Re: [Xen-devel] [PATCH] xen: credit2: don't let b_avgload go negative.

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 22/07/16 13:04, Dario Faggioli wrote:
> The ASSERT() made effective by b5b5876619bd8ec2e
> ("xen: credit2: fix two s_time_t handling issues
> in load balancing") triggers for b_avgload (spotted
> by OSSTest).
> 
> b_avgload is where we store the prediction of how
> the load of a runqueue will look like in the medium
> to long term, because of a vcpu being added to or
> removed from there.
> 
> On vcpu removal, saturate down b_avgload to zero,
> as it makes very few sense to predict that the
> load of a runqueue will at some point become negative!
> 
> Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>

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

Wei, do you want to apply it?

Thanks,
 -George

> ---
> Cc: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
> Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
> ---
>  xen/common/sched_credit2.c |    8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/xen/common/sched_credit2.c b/xen/common/sched_credit2.c
> index b92226c..1d79de0 100644
> --- a/xen/common/sched_credit2.c
> +++ b/xen/common/sched_credit2.c
> @@ -1323,14 +1323,16 @@ runq_assign(const struct scheduler *ops, struct vcpu 
> *vc)
>  static void
>  __runq_deassign(struct csched2_vcpu *svc)
>  {
> +    struct csched2_runqueue_data *rqd = svc->rqd;
> +
>      ASSERT(!__vcpu_on_runq(svc));
>      ASSERT(!(svc->flags & CSFLAG_scheduled));
>  
>      list_del_init(&svc->rqd_elem);
> -    update_max_weight(svc->rqd, 0, svc->weight);
> +    update_max_weight(rqd, 0, svc->weight);
>  
>      /* Expected new load based on removing this vcpu */
> -    svc->rqd->b_avgload -= svc->avgload;
> +    rqd->b_avgload = max_t(s_time_t, rqd->b_avgload - svc->avgload, 0);
>  
>      svc->rqd = NULL;
>  }
> @@ -1590,7 +1592,7 @@ csched2_cpu_pick(const struct scheduler *ops, struct 
> vcpu *vc)
>          if ( rqd == svc->rqd )
>          {
>              if ( cpumask_intersects(vc->cpu_hard_affinity, &rqd->active) )
> -                rqd_avgload = rqd->b_avgload - svc->avgload;
> +                rqd_avgload = max_t(s_time_t, rqd->b_avgload - svc->avgload, 
> 0);
>          }
>          else if ( spin_trylock(&rqd->lock) )
>          {
> 


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