[PATCH v2 0/3] sched: Simplify ifdeffery around CONFIG_SCHED_SMT

[PATCH v2 0/3] sched: Simplify ifdeffery around CONFIG_SCHED_SMT

[Shrikanth Hegde]

Semantics
=========
- For CONFIG_SCHED_SMT=y:
    No functional change.
- For CONFIG_SCHED_SMT=n:
    - cpu_smt_mask(cpu) becomes cpumask_of(cpu), effectively making it
      per CPU with no siblings.
    - sched_smt_present remains defined, but never becomes active:
    	 Since cpumask_weight(cpumask_of(cpu)) == 1

Performance impact
==================
- CONFIG_SCHED_SMT=y:
    No change in generated code.
- CONFIG_SCHED_SMT=n:
    - Small increase in text size (~0.01%) due to removal of compile-time
      stubs. Most paths remain effectively dead due to static keys.
    - Fast paths are protected using IS_ENABLED(CONFIG_SCHED_SMT).

With that, cpu_smt_mask() to be used unconditionally and reduces
CONFIG_SCHED_SMT-specific code paths, improving readability and
maintainability.

This leaves the remaining use of CONFIG_SCHED_SMT mainly for topology
handling bits.

Since v1[1]:
- Dropped changes for stop_core_cpuslocked. intel_ifs is the only
  user and it gets compiled with SCHED_SMT always. Added comments
  around it why ifdefs are still being kept.
- Decided not to add sched_smt_active checks for sched_core_cpu_deactivate
  and sched_core_cpu_starting, they bail out if weight(smt_mask) == 1.
  Hence core_lock won't be held for long. It is not fastpath either.
- Thanks to Valentin Schneider and Tejun Heo for checking out the series
  and the feedback.

Shrikanth Hegde (3):
  topology: Introduce cpu_smt_mask for CONFIG_SCHED_SMT=n
  sched: Simplify ifdeffery around cpu_smt_mask
  sched/fair: Add compile time check in fastpaths for CONFIG_SCHED_SMT=n

 include/linux/sched/smt.h |  4 ----
 include/linux/topology.h  | 15 +++++++++++++-
 kernel/sched/core.c       |  6 ------
 kernel/sched/ext_idle.c   |  6 ------
 kernel/sched/fair.c       | 41 +++++----------------------------------
 kernel/sched/sched.h      |  6 ------
 kernel/sched/topology.c   |  2 --
 kernel/stop_machine.c     |  5 +++++
 kernel/workqueue.c        |  4 ----
 9 files changed, 24 insertions(+), 65 deletions(-)

-- 
2.47.3

[PATCH v2 1/3] topology: Introduce cpu_smt_mask for CONFIG_SCHED_SMT=n

[Shrikanth Hegde]

Define cpu_smt_mask in case of CONFIG_SCHED_SMT=n as cpumask_of that
CPU. With that config, it is expected that kernel treats each CPU
as individual core. Using cpumask_of(cpu) reflects that.

This would help to get rid of the ifdeffery that is spread across
the codebase since cpu_smt_mask is defined only in case of
CONFIG_SCHED_SMT=y.

Note: There is no arch today which defines cpu_smt_mask unconditionally.
So likely defining the cpu_smt_mask shouldn't lead redefintion errors.

Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
---
 include/linux/topology.h | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/include/linux/topology.h b/include/linux/topology.h
index 6575af39fd10..709a2dcf4c73 100644
--- a/include/linux/topology.h
+++ b/include/linux/topology.h
@@ -230,11 +230,24 @@ static inline int cpu_to_mem(int cpu)
 #define topology_drawer_cpumask(cpu)		cpumask_of(cpu)
 #endif
 
-#if defined(CONFIG_SCHED_SMT) && !defined(cpu_smt_mask)
+/*
+ * Defining cpu_smt_mask as cpumask_of that CPU helps to get
+ * rid of lot of ifdeffery all around the codebase in case of
+ * CONFIG_SCHED_SMT=n. It just means there are no other siblings, which
+ * is what is expected.
+ */
+#if defined(CONFIG_SCHED_SMT)
+# if !defined(cpu_smt_mask)
 static inline const struct cpumask *cpu_smt_mask(int cpu)
 {
 	return topology_sibling_cpumask(cpu);
 }
+# endif
+#else	/* !CONFIG_SCHED_SMT */
+static inline const struct cpumask *cpu_smt_mask(int cpu)
+{
+	return cpumask_of(cpu);
+}
 #endif
 
