linux-kernel - Re: [PATCH RFC] sched: Micro optimization in pick_next_task() and in check_preempt

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Date:   Thu, 19 Dec 2019 09:59:41 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, bsegall@...gle.com, mgorman@...e.de,
        linux-kernel@...r.kernel.org
Subject: Re: [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@...tuozzo.com> wrote:

> On 19.12.2019 17:43, Steven Rostedt wrote:
> > On Thu, 19 Dec 2019 17:25:40 +0300
> > Kirill Tkhai <ktkhai@...tuozzo.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,
> >