linux-kernel - Re: [PATCH] sched/fair: Add SMT4 group_smt

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Date:   Tue, 05 Sep 2023 10:54:09 -0700
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>, bristot@...hat.com,
        bsegall@...gle.com, dietmar.eggemann@....com, hdanton@...a.com,
        ionela.voinescu@....com, juri.lelli@...hat.com,
        len.brown@...el.com, linux-kernel@...r.kernel.org, mgorman@...e.de,
        naveen.n.rao@...ux.vnet.ibm.com, rafael.j.wysocki@...el.com,
        ravi.v.shankar@...el.com, ricardo.neri@...el.com,
        rostedt@...dmis.org, srikar@...ux.vnet.ibm.com,
        srinivas.pandruvada@...ux.intel.com, v-songbaohua@...o.com,
        vincent.guittot@...aro.org, vschneid@...hat.com, x86@...nel.org,
        yangyicong@...ilicon.com, yu.c.chen@...el.com
Subject: Re: [PATCH] sched/fair: Add SMT4 group_smt_balance handling

On Tue, 2023-09-05 at 12:41 +0200, Peter Zijlstra wrote:
> On Thu, Jul 27, 2023 at 06:32:44AM -0700, Tim Chen wrote:
> 
> >  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;
> 
> This is really daft; why can't this simply be: fallthrough; ? At the
> very least that break must go.
> 
> 

Yes, the break should go.  I was adding the goto to prevent compiler
from complaining about fall through code.  The break no longer is needed.

Tim

>From 81971a0b1eb64059756f00d8497b1865af2c0792 Mon Sep 17 00:00:00 2001
From: Tim Chen <tim.c.chen@...ux.intel.com>
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.

Fixes: fee1759e4f04 ("sched/fair: Determine active load balance for SMT sched groups")
Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
---
 kernel/sched/fair.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0b7445cd5af9..6e7ee2efc1ba 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9575,7 +9575,7 @@ 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;
 
@@ -9763,6 +9763,19 @@ 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;
+
 	case group_fully_busy:
 		/*
 		 * Select the fully busy group with highest avg_load. In
@@ -9775,7 +9788,7 @@ 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;
 
-- 
2.32.0