linux-kernel - [PATCH 5/6] sched/fair: Consider the idle state of the whole core for load balance

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Message-Id: <849d3fee51d218de71ecc9d557667ff6b137ac2d.1683156492.git.tim.c.chen@linux.intel.com>
Date:   Thu,  4 May 2023 09:09:55 -0700
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ricardo Neri <ricardo.neri@...el.com>,
        "Ravi V . Shankar" <ravi.v.shankar@...el.com>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Len Brown <len.brown@...el.com>, Mel Gorman <mgorman@...e.de>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Ionela Voinescu <ionela.voinescu@....com>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        naveen.n.rao@...ux.vnet.ibm.com,
        Yicong Yang <yangyicong@...ilicon.com>,
        Barry Song <v-songbaohua@...o.com>
Subject: [PATCH 5/6] sched/fair: Consider the idle state of the whole core for load balance

From: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>

should_we_balance() traverses the group_balance_mask (AND'ed with lb_env::
cpus) starting from lower numbered CPUs looking for the first idle CPU.

In hybrid x86 systems, the siblings of SMT cores get CPU numbers, before
non-SMT cores:

	[0, 1] [2, 3] [4, 5] 5 6 7 8
         b  i   b  i   b  i  b i i i

In the figure above, CPUs in brackets are siblings of an SMT core. The
rest are non-SMT cores. 'b' indicates a busy CPU, 'i' indicates an
idle CPU.

We should let a CPU on a fully idle core get the first chance to idle
load balance as it has more CPU capacity than a CPU on an idle SMT
CPU with busy sibling.  So for the figure above, if we are running
should_we_balance() to CPU 1, we should return false to let CPU 6 on
idle core to have a chance first to idle load balance.

A partially busy (i.e., of type group_has_spare) local group with SMT 
cores will often have only one SMT sibling busy. If the destination CPU
is a non-SMT core, partially busy, lower-numbered, SMT cores should not
be considered when finding the first idle CPU. 

However, in should_we_balance(), when we encounter idle SMT first in partially
busy core, we prematurely break the search for the first idle CPU.

Higher-numbered, non-SMT cores is not given the chance to have
idle balance done on their behalf. Those CPUs will only be considered
for idle balancing by chance via CPU_NEWLY_IDLE.

Instead, consider the idle state of the whole SMT core.

Signed-off-by: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Co-developed-by: Tim Chen <tim.c.chen@...ux.intel.com>
Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
---
 kernel/sched/fair.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 58ef7d529731..c77fadba063d 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10683,7 +10683,7 @@ static int active_load_balance_cpu_stop(void *data);
 static int should_we_balance(struct lb_env *env)
 {
 	struct sched_group *sg = env->sd->groups;
-	int cpu;
+	int cpu, idle_smt = -1;
 
 	/*
 	 * Ensure the balancing environment is consistent; can happen
@@ -10709,11 +10709,26 @@ static int should_we_balance(struct lb_env *env)
 	for_each_cpu_and(cpu, group_balance_mask(sg), env->cpus) {
 		if (!idle_cpu(cpu))
 			continue;
+		else {
+			/*
+			 * Don't balance to idle SMT in busy core right away when
+			 * balancing cores, but remember the first idle SMT CPU for
+			 * later consideration.  Find CPU on an idle core first.
+			 */
+			if (!(env->sd->flags & SD_SHARE_CPUCAPACITY) && !is_core_idle(cpu)) {
+				if (idle_smt == -1)
+					idle_smt = cpu;
+				continue;
+			}
+		}
 
 		/* Are we the first idle CPU? */
 		return cpu == env->dst_cpu;
 	}
 
+	if (idle_smt == env->dst_cpu)
+		return true;
+
 	/* Are we the first CPU of this group ? */
 	return group_balance_cpu(sg) == env->dst_cpu;
 }
-- 
2.32.0