assigned
> +     * to a pCPU, nor in the waitqueue.
> +     *
> +     * If it was on a cpu, we've removed it from there above. If it is
> +     * in the waitqueue, we remove it from there now. And then we bail.
> +     */
> +    if ( unlikely(!is_vcpu_online(v)) )
> +    {
> +        spin_lock(&prv->waitq_lock);
> +        list_del_init(&nvc->waitq_elem);
> +        spin_unlock(&prv->waitq_lock);
> +        goto out;
> +    }
> +
>      /*
>       * Let's now consider new_cpu, which is where v is being sent. It can be
>       * either free, or have a vCPU already assigned to it.
> @@ -646,6 +697,7 @@ static void null_vcpu_migrate(const struct scheduler 
> *ops, struct vcpu *v,
>       * at least. In case of suspend, any temporary inconsistency caused
>       * by this, will be fixed-up during resume.
>       */
> + out:
>      v->processor = new_cpu;
>  }
>  
> 


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