Re: [Patch v3 1/6] sched/fair: Determine active load balance for SMT sched groups

[Peter Zijlstra]

On Mon, Jul 17, 2023 at 01:06:59AM +0530, Shrikanth Hegde wrote:
> 
> 
> On 7/15/23 4:35 AM, Tim Chen wrote:
> > On Fri, 2023-07-14 at 18:36 +0530, Shrikanth Hegde wrote:
> > 
> >>
> >>
> >> If we consider symmetric platforms which have SMT4 such as power10. 
> >> we have a topology like below. multiple such MC will form DIE(PKG)
> >>
> >>
> >> [0 2 4 6][1 3 5 7][8 10 12 14][9 11 13 15]
> >> [--SMT--][--SMT--][----SMT---][---SMT----]
> >> [--sg1--][--sg1--][---sg1----][---sg1----]
> >> [--------------MC------------------------]
> >>
> >> In case of SMT4, if there is any group which has 2 or more tasks, that 
> >> group will be marked as group_smt_balance. previously, if that group had 2
> >> or 3 tasks, it would have been marked as group_has_spare. Since all the groups have 
> >> SMT that means behavior would be same fully busy right? That can cause some 
> >> corner cases. No?
> > 
> > You raised a good point. I was looking from SMT2
> > perspective so group_smt_balance implies group_fully_busy.
> > That is no longer true for SMT4.
> > 
> > I am thinking of the following fix on the current patch
> > to take care of SMT4. Do you think this addresses
> 
> Thanks Tim for taking a look at it again. 
> 
> Yes. I think this would address some of the corner cases. 
> Any SMT4 group having 2,3,4 will have smt_balance as the group type, and busiest one 
> is the one which has least number of idle cpu's. (same conditions as group_has_spare)
> 
> 
> 
> 
> > concerns from you and Tobias?
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 294a662c9410..3fc8d3a3bd22 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -9588,6 +9588,17 @@ static bool update_sd_pick_busiest(struct lb_env *env,
> >                 break;
> >  
> >         case group_smt_balance:
> > +               /* no idle cpus on both groups handled by group_fully_busy below */
> > +               if (sgs->idle_cpus != 0 || busiest->idle_cpus != 0) {
> > +                       if (sgs->idle_cpus > busiest->idle_cpus)
> > +                               return false;
> > +                       if (sgs->idle_cpus < busiest->idle_cpus)
> > +                               return true;
> > +                       if (sgs->sum_nr_running <= busiest_sum_nr_running)
> > +                               return false;
> > +                       else
> > +                               return true;
> > +               }
> > 
> > 
> > I will be on vacation next three weeks so my response will be slow.
> > 
> > Tim
> > 
> >>
> 
> Small suggestion to above code to avoid compiler warning of switch case falling
> through and else case can be removed, since update_sd_pick_busiest by default returns true.
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e5a75c76bcaa..ae364ac6f22e 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9728,9 +9728,9 @@ static bool update_sd_pick_busiest(struct lb_env *env,
>                                 return true;
>                         if (sgs->sum_nr_running <= busiest->sum_nr_running)
>                                 return false;
> -                       else
> -                               return true;
>                 }
> +               break;
> +
>         case group_fully_busy:
>                 /*
>                  * Select the fully busy group with highest avg_load. In
> 
> 

Can someone please send a full patch for this? I've already queued Tim's
patches in tip/sched/core (tip-bot seems to have died somewhere last
week, it's being worked on).