Patch "sched/fair: Take the scheduling domain into account in select_idle_smt()" has been added to the 6.1-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    sched/fair: Take the scheduling domain into account in select_idle_smt()

to the 6.1-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     sched-fair-take-the-scheduling-domain-into-account-i.patch
and it can be found in the queue-6.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 768f14c81ca6c8eaa18beca0c84defa3df997b8c
Author: Keisuke Nishimura <keisuke.nishimura@xxxxxxxx>
Date:   Wed Jan 10 14:17:06 2024 +0100

    sched/fair: Take the scheduling domain into account in select_idle_smt()
    
    [ Upstream commit 8aeaffef8c6eceab0e1498486fdd4f3dc3b7066c ]
    
    When picking a CPU on task wakeup, select_idle_smt() has to take
    into account the scheduling domain of @target. This is because the
    "isolcpus" kernel command line option can remove CPUs from the domain to
    isolate them from other SMT siblings.
    
    This fix checks if the candidate CPU is in the target scheduling domain.
    
    Commit:
    
      df3cb4ea1fb6 ("sched/fair: Fix wrong cpu selecting from isolated domain")
    
    ... originally introduced this fix by adding the check of the scheduling
    domain in the loop.
    
    However, commit:
    
      3e6efe87cd5cc ("sched/fair: Remove redundant check in select_idle_smt()")
    
    ... accidentally removed the check. Bring it back.
    
    Fixes: 3e6efe87cd5c ("sched/fair: Remove redundant check in select_idle_smt()")
    Signed-off-by: Keisuke Nishimura <keisuke.nishimura@xxxxxxxx>
    Signed-off-by: Julia Lawall <julia.lawall@xxxxxxxx>
    Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
    Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
    Link: https://lore.kernel.org/r/20240110131707.437301-1-keisuke.nishimura@xxxxxxxx
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 2558ab9033bee..1c4e54fffb8b6 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6678,13 +6678,19 @@ static int select_idle_core(struct task_struct *p, int core, struct cpumask *cpu
 /*
  * Scan the local SMT mask for idle CPUs.
  */
-static int select_idle_smt(struct task_struct *p, int target)
+static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int target)
 {
 	int cpu;
 
 	for_each_cpu_and(cpu, cpu_smt_mask(target), p->cpus_ptr) {
 		if (cpu == target)
 			continue;
+		/*
+		 * Check if the CPU is in the LLC scheduling domain of @target.
+		 * Due to isolcpus, there is no guarantee that all the siblings are in the domain.
+		 */
+		if (!cpumask_test_cpu(cpu, sched_domain_span(sd)))
+			continue;
 		if (available_idle_cpu(cpu) || sched_idle_cpu(cpu))
 			return cpu;
 	}
@@ -6708,7 +6714,7 @@ static inline int select_idle_core(struct task_struct *p, int core, struct cpuma
 	return __select_idle_cpu(core, p);
 }
 
-static inline int select_idle_smt(struct task_struct *p, int target)
+static inline int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int target)
 {
 	return -1;
 }
@@ -6970,7 +6976,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
 		has_idle_core = test_idle_cores(target);
 
 		if (!has_idle_core && cpus_share_cache(prev, target)) {
-			i = select_idle_smt(p, prev);
+			i = select_idle_smt(p, sd, prev);
 			if ((unsigned int)i < nr_cpumask_bits)
 				return i;
 		}