 #ifndef topology_is_primary_thread
-- 
2.47.3

Re: [PATCH v2 1/3] topology: Introduce cpu_smt_mask for CONFIG_SCHED_SMT=n

[Phil Auld]

On Tue, May 12, 2026 at 08:51:23PM +0530 Shrikanth Hegde wrote:
> Define cpu_smt_mask in case of CONFIG_SCHED_SMT=n as cpumask_of that
> CPU. With that config, it is expected that kernel treats each CPU
> as individual core. Using cpumask_of(cpu) reflects that.
> 
> This would help to get rid of the ifdeffery that is spread across
> the codebase since cpu_smt_mask is defined only in case of
> CONFIG_SCHED_SMT=y.
> 
> Note: There is no arch today which defines cpu_smt_mask unconditionally.
> So likely defining the cpu_smt_mask shouldn't lead redefintion errors.
> 
> Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>


I like this whole cleanup, fwiw.

Reviewed-by: Phil Auld <pauld@redhat.com>


Cheers,
Phil

> ---
>  include/linux/topology.h | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/topology.h b/include/linux/topology.h
> index 6575af39fd10..709a2dcf4c73 100644
> --- a/include/linux/topology.h
> +++ b/include/linux/topology.h
> @@ -230,11 +230,24 @@ static inline int cpu_to_mem(int cpu)
>  #define topology_drawer_cpumask(cpu)		cpumask_of(cpu)
>  #endif
>  
> -#if defined(CONFIG_SCHED_SMT) && !defined(cpu_smt_mask)
> +/*
> + * Defining cpu_smt_mask as cpumask_of that CPU helps to get
> + * rid of lot of ifdeffery all around the codebase in case of
> + * CONFIG_SCHED_SMT=n. It just means there are no other siblings, which
> + * is what is expected.
> + */
> +#if defined(CONFIG_SCHED_SMT)
> +# if !defined(cpu_smt_mask)
>  static inline const struct cpumask *cpu_smt_mask(int cpu)
>  {
>  	return topology_sibling_cpumask(cpu);
>  }
> +# endif
> +#else	/* !CONFIG_SCHED_SMT */
> +static inline const struct cpumask *cpu_smt_mask(int cpu)
> +{
> +	return cpumask_of(cpu);
> +}
>  #endif
>  
>  #ifndef topology_is_primary_thread
> -- 
> 2.47.3
> 
> 

-- 

[PATCH v2 2/3] sched: Simplify ifdeffery around cpu_smt_mask

[Shrikanth Hegde]

Now, that cpu_smt_mask is defined as cpumask_of(cpu) for
CONFIG_SCHED_SMT=n, it is possible to get rid of the ifdeffery.

Effectively,
- This makes sched_smt_present is defined always

- cpumask_weight(cpumask_of(cpu)) == 1. So sched_smt_present_inc/dec
  will never enable the sched_smt_present. Which is expected.

- Paths that were compile-time eliminated become runtime guarded
  using static keys.

- Defines set_idle_cores, test_idle_cores etc which could likely benefit
  the CONFIG_SCHED_SMT=n systems to use the same optimizations within the
  LLC at wakeups.

- This will expose sched_smt_present symbol for CONFIG_SCHED_SMT=n.
  Likely not a concern.

- There a bloat of code CONFIG_SCHED_SMT=n. (NR_CPUS=2048)
  add/remove: 24/18 grow/shrink: 26/28 up/down: 6396/-3188 (3208)
  Total: Before=30629880, After=30633088, chg +0.01%

- No code bloat for CONFIG_SCHED_SMT=y, which is expected.

- Add comments around stop_core_cpuslocked on why ifdefs are not
  removed.

- This leaves the remaining uses of CONFIG_SCHED_SMT mainly for
  topology building bits which have a policy based decision.

Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
---
 include/linux/sched/smt.h |  4 ----
 kernel/sched/core.c       |  6 ------
 kernel/sched/ext_idle.c   |  6 ------
 kernel/sched/fair.c       | 35 -----------------------------------
 kernel/sched/sched.h      |  6 ------
 kernel/sched/topology.c   |  2 --
 kernel/stop_machine.c     |  5 +++++
 kernel/workqueue.c        |  4 ----
 8 files changed, 5 insertions(+), 63 deletions(-)

diff --git a/include/linux/sched/smt.h b/include/linux/sched/smt.h
index 166b19af956f..cde6679c0278 100644
--- a/include/linux/sched/smt.h
+++ b/include/linux/sched/smt.h
@@ -4,16 +4,12 @@
 
 #include <linux/static_key.h>
 
-#ifdef CONFIG_SCHED_SMT
 extern struct static_key_false sched_smt_present;
 
 static __always_inline bool sched_smt_active(void)
 {
 	return static_branch_likely(&sched_smt_present);
 }
-#else
-static __always_inline bool sched_smt_active(void) { return false; }
-#endif
 
 void arch_smt_update(void);
 
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b905805bbcbe..3ae5f19c1b7e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8612,18 +8612,14 @@ static void cpuset_cpu_inactive(unsigned int cpu)
 
 static inline void sched_smt_present_inc(int cpu)
 {
-#ifdef CONFIG_SCHED_SMT
 	if (cpumask_weight(cpu_smt_mask(cpu)) == 2)
 		static_branch_inc_cpuslocked(&sched_smt_present);
-#endif
 }
 
 static inline void sched_smt_present_dec(int cpu)
 {
-#ifdef CONFIG_SCHED_SMT
 	if (cpumask_weight(cpu_smt_mask(cpu)) == 2)
 		static_branch_dec_cpuslocked(&sched_smt_present);
-#endif
 }
 
 int sched_cpu_activate(unsigned int cpu)
@@ -8711,9 +8707,7 @@ int sched_cpu_deactivate(unsigned int cpu)
 	 */
 	sched_smt_present_dec(cpu);
 
-#ifdef CONFIG_SCHED_SMT
 	sched_core_cpu_deactivate(cpu);
-#endif
 
 	if (!sched_smp_initialized)
 		return 0;
diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c
index 6e1980763270..9f5ad6b071f9 100644
--- a/kernel/sched/ext_idle.c
+++ b/kernel/sched/ext_idle.c
@@ -79,7 +79,6 @@ static bool scx_idle_test_and_clear_cpu(int cpu)
 	int node = scx_cpu_node_if_enabled(cpu);
 	struct cpumask *idle_cpus = idle_cpumask(node)->cpu;
 
-#ifdef CONFIG_SCHED_SMT
 	/*
 	 * SMT mask should be cleared whether we can claim @cpu or not. The SMT
 	 * cluster is not wholly idle either way. This also prevents
@@ -104,7 +103,6 @@ static bool scx_idle_test_and_clear_cpu(int cpu)
 		else if (cpumask_test_cpu(cpu, idle_smts))
 			__cpumask_clear_cpu(cpu, idle_smts);
 	}
-#endif
 
 	return cpumask_test_and_clear_cpu(cpu, idle_cpus);
 }
@@ -622,7 +620,6 @@ s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
 		goto out_unlock;
 	}
 
-#ifdef CONFIG_SCHED_SMT
 	/*
 	 * Use @prev_cpu's sibling if it's idle.
 	 */
@@ -634,7 +631,6 @@ s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
 				goto out_unlock;
 		}
 	}
-#endif
 
 	/*
 	 * Search for any idle CPU in the same LLC domain.
@@ -714,7 +710,6 @@ static void update_builtin_idle(int cpu, bool idle)
 
 	assign_cpu(cpu, idle_cpus, idle);
 
-#ifdef CONFIG_SCHED_SMT
 	if (sched_smt_active()) {
 		const struct cpumask *smt = cpu_smt_mask(cpu);
 		struct cpumask *idle_smts = idle_cpumask(node)->smt;
@@ -731,7 +726,6 @@ static void update_builtin_idle(int cpu, bool idle)
 			cpumask_andnot(idle_smts, idle_smts, smt);
 		}
 	}
-#endif
 }
 
 /*
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 3ebec186f982..353e31ecaadc 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1584,7 +1584,6 @@ update_stats_curr_start(struct cfs_rq *cfs_rq, struct sched_entity *se)
 
 static inline bool is_core_idle(int cpu)
 {
-#ifdef CONFIG_SCHED_SMT
 	int sibling;
 
 	for_each_cpu(sibling, cpu_smt_mask(cpu)) {
@@ -1594,7 +1593,6 @@ static inline bool is_core_idle(int cpu)
 		if (!idle_cpu(sibling))
 			return false;
 	}
-#endif
 
 	return true;
 }
@@ -2277,7 +2275,6 @@ numa_type numa_classify(unsigned int imbalance_pct,
 	return node_fully_busy;
 }
 
