On Tue, Jun 30, 2020 at 04:02:31PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 30, 2020 at 03:55:05PM +0200, Rasmus Villemoes wrote:
> 
> > > Consistently so with GCC-4.9. Any other GCC I tried does the sane thing.
> > 
> > Does that include gcc 4.8, or is it only "anything newer than 4.9"?
> 
> It includes 4.8 :-)
> 
> > so the section it was put in has an alignment of 64. The generated
> > assembly is indeed
> > 
> >         .globl  fair_sched_class
> >         .section        __fair_sched_class,"a",@progbits
> >         .align 64
> > 
> > /me goes brew coffee
> 
> Right.. so I now have the below patch, and with that I get:
> 
> 62931: c1e62c20     0 NOTYPE  GLOBAL DEFAULT    2 __begin_sched_classes
> 65736: c1e62e40   128 OBJECT  GLOBAL DEFAULT    2 stop_sched_class
> 71813: c1e62cc0   128 OBJECT  GLOBAL DEFAULT    2 fair_sched_class
> 78689: c1e62c40   128 OBJECT  GLOBAL DEFAULT    2 idle_sched_class
> 78953: c1e62ec0     0 NOTYPE  GLOBAL DEFAULT    2 __end_sched_classes
> 79090: c1e62d40   128 OBJECT  GLOBAL DEFAULT    2 rt_sched_class
> 79431: c1e62dc0   128 OBJECT  GLOBAL DEFAULT    2 dl_sched_class
> 
> 
> Which has me stumped on __begin_sched_classes being on a 32byte edge
> (and crashes differently due to that).
> 
> Argh!!

Steve suggested adding a dummy variable before the lot and this actually
works... But this just cannot be right :-(


---
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index 66fb84c3dc7ee..9c0ee5cf73a50 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -108,6 +108,17 @@
 #define SBSS_MAIN .sbss
 #endif
 
+/*
+ * Align to a 32 byte boundary equal to the
+ * alignment gcc 4.5 uses for a struct
+ */
+#if GCC_VERSION >= 40900 && GCC_VERSION < 50000
+#define STRUCT_ALIGNMENT 64
+#else
+#define STRUCT_ALIGNMENT 32
+#endif
+#define STRUCT_ALIGN() . = ALIGN(STRUCT_ALIGNMENT)
+
 /*
  * The order of the sched class addresses are important, as they are
  * used to determine the order of the priority of each sched class in
@@ -115,6 +126,7 @@
  */
 #define SCHED_DATA				\
 	STRUCT_ALIGN();				\
+	*(__dummy_sched_class)			\
 	__begin_sched_classes = .;		\
 	*(__idle_sched_class)			\
 	*(__fair_sched_class)			\
@@ -123,13 +135,6 @@
 	*(__stop_sched_class)			\
 	__end_sched_classes = .;
 
-/*
- * Align to a 32 byte boundary equal to the
- * alignment gcc 4.5 uses for a struct
- */
-#define STRUCT_ALIGNMENT 32
-#define STRUCT_ALIGN() . = ALIGN(STRUCT_ALIGNMENT)
-
 /* The actual configuration determine if the init/exit sections
  * are handled as text/data or they can be discarded (which
  * often happens at runtime)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 81640fe0eae8f..f8535a3438819 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6641,6 +6641,9 @@ static struct kmem_cache *task_group_cache __read_mostly;
 DECLARE_PER_CPU(cpumask_var_t, load_balance_mask);
 DECLARE_PER_CPU(cpumask_var_t, select_idle_mask);
 
+const struct sched_class dummy_sched_class
+	__attribute__((section("__dummy_sched_class")));
+
 void __init sched_init(void)
 {
 	unsigned long ptr = 0;
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 4165c06d1d7bd..33251d0ab62e7 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -67,6 +67,7 @@
 #include <linux/tsacct_kern.h>
 
 #include <asm/tlb.h>
+#include <asm-generic/vmlinux.lds.h>
 
 #ifdef CONFIG_PARAVIRT
 # include <asm/paravirt.h>
@@ -1811,7 +1812,7 @@ struct sched_class {
 #ifdef CONFIG_FAIR_GROUP_SCHED
 	void (*task_change_group)(struct task_struct *p, int type);
 #endif
-} __aligned(32); /* STRUCT_ALIGN(), vmlinux.lds.h */
+} __aligned(STRUCT_ALIGNMENT); /* STRUCT_ALIGN(), vmlinux.lds.h */
 
 static inline void put_prev_task(struct rq *rq, struct task_struct *prev)
 {