Re: Fwd: Intel hybrid CPU scheduler always prefers E cores

[Tim Chen <tim.c.chen@linux.intel.com>]

On Tue, 2023-11-28 at 20:22 +0700, Bagas Sanjaya wrote:
> Hi,
> 
> I come across an interesting bug report on Bugzilla [1]. The reporter
> wrote:
> 
> > I am running an intel alder lake system (Core i7-1260P), with a mix of P and E cores.
> > 
> > Since Linux 6.6, and also on the current 6.7 RC, the scheduler seems to have a strong preference for the E cores, and single threaded workloads are consistently scheduled on one of the E cores.
> > 
> > With Linux 6.4 and before, when I ran a single threaded CPU-bound process, it was scheduled on a P core. With 6.5, it seems that the choice of P or E seemed rather random.
> > 
> > I tested these by running "stress" with different amounts of threads. With a single thread on Linux 6.6 and 6.7, I always have an E core at 100% and no load on the P cores. Starting from 3 threads I get some load on the P cores as well, but the E cores stay more heavily loaded.
> > With "taskset" I can force a process to run on a P core, but clearly it's not very practical to have to do CPU scheduling manually.
> > 
> > This severely affects single-threaded performance of my CPU since the E cores are considerably slower. Several of my workflows are now a lot slower due to them being single-threaded and heavily CPU-bound and being scheduled on E cores whereas they would run on P cores before.
> > 
> > I am not sure what the exact desired behaviour is here, to balance power consumption and performance, but currently my P cores are barely used for single-threaded workloads.
> > 
> > Is this intended behaviour or is this indeed a regression? Or is there perhaps any configuration that I should have done from my side? Is there any further info that I can provide to help you figure out what's going on?
> 
> PM and scheduler people, is this a regression or works as intended?
> 
> Thanks.
> 
> [1]: https://bugzilla.kernel.org/show_bug.cgi?id=218195
> 

I have noticed that the current code sometimes is quite trigger happy
moving tasks off P-core, whenever there are more than 2 tasks on a core.
Sometimes, Short running house keeping tasks
could disturb the running task on P-core as a result.

Can you try the following patch?  On my Alder Lake system, I see as I add single
threaded tasks, they first run on P-cores, then followed by E-cores with this
patch on 6.6.

Tim

From 68a15ef01803c252261ebb47d86dfc1f2c68ae1e Mon Sep 17 00:00:00 2001
From: Tim Chen <tim.c.chen@linux.intel.com>
Date: Fri, 6 Oct 2023 15:58:56 -0700
Subject: [PATCH] sched/fair: Don't force smt balancing when CPU has spare
 capacity

Currently group_smt_balance is picked whenever there are more
than two tasks on a core with two SMT.  However, the utilization
of those tasks may be low and do not warrant a task
migration to a CPU of lower priority.

Adjust sched group clssification and sibling_imbalance()
to reflect this consideration.  Use sibling_imbalance() to
compute imbalance in calculate_imbalance() for the group_smt_balance
case.

Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>

---
 kernel/sched/fair.c | 23 +++++++++++------------
 1 file changed, 11 insertions(+), 12 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ef7490c4b8b4..7dd7c2d2367a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9460,14 +9460,15 @@ group_type group_classify(unsigned int imbalance_pct,
 	if (sgs->group_asym_packing)
 		return group_asym_packing;
 
-	if (sgs->group_smt_balance)
-		return group_smt_balance;
-
 	if (sgs->group_misfit_task_load)
 		return group_misfit_task;
 
-	if (!group_has_capacity(imbalance_pct, sgs))
-		return group_fully_busy;
+	if (!group_has_capacity(imbalance_pct, sgs)) {
+		if (sgs->group_smt_balance)
+			return group_smt_balance;
+		else
+			return group_fully_busy;
+	}
 
 	return group_has_spare;
 }
@@ -9573,6 +9574,11 @@ static inline long sibling_imbalance(struct lb_env *env,
 	if (env->idle == CPU_NOT_IDLE || !busiest->sum_nr_running)
 		return 0;
 
+	/* Do not pull tasks off preferred group with spare capacity */
+	if (busiest->group_type == group_has_spare &&
+	    sched_asym_prefer(sds->busiest->asym_prefer_cpu, env->dst_cpu))
+		return 0;
+
 	ncores_busiest = sds->busiest->cores;
 	ncores_local = sds->local->cores;
 
@@ -10411,13 +10417,6 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
 		return;
 	}
 
-	if (busiest->group_type == group_smt_balance) {
-		/* Reduce number of tasks sharing CPU capacity */
-		env->migration_type = migrate_task;
-		env->imbalance = 1;
-		return;
-	}
-
 	if (busiest->group_type == group_imbalanced) {
 		/*
 		 * In the group_imb case we cannot rely on group-wide averages
-- 
2.32.0