-#ifdef CONFIG_SCHED_SMT
 /* Forward declarations of select_idle_sibling helpers */
 static inline bool test_idle_cores(int cpu);
 static inline int numa_idle_core(int idle_core, int cpu)
@@ -2295,12 +2292,6 @@ static inline int numa_idle_core(int idle_core, int cpu)
 
 	return idle_core;
 }
-#else /* !CONFIG_SCHED_SMT: */
-static inline int numa_idle_core(int idle_core, int cpu)
-{
-	return idle_core;
-}
-#endif /* !CONFIG_SCHED_SMT */
 
 /*
  * Gather all necessary information to make NUMA balancing placement
@@ -7811,7 +7802,6 @@ static inline int __select_idle_cpu(int cpu, struct task_struct *p)
 	return -1;
 }
 
-#ifdef CONFIG_SCHED_SMT
 DEFINE_STATIC_KEY_FALSE(sched_smt_present);
 EXPORT_SYMBOL_GPL(sched_smt_present);
 
@@ -7921,29 +7911,6 @@ static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int t
 	return -1;
 }
 
-#else /* !CONFIG_SCHED_SMT: */
-
-static inline void set_idle_cores(int cpu, int val)
-{
-}
-
-static inline bool test_idle_cores(int cpu)
-{
-	return false;
-}
-
-static inline int select_idle_core(struct task_struct *p, int core, struct cpumask *cpus, int *idle_cpu)
-{
-	return __select_idle_cpu(core, p);
-}
-
-static inline int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int target)
-{
-	return -1;
-}
-
-#endif /* !CONFIG_SCHED_SMT */
-
 /*
  * Scan the LLC domain for idle CPUs; this is dynamically regulated by
  * comparing the average scan cost (tracked in sd->avg_scan_cost) against the
@@ -12036,9 +12003,7 @@ static int should_we_balance(struct lb_env *env)
 			 * idle has been found, then its not needed to check other
 			 * SMT siblings for idleness:
 			 */
-#ifdef CONFIG_SCHED_SMT
 			cpumask_andnot(swb_cpus, swb_cpus, cpu_smt_mask(cpu));
-#endif
 			continue;
 		}
 
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 9f63b15d309d..e476623a0c2a 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1667,7 +1667,6 @@ do {						\
 	flags = _raw_spin_rq_lock_irqsave(rq);	\
 } while (0)
 
-#ifdef CONFIG_SCHED_SMT
 extern void __update_idle_core(struct rq *rq);
 
 static inline void update_idle_core(struct rq *rq)
@@ -1676,12 +1675,7 @@ static inline void update_idle_core(struct rq *rq)
 		__update_idle_core(rq);
 }
 
-#else /* !CONFIG_SCHED_SMT: */
-static inline void update_idle_core(struct rq *rq) { }
-#endif /* !CONFIG_SCHED_SMT */
-
 #ifdef CONFIG_FAIR_GROUP_SCHED
-
 static inline struct task_struct *task_of(struct sched_entity *se)
 {
 	WARN_ON_ONCE(!entity_is_task(se));
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 5847b83d9d55..a1f46e3f4ede 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1310,9 +1310,7 @@ static void init_sched_groups_capacity(int cpu, struct sched_domain *sd)
 		cpumask_copy(mask, sched_group_span(sg));
 		for_each_cpu(cpu, mask) {
 			cores++;
-#ifdef CONFIG_SCHED_SMT
 			cpumask_andnot(mask, mask, cpu_smt_mask(cpu));
-#endif
 		}
 		sg->cores = cores;
 
diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index 3fe6b0c99f3d..773d8e9ae30c 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -633,6 +633,11 @@ int stop_machine(cpu_stop_fn_t fn, void *data, const struct cpumask *cpus)
 EXPORT_SYMBOL_GPL(stop_machine);
 
