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SubjectRe: [PATCH RFC] sched: Micro optimization in pick_next_task() and in check_preempt_curr()
On Thu, 19 Dec 2019 17:46:37 +0300
Kirill Tkhai <ktkhai@virtuozzo.com> wrote:

> On 19.12.2019 17:43, Steven Rostedt wrote:
> > On Thu, 19 Dec 2019 17:25:40 +0300
> > Kirill Tkhai <ktkhai@virtuozzo.com> wrote:
> >
> >> (17:19:25) nbjoerg: but it is not guarenteed behavior
> >> (17:19:50) nbjoerg: if for some strange reason you really need to enforce relative orders of global objects, put them in consecutively named sections
> >
> > Which appears to work. I tried this patch on top of yours:
> >
> > Not sure how this does with locality though.
>
> Hm, I'm not sure, but AFAIR some (all?) sections are aligned at 4K.
> Will this bring holes (4K-sizeof(struct sched_class)) in address
> space?

I believe only if you set the align attribute in the linker script.
With this and your patch:

# grep sched_class /proc/kallsyms
ffffffff8e760900 D idle_sched_class
ffffffff8e7609e0 D fair_sched_class
ffffffff8e760ac0 D rt_sched_class
ffffffff8e760ba0 D dl_sched_class
ffffffff8e760c80 D stop_sched_class

-- Steve

>
>
> > -- Steve
> >
> > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> > index e00f41aa8ec4..ff12a422ff19 100644
> > --- a/include/asm-generic/vmlinux.lds.h
> > +++ b/include/asm-generic/vmlinux.lds.h
> > @@ -108,6 +108,13 @@
> > #define SBSS_MAIN .sbss
> > #endif
> >
> > +#define SCHED_DATA \
> > + *(__idle_sched_class) \
> > + *(__fair_sched_class) \
> > + *(__rt_sched_class) \
> > + *(__dl_sched_class) \
> > + *(__stop_sched_class)
> > +
> > /*
> > * Align to a 32 byte boundary equal to the
> > * alignment gcc 4.5 uses for a struct
> > @@ -308,6 +315,7 @@
> > #define DATA_DATA \
> > *(.xiptext) \
> > *(DATA_MAIN) \
> > + SCHED_DATA \
> > *(.ref.data) \
> > *(.data..shared_aligned) /* percpu related */ \
> > MEM_KEEP(init.data*) \
> > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > index 43323f875cb9..5abdbe569f93 100644
> > --- a/kernel/sched/deadline.c
> > +++ b/kernel/sched/deadline.c
> > @@ -2428,7 +2428,8 @@ static void prio_changed_dl(struct rq *rq, struct task_struct *p,
> > }
> > }
> >
> > -const struct sched_class dl_sched_class = {
> > +const struct sched_class dl_sched_class
> > + __attribute__((section("__dl_sched_class"))) = {
> > .next = &rt_sched_class,
> > .enqueue_task = enqueue_task_dl,
> > .dequeue_task = dequeue_task_dl,
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 08a233e97a01..e745fe0e0cd3 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -10745,7 +10745,8 @@ 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 = {
> > +const struct sched_class fair_sched_class
> > + __attribute__((section("__fair_sched_class"))) = {
> > .next = &idle_sched_class,
> > .enqueue_task = enqueue_task_fair,
> > .dequeue_task = dequeue_task_fair,
> > diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> > index ffa959e91227..700a9c826f0e 100644
> > --- a/kernel/sched/idle.c
> > +++ b/kernel/sched/idle.c
> > @@ -454,7 +454,8 @@ 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 = {
> > +const struct sched_class idle_sched_class
> > + __attribute__((section("__idle_sched_class"))) = {
> > /* .next is NULL */
> > /* no enqueue/yield_task for idle tasks */
> >
> > diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> > index e591d40fd645..5d3f9bcddaeb 100644
> > --- a/kernel/sched/rt.c
> > +++ b/kernel/sched/rt.c
> > @@ -2354,7 +2354,8 @@ static unsigned int get_rr_interval_rt(struct rq *rq, struct task_struct *task)
> > return 0;
> > }
> >
> > -const struct sched_class rt_sched_class = {
> > +const struct sched_class rt_sched_class
> > + __attribute__((section("__rt_sched_class"))) = {
> > .next = &fair_sched_class,
> > .enqueue_task = enqueue_task_rt,
> > .dequeue_task = dequeue_task_rt,
> > diff --git a/kernel/sched/stop_task.c b/kernel/sched/stop_task.c
> > index 4c9e9975684f..03bc7530ff75 100644
> > --- a/kernel/sched/stop_task.c
> > +++ b/kernel/sched/stop_task.c
> > @@ -115,7 +115,8 @@ 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 = {
> > +const struct sched_class stop_sched_class
> > + __attribute__((section("__stop_sched_class"))) = {
> > .next = &dl_sched_class,
> >
> > .enqueue_task = enqueue_task_stop,
> >

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