Re: GCC section alignment, and GCC-4.9 being a weird one

[Nick Desaulniers]

On Wed, Oct 21, 2020 at 6:45 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> Ah, thanks!
>
> In that case something like the below ought to make it good.
>
> I'll go feed it to the robots, see if anything falls over.
>
> ---
>  kernel/sched/deadline.c  | 4 +++-
>  kernel/sched/fair.c      | 4 +++-
>  kernel/sched/idle.c      | 4 +++-
>  kernel/sched/rt.c        | 4 +++-
>  kernel/sched/sched.h     | 3 +--
>  kernel/sched/stop_task.c | 3 ++-
>  6 files changed, 15 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 6d93f4518734..f4855203143d 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -2504,7 +2504,9 @@ static void prio_changed_dl(struct rq *rq, struct task_struct *p,
>  }
>
>  const struct sched_class dl_sched_class
> -       __attribute__((section("__dl_sched_class"))) = {
> +       __attribute__((section("__dl_sched_class")))
> +       __attribute__((aligned(__alignof__(struct sched_class)))) = {

If you used some of the macros from
include/linux/compiler_attributes.h like __section and __aligned, I
would prefer it.  Please consider spelling out __attribute__(()) an
antipattern.

> +
>         .enqueue_task           = enqueue_task_dl,
>         .dequeue_task           = dequeue_task_dl,
>         .yield_task             = yield_task_dl,
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index aa4c6227cd6d..9bfa9f545b9a 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -11159,7 +11159,9 @@ static unsigned int get_rr_interval_fair(struct rq *rq, struct task_struct *task
>   * All the scheduling class methods:
>   */
>  const struct sched_class fair_sched_class
> -       __attribute__((section("__fair_sched_class"))) = {
> +       __attribute__((section("__fair_sched_class")))
> +       __attribute__((aligned(__alignof__(struct sched_class)))) = {
> +
>         .enqueue_task           = enqueue_task_fair,
>         .dequeue_task           = dequeue_task_fair,
>         .yield_task             = yield_task_fair,
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index f324dc36fc43..c74ded2cabd2 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -458,7 +458,9 @@ static void update_curr_idle(struct rq *rq)
>   * Simple, special scheduling class for the per-CPU idle tasks:
>   */
>  const struct sched_class idle_sched_class
> -       __attribute__((section("__idle_sched_class"))) = {
> +       __attribute__((section("__idle_sched_class")))
> +       __attribute__((aligned(__alignof__(struct sched_class)))) = {
> +
>         /* no enqueue/yield_task for idle tasks */
>
>         /* dequeue is not valid, we print a debug message there: */
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index f215eea6a966..002cdbfa5308 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -2430,7 +2430,9 @@ static unsigned int get_rr_interval_rt(struct rq *rq, struct task_struct *task)
>  }
>
>  const struct sched_class rt_sched_class
> -       __attribute__((section("__rt_sched_class"))) = {
> +       __attribute__((section("__rt_sched_class")))
> +       __attribute__((aligned(__alignof__(struct sched_class)))) = {
> +
>         .enqueue_task           = enqueue_task_rt,
>         .dequeue_task           = dequeue_task_rt,
>         .yield_task             = yield_task_rt,
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 28709f6b0975..42cf1e0cbf16 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -67,7 +67,6 @@
>  #include <linux/tsacct_kern.h>
>
>  #include <asm/tlb.h>
> -#include <asm-generic/vmlinux.lds.h>
>
>  #ifdef CONFIG_PARAVIRT
>  # include <asm/paravirt.h>
> @@ -1826,7 +1825,7 @@ struct sched_class {
>  #ifdef CONFIG_FAIR_GROUP_SCHED
>         void (*task_change_group)(struct task_struct *p, int type);
>  #endif
> -} __aligned(STRUCT_ALIGNMENT); /* STRUCT_ALIGN(), vmlinux.lds.h */
> +};
>
>  static inline void put_prev_task(struct rq *rq, struct task_struct *prev)
>  {
> diff --git a/kernel/sched/stop_task.c b/kernel/sched/stop_task.c
> index 394bc8126a1e..7bac6e0a9212 100644
> --- a/kernel/sched/stop_task.c
> +++ b/kernel/sched/stop_task.c
> @@ -110,7 +110,8 @@ static void update_curr_stop(struct rq *rq)
>   * Simple, special scheduling class for the per-CPU stop tasks:
>   */
>  const struct sched_class stop_sched_class
> -       __attribute__((section("__stop_sched_class"))) = {
> +       __attribute__((section("__stop_sched_class")))
> +       __attribute__((aligned(__alignof__(struct sched_class)))) = {
>
>         .enqueue_task           = enqueue_task_stop,
>         .dequeue_task           = dequeue_task_stop,



-- 
Thanks,
~Nick Desaulniers