 #ifdef CONFIG_SCHED_SMT
+/*
+ * INTEL_IFS is the only user of this API. That selftest can
+ * only be compiled if SMP=y. On x86 it selects SCHED_SMT.
+ * Keep the ifdefs for now.
+ */
 int stop_core_cpuslocked(unsigned int cpu, cpu_stop_fn_t fn, void *data)
 {
 	const struct cpumask *smt_mask = cpu_smt_mask(cpu);
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 3d2e3b2ec528..c911fdcb4428 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -8198,11 +8198,7 @@ static bool __init cpus_dont_share(int cpu0, int cpu1)
 
 static bool __init cpus_share_smt(int cpu0, int cpu1)
 {
-#ifdef CONFIG_SCHED_SMT
 	return cpumask_test_cpu(cpu0, cpu_smt_mask(cpu1));
-#else
-	return false;
-#endif
 }
 
 static bool __init cpus_share_numa(int cpu0, int cpu1)
-- 
2.47.3

Re: [PATCH v2 2/3] sched: Simplify ifdeffery around cpu_smt_mask

[Phil Auld]

On Tue, May 12, 2026 at 08:51:24PM +0530 Shrikanth Hegde wrote:
> Now, that cpu_smt_mask is defined as cpumask_of(cpu) for
> CONFIG_SCHED_SMT=n, it is possible to get rid of the ifdeffery.
> 
> Effectively,
> - This makes sched_smt_present is defined always
> 
> - cpumask_weight(cpumask_of(cpu)) == 1. So sched_smt_present_inc/dec
>   will never enable the sched_smt_present. Which is expected.
> 
> - Paths that were compile-time eliminated become runtime guarded
>   using static keys.
> 
> - Defines set_idle_cores, test_idle_cores etc which could likely benefit
>   the CONFIG_SCHED_SMT=n systems to use the same optimizations within the
>   LLC at wakeups.
> 
> - This will expose sched_smt_present symbol for CONFIG_SCHED_SMT=n.
>   Likely not a concern.
> 
> - There a bloat of code CONFIG_SCHED_SMT=n. (NR_CPUS=2048)
>   add/remove: 24/18 grow/shrink: 26/28 up/down: 6396/-3188 (3208)
>   Total: Before=30629880, After=30633088, chg +0.01%
> 
> - No code bloat for CONFIG_SCHED_SMT=y, which is expected.
> 
> - Add comments around stop_core_cpuslocked on why ifdefs are not
>   removed.
> 
> - This leaves the remaining uses of CONFIG_SCHED_SMT mainly for
>   topology building bits which have a policy based decision.
> 
> Acked-by: Tejun Heo <tj@kernel.org>
> Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>

LGTM.

