Re: [PATCH] sched/fair: Add SMT4 group_smt_balance handling

[Tim Chen]

On Tue, 2023-09-05 at 13:33 +0530, Shrikanth Hegde wrote:
> 
> On 8/22/23 12:49 AM, Tim Chen wrote:
> > On Mon, 2023-08-07 at 15:06 +0530, Shrikanth Hegde wrote:
> > > > 
> > > > From: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> > > > Date: Fri, 14 Jul 2023 16:09:30 -0700
> > > > Subject: [PATCH] sched/fair: Add SMT4 group_smt_balance handling
> > > > 
> > > > For SMT4, any group with more than 2 tasks will be marked as
> > > > group_smt_balance. Retain the behaviour of group_has_spare by marking
> > > > the busiest group as the group which has the least number of idle_cpus.
> > > > 
> > > > Also, handle rounding effect of adding (ncores_local + ncores_busy)
> > > > when the local is fully idle and busy group has more than 2 tasks.
> > > > Local group should try to pull at least 1 task in this case.
> > > > 
> > > > Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> > > > ---
> > > >  kernel/sched/fair.c | 18 ++++++++++++++++--
> > > >  1 file changed, 16 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > > index a87988327f88..566686c5f2bd 100644
> > > > --- a/kernel/sched/fair.c
> > > > +++ b/kernel/sched/fair.c
> > > > @@ -9563,7 +9563,7 @@ static inline long sibling_imbalance(struct lb_env *env,
> > > >  	imbalance /= ncores_local + ncores_busiest;
> > > >  
> > > >  	/* Take advantage of resource in an empty sched group */
> > > > -	if (imbalance == 0 && local->sum_nr_running == 0 &&
> > > > +	if (imbalance <= 1 && local->sum_nr_running == 0 &&
> > > >  	    busiest->sum_nr_running > 1)
> > > >  		imbalance = 2;
> > > >  
> > > > @@ -9751,6 +9751,20 @@ 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;
> > > > +		}
> > > > +		goto fully_busy;
> > > > +		break;
> > > > +
> > > >  	case group_fully_busy:
> > > >  		/*
> > > >  		 * Select the fully busy group with highest avg_load. In
> > > > @@ -9763,7 +9777,7 @@ static bool update_sd_pick_busiest(struct lb_env *env,
> > > >  		 * select the 1st one, except if @sg is composed of SMT
> > > >  		 * siblings.
> > > >  		 */
> > > > -
> > > > +fully_busy:
> > > >  		if (sgs->avg_load < busiest->avg_load)
> > > >  			return false;
> > > >  
> > > 
> > > Hi Tim, Peter. 
> > > 
> > > group_smt_balance(cluster scheduling), patches are in tip/sched/core. I dont 
> > > see this above patch there yet. Currently as is, this can cause function difference 
> > > in SMT4 systems( such as Power10). 
> > > 
> > > Can we please have the above patch as well in tip/sched/core?
> > > 
> > > Acked-by: Shrikanth Hegde <sshegde@xxxxxxxxxxxxxxxxxx>
> > 
> > Hi Peter,
> > 
> > Just back from my long vacation.  Wonder if you have any comments on the above patch
> > for fixing the SMT4 case?
> > 
> > Tim
> 
> Hi Tim, Peter. 
> 
> are there any concerns with the above patch for fixing the SMT4 case. 
> Currently the behavior is group_smt_balance is set for having even 2 tasks in 
> SMT4, ideally it should be same as the group_has_spare. 
> 
> The above patch copies the same behavior to group_smt_balance. 
> > 

You mean simplify the patch as below?  I think that should be fine.  Can you
make sure it works for SMT4?  And I can update the patch once you confirm it
works properly.

Tim

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6e7ee2efc1ba..48e9ab7f8a87 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9764,16 +9764,9 @@ static bool update_sd_pick_busiest(struct lb_env *env,
 
        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;
-               }
+               if (sgs->idle_cpus != 0 || busiest->idle_cpus != 0)
+                       goto has_spare;
+
                goto fully_busy;
 
        case group_fully_busy:
@@ -9809,6 +9802,7 @@ static bool update_sd_pick_busiest(struct lb_env *env,
                 * as we do not want to pull task off SMT core with one task
                 * and make the core idle.
                 */
+has_spare:
                if (smt_vs_nonsmt_groups(sds->busiest, sg)) {
                        if (sg->flags & SD_SHARE_CPUCAPACITY && sgs->sum_h_nr_running <= 1)
                                return false;