Reviewed-by: Phil Auld <pauld@redhat.com>

Cheers,
Phil


> ---
>  include/linux/sched/smt.h |  4 ----
>  kernel/sched/core.c       |  6 ------
>  kernel/sched/ext_idle.c   |  6 ------
>  kernel/sched/fair.c       | 35 -----------------------------------
>  kernel/sched/sched.h      |  6 ------
>  kernel/sched/topology.c   |  2 --
>  kernel/stop_machine.c     |  5 +++++
>  kernel/workqueue.c        |  4 ----
>  8 files changed, 5 insertions(+), 63 deletions(-)
> 
> diff --git a/include/linux/sched/smt.h b/include/linux/sched/smt.h
> index 166b19af956f..cde6679c0278 100644
> --- a/include/linux/sched/smt.h
> +++ b/include/linux/sched/smt.h
> @@ -4,16 +4,12 @@
>  
>  #include <linux/static_key.h>
>  
> -#ifdef CONFIG_SCHED_SMT
>  extern struct static_key_false sched_smt_present;
>  
>  static __always_inline bool sched_smt_active(void)
>  {
>  	return static_branch_likely(&sched_smt_present);
>  }
> -#else
> -static __always_inline bool sched_smt_active(void) { return false; }
> -#endif
>  
>  void arch_smt_update(void);
>  
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index b905805bbcbe..3ae5f19c1b7e 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -8612,18 +8612,14 @@ static void cpuset_cpu_inactive(unsigned int cpu)
>  
>  static inline void sched_smt_present_inc(int cpu)
>  {
> -#ifdef CONFIG_SCHED_SMT
>  	if (cpumask_weight(cpu_smt_mask(cpu)) == 2)
>  		static_branch_inc_cpuslocked(&sched_smt_present);
> -#endif
>  }
>  
>  static inline void sched_smt_present_dec(int cpu)
>  {
> -#ifdef CONFIG_SCHED_SMT
>  	if (cpumask_weight(cpu_smt_mask(cpu)) == 2)
>  		static_branch_dec_cpuslocked(&sched_smt_present);
> -#endif
>  }
>  
>  int sched_cpu_activate(unsigned int cpu)
> @@ -8711,9 +8707,7 @@ int sched_cpu_deactivate(unsigned int cpu)
>  	 */
>  	sched_smt_present_dec(cpu);
>  
> -#ifdef CONFIG_SCHED_SMT
>  	sched_core_cpu_deactivate(cpu);
> -#endif
>  
>  	if (!sched_smp_initialized)
>  		return 0;
> diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c
> index 6e1980763270..9f5ad6b071f9 100644
> --- a/kernel/sched/ext_idle.c
> +++ b/kernel/sched/ext_idle.c
> @@ -79,7 +79,6 @@ static bool scx_idle_test_and_clear_cpu(int cpu)
>  	int node = scx_cpu_node_if_enabled(cpu);
>  	struct cpumask *idle_cpus = idle_cpumask(node)->cpu;
>  
> -#ifdef CONFIG_SCHED_SMT
>  	/*
>  	 * SMT mask should be cleared whether we can claim @cpu or not. The SMT
>  	 * cluster is not wholly idle either way. This also prevents
> @@ -104,7 +103,6 @@ static bool scx_idle_test_and_clear_cpu(int cpu)
>  		else if (cpumask_test_cpu(cpu, idle_smts))
>  			__cpumask_clear_cpu(cpu, idle_smts);
>  	}
> -#endif
>  
>  	return cpumask_test_and_clear_cpu(cpu, idle_cpus);
>  }
> @@ -622,7 +620,6 @@ s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
>  		goto out_unlock;
>  	}
>  
> -#ifdef CONFIG_SCHED_SMT
>  	/*
>  	 * Use @prev_cpu's sibling if it's idle.
>  	 */
> @@ -634,7 +631,6 @@ s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
>  				goto out_unlock;
>  		}
>  	}
> -#endif
>  
>  	/*
>  	 * Search for any idle CPU in the same LLC domain.
> @@ -714,7 +710,6 @@ static void update_builtin_idle(int cpu, bool idle)
>  
>  	assign_cpu(cpu, idle_cpus, idle);
>  
> -#ifdef CONFIG_SCHED_SMT
>  	if (sched_smt_active()) {
>  		const struct cpumask *smt = cpu_smt_mask(cpu);
>  		struct cpumask *idle_smts = idle_cpumask(node)->smt;
> @@ -731,7 +726,6 @@ static void update_builtin_idle(int cpu, bool idle)
>  			cpumask_andnot(idle_smts, idle_smts, smt);
>  		}
>  	}
> -#endif
>  }
>  
>  /*
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 3ebec186f982..353e31ecaadc 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1584,7 +1584,6 @@ update_stats_curr_start(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  
>  static inline bool is_core_idle(int cpu)
>  {
> -#ifdef CONFIG_SCHED_SMT
>  	int sibling;
>  
>  	for_each_cpu(sibling, cpu_smt_mask(cpu)) {
> @@ -1594,7 +1593,6 @@ static inline bool is_core_idle(int cpu)
>  		if (!idle_cpu(sibling))
>  			return false;
>  	}
> -#endif
>  
>  	return true;
>  }
> @@ -2277,7 +2275,6 @@ numa_type numa_classify(unsigned int imbalance_pct,
>  	return node_fully_busy;
>  }
>  
> -#ifdef CONFIG_SCHED_SMT
>  /* Forward declarations of select_idle_sibling helpers */
>  static inline bool test_idle_cores(int cpu);
>  static inline int numa_idle_core(int idle_core, int cpu)
> @@ -2295,12 +2292,6 @@ static inline int numa_idle_core(int idle_core, int cpu)
>  
>  	return idle_core;
>  }
> -#else /* !CONFIG_SCHED_SMT: */
> -static inline int numa_idle_core(int idle_core, int cpu)
> -{
> -	return idle_core;
> -}
> -#endif /* !CONFIG_SCHED_SMT */
>  
>  /*
>   * Gather all necessary information to make NUMA balancing placement
> @@ -7811,7 +7802,6 @@ static inline int __select_idle_cpu(int cpu, struct task_struct *p)
>  	return -1;
>  }
>  
> -#ifdef CONFIG_SCHED_SMT
>  DEFINE_STATIC_KEY_FALSE(sched_smt_present);
>  EXPORT_SYMBOL_GPL(sched_smt_present);
>  
> @@ -7921,29 +7911,6 @@ static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int t
>  	return -1;
>  }
>  
> -#else /* !CONFIG_SCHED_SMT: */
> -
> -static inline void set_idle_cores(int cpu, int val)
> -{
> -}
> -
> -static inline bool test_idle_cores(int cpu)
> -{
> -	return false;
> -}
> -
> -static inline int select_idle_core(struct task_struct *p, int core, struct cpumask *cpus, int *idle_cpu)
> -{
> -	return __select_idle_cpu(core, p);
> -}
> -
> -static inline int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int target)
> -{
> -	return -1;
> -}
> -
> -#endif /* !CONFIG_SCHED_SMT */
> -
>  /*
>   * Scan the LLC domain for idle CPUs; this is dynamically regulated by
>   * comparing the average scan cost (tracked in sd->avg_scan_cost) against the
> @@ -12036,9 +12003,7 @@ static int should_we_balance(struct lb_env *env)
>  			 * idle has been found, then its not needed to check other
>  			 * SMT siblings for idleness:
>  			 */
> -#ifdef CONFIG_SCHED_SMT
>  			cpumask_andnot(swb_cpus, swb_cpus, cpu_smt_mask(cpu));
> -#endif
>  			continue;
>  		}
>  
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 9f63b15d309d..e476623a0c2a 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1667,7 +1667,6 @@ do {						\
>  	flags = _raw_spin_rq_lock_irqsave(rq);	\
>  } while (0)
>  
> -#ifdef CONFIG_SCHED_SMT
>  extern void __update_idle_core(struct rq *rq);
>  
>  static inline void update_idle_core(struct rq *rq)
> @@ -1676,12 +1675,7 @@ static inline void update_idle_core(struct rq *rq)
>  		__update_idle_core(rq);
>  }
>  
> -#else /* !CONFIG_SCHED_SMT: */
> -static inline void update_idle_core(struct rq *rq) { }
> -#endif /* !CONFIG_SCHED_SMT */
> -
>  #ifdef CONFIG_FAIR_GROUP_SCHED
> -
>  static inline struct task_struct *task_of(struct sched_entity *se)
>  {
>  	WARN_ON_ONCE(!entity_is_task(se));
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 5847b83d9d55..a1f46e3f4ede 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -1310,9 +1310,7 @@ static void init_sched_groups_capacity(int cpu, struct sched_domain *sd)
>  		cpumask_copy(mask, sched_group_span(sg));
>  		for_each_cpu(cpu, mask) {
>  			cores++;
> -#ifdef CONFIG_SCHED_SMT
>  			cpumask_andnot(mask, mask, cpu_smt_mask(cpu));
> -#endif
>  		}
>  		sg->cores = cores;
>  
> diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
> index 3fe6b0c99f3d..773d8e9ae30c 100644
> --- a/kernel/stop_machine.c
> +++ b/kernel/stop_machine.c
> @@ -633,6 +633,11 @@ int stop_machine(cpu_stop_fn_t fn, void *data, const struct cpumask *cpus)
>  EXPORT_SYMBOL_GPL(stop_machine);
>  
>  #ifdef CONFIG_SCHED_SMT
> +/*
> + * INTEL_IFS is the only user of this API. That selftest can
> + * only be compiled if SMP=y. On x86 it selects SCHED_SMT.
> + * Keep the ifdefs for now.
> + */
>  int stop_core_cpuslocked(unsigned int cpu, cpu_stop_fn_t fn, void *data)
>  {
>  	const struct cpumask *smt_mask = cpu_smt_mask(cpu);
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index 3d2e3b2ec528..c911fdcb4428 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -8198,11 +8198,7 @@ static bool __init cpus_dont_share(int cpu0, int cpu1)
>  
>  static bool __init cpus_share_smt(int cpu0, int cpu1)
>  {
> -#ifdef CONFIG_SCHED_SMT
>  	return cpumask_test_cpu(cpu0, cpu_smt_mask(cpu1));
> -#else
> -	return false;
> -#endif
>  }
>  
>  static bool __init cpus_share_numa(int cpu0, int cpu1)
> -- 
> 2.47.3
> 
> 

-- 

[PATCH v2 3/3] sched/fair: Add compile time check in fastpaths for CONFIG_SCHED_SMT=n

[Shrikanth Hegde]

For fastpaths such as wakeup, load balance even a minimal code additons
can pop up. Add IS_ENABLED checks there to ensure there is no overhead.

Other places are either have sched_smt_active() check or they are not in
fast paths.

Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
---
 kernel/sched/fair.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 353e31ecaadc..b6f9592b31fd 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1586,6 +1586,9 @@ static inline bool is_core_idle(int cpu)
 {
 	int sibling;
 
+	if (!IS_ENABLED(CONFIG_SCHED_SMT))
+		return true;
+
 	for_each_cpu(sibling, cpu_smt_mask(cpu)) {
 		if (cpu == sibling)
 			continue;
@@ -12003,7 +12006,8 @@ static int should_we_balance(struct lb_env *env)
 			 * idle has been found, then its not needed to check other
 			 * SMT siblings for idleness:
 			 */
-			cpumask_andnot(swb_cpus, swb_cpus, cpu_smt_mask(cpu));
+			if (IS_ENABLED(CONFIG_SCHED_SMT))
+				cpumask_andnot(swb_cpus, swb_cpus, cpu_smt_mask(cpu));
 			continue;
 		}
 
-- 
2.47.3

Re: [PATCH v2 3/3] sched/fair: Add compile time check in fastpaths for CONFIG_SCHED_SMT=n

[Phil Auld]

Hi Shrikanth,

On Tue, May 12, 2026 at 08:51:25PM +0530 Shrikanth Hegde wrote:
> For fastpaths such as wakeup, load balance even a minimal code additons
> can pop up. Add IS_ENABLED checks there to ensure there is no overhead.

"For fastpaths such as wakeup and load balance, even minimal code additions
can add up. Add IS_ENABLED() checks..."  ?

> 
> Other places are either have sched_smt_active() check or they are not in

s/are//

"have a sched_smt_active_check()" or drop "check" ? 


Otherwise lgtm.

Reviewed-by: Phil Auld <pauld@redhat.com>


Cheers,
Phil



> fast paths.
> 
> Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
> ---
>  kernel/sched/fair.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 353e31ecaadc..b6f9592b31fd 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1586,6 +1586,9 @@ static inline bool is_core_idle(int cpu)
>  {
>  	int sibling;
>  
> +	if (!IS_ENABLED(CONFIG_SCHED_SMT))
> +		return true;
> +
>  	for_each_cpu(sibling, cpu_smt_mask(cpu)) {
>  		if (cpu == sibling)
>  			continue;
> @@ -12003,7 +12006,8 @@ static int should_we_balance(struct lb_env *env)
>  			 * idle has been found, then its not needed to check other
>  			 * SMT siblings for idleness:
>  			 */
> -			cpumask_andnot(swb_cpus, swb_cpus, cpu_smt_mask(cpu));
> +			if (IS_ENABLED(CONFIG_SCHED_SMT))
> +				cpumask_andnot(swb_cpus, swb_cpus, cpu_smt_mask(cpu));
>  			continue;
>  		}
>  
> -- 
> 2.47.3
> 
> 

-- 

Re: [PATCH v2 0/3] sched: Simplify ifdeffery around CONFIG_SCHED_SMT

[Valentin Schneider]

On 12/05/26 20:51, Shrikanth Hegde wrote:
> Semantics
> =========
> - For CONFIG_SCHED_SMT=y:
>     No functional change.
> - For CONFIG_SCHED_SMT=n:
>     - cpu_smt_mask(cpu) becomes cpumask_of(cpu), effectively making it
>       per CPU with no siblings.
>     - sched_smt_present remains defined, but never becomes active:
>        Since cpumask_weight(cpumask_of(cpu)) == 1
>
> Performance impact
> ==================
> - CONFIG_SCHED_SMT=y:
>     No change in generated code.
> - CONFIG_SCHED_SMT=n:
>     - Small increase in text size (~0.01%) due to removal of compile-time
>       stubs. Most paths remain effectively dead due to static keys.
>     - Fast paths are protected using IS_ENABLED(CONFIG_SCHED_SMT).
>
> With that, cpu_smt_mask() to be used unconditionally and reduces
> CONFIG_SCHED_SMT-specific code paths, improving readability and
> maintainability.
>
> This leaves the remaining use of CONFIG_SCHED_SMT mainly for topology
> handling bits.
>

Reviewed-by: Valentin Schneider <vschneid@redhat.com>