Re: [PATCH] sched/core: Minor optimize pick_next_task() when core-sched enable

[Joel Fernandes]

On Mon, Mar 20, 2023 at 4:55 AM Hao Jia <jiahao.os@xxxxxxxxxxxxx> wrote:
>
> kindly ping...
>
> On 2023/3/8 Hao Jia wrote:
> > If core-sched is enabled, sometimes we will traverse each CPU on the core
> > to find the highest priority task 'max' on the entire core, and then try
> > and find a runnable task that matches @max.
> > But in the following case, we choose the runnable task is not the best.
> >
> > core max: task2 (cookie 0)
> >
> >       rq0                             rq1
> > task0(cookie non-zero)                task2(cookie 0)
> > task1(cookie 0)
> > task3(cookie 0)
> > ...
> >
> > pick-task: idle                       pick-task: task2
> >
> > CPU0 and CPU1 are two CPUs on the same core, task0 and task2 are the
> > highest priority tasks on rq0 and rq1 respectively, task2 is @max
> > on the entire core.
> >
> > In the case that 'max' has a zero cookie, instead of continuing to
> > search for a runnable task on rq0 that matches @max's cookie, we
> > choose idle for rq0 directly.
> > At this time, it is obviously better to choose task1 to run for rq0,
> > which will increase the CPU utilization.
> > Therefore, we queue tasks with zero cookies in core_tree, and record
> > the number of non-zero cookie tasks of each rq to detect the status
> > of the sched-core.

I do remember this as a known issue (more of a known but unimplemented
optimization) which happens when you have a high priority non-cookie
task which is in front of several low priority ones on the same
thread/rq. Adding +Vineeth Pillai to see if he remembers the issue.

I can try to take a look at it this week to make sense of your patch.
The code in upstream has changed quite a bit since we did this work on
the older kernels, so allow some time to page it all in.

Meanwhile, could you please provide some more details of your
usecase/workload and how this patch improves it? Also out of
curiosity, is bytedance using core scheduling for security or
something else?

Thanks,

 - Joel


> >
> > Signed-off-by: Hao Jia <jiahao.os@xxxxxxxxxxxxx>
> > ---
> >   kernel/sched/core.c       | 29 +++++++++++++++--------------
> >   kernel/sched/core_sched.c |  9 ++++-----
> >   kernel/sched/sched.h      |  1 +
> >   3 files changed, 20 insertions(+), 19 deletions(-)
> >
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index af017e038b48..765cd14c52e1 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -236,8 +236,8 @@ void sched_core_enqueue(struct rq *rq, struct task_struct *p)
> >   {
> >       rq->core->core_task_seq++;
> >
> > -     if (!p->core_cookie)
> > -             return;
> > +     if (p->core_cookie)
> > +             rq->cookied_count++;
> >
> >       rb_add(&p->core_node, &rq->core_tree, rb_sched_core_less);
> >   }
> > @@ -246,11 +246,16 @@ void sched_core_dequeue(struct rq *rq, struct task_struct *p, int flags)
> >   {
> >       rq->core->core_task_seq++;
> >
> > +     if (p->core_cookie)
> > +             rq->cookied_count--;
> > +
> >       if (sched_core_enqueued(p)) {
> >               rb_erase(&p->core_node, &rq->core_tree);
> >               RB_CLEAR_NODE(&p->core_node);
> >       }
> >
> > +     if (!sched_core_enabled(rq))
> > +             return;
> >       /*
> >        * Migrating the last task off the cpu, with the cpu in forced idle
> >        * state. Reschedule to create an accounting edge for forced idle,
> > @@ -370,7 +375,7 @@ static void sched_core_assert_empty(void)
> >       int cpu;
> >
> >       for_each_possible_cpu(cpu)
> > -             WARN_ON_ONCE(!RB_EMPTY_ROOT(&cpu_rq(cpu)->core_tree));
> > +             WARN_ON_ONCE(cpu_rq(cpu)->cookied_count);
> >   }
> >
> >   static void __sched_core_enable(void)
> > @@ -2061,14 +2066,12 @@ static inline void enqueue_task(struct rq *rq, struct task_struct *p, int flags)
> >       uclamp_rq_inc(rq, p);
> >       p->sched_class->enqueue_task(rq, p, flags);
> >
> > -     if (sched_core_enabled(rq))
> > -             sched_core_enqueue(rq, p);
> > +     sched_core_enqueue(rq, p);
> >   }
> >
> >   static inline void dequeue_task(struct rq *rq, struct task_struct *p, int flags)
> >   {
> > -     if (sched_core_enabled(rq))
> > -             sched_core_dequeue(rq, p, flags);
> > +     sched_core_dequeue(rq, p, flags);
> >
> >       if (!(flags & DEQUEUE_NOCLOCK))
> >               update_rq_clock(rq);
> > @@ -6126,13 +6129,8 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> >               rq_i = cpu_rq(i);
> >               p = rq_i->core_pick;
> >
> > -             if (!cookie_equals(p, cookie)) {
> > -                     p = NULL;
> > -                     if (cookie)
> > -                             p = sched_core_find(rq_i, cookie);
> > -                     if (!p)
> > -                             p = idle_sched_class.pick_task(rq_i);
> > -             }
> > +             if (!cookie_equals(p, cookie))
> > +                     p = sched_core_find(rq_i, cookie);
> >
> >               rq_i->core_pick = p;
> >
> > @@ -6333,6 +6331,7 @@ static void sched_core_cpu_starting(unsigned int cpu)
> >       sched_core_lock(cpu, &flags);
> >
> >       WARN_ON_ONCE(rq->core != rq);
> > +     WARN_ON_ONCE(rq->cookied_count);
> >
> >       /* if we're the first, we'll be our own leader */
> >       if (cpumask_weight(smt_mask) == 1)
> > @@ -6425,6 +6424,7 @@ static inline void sched_core_cpu_dying(unsigned int cpu)
> >   {
> >       struct rq *rq = cpu_rq(cpu);
> >
> > +     WARN_ON_ONCE(rq->cookied_count);
> >       if (rq->core != rq)
> >               rq->core = rq;
> >   }
> > @@ -9917,6 +9917,7 @@ void __init sched_init(void)
> >               rq->core = rq;
> >               rq->core_pick = NULL;
> >               rq->core_enabled = 0;
> > +             rq->cookied_count = 0;
> >               rq->core_tree = RB_ROOT;
> >               rq->core_forceidle_count = 0;
> >               rq->core_forceidle_occupation = 0;
> > diff --git a/kernel/sched/core_sched.c b/kernel/sched/core_sched.c
> > index a57fd8f27498..70f424abcc2b 100644
> > --- a/kernel/sched/core_sched.c
> > +++ b/kernel/sched/core_sched.c
> > @@ -56,6 +56,7 @@ static unsigned long sched_core_update_cookie(struct task_struct *p,
> >       unsigned long old_cookie;
> >       struct rq_flags rf;
> >       struct rq *rq;
> > +     bool enqueued;
> >
> >       rq = task_rq_lock(p, &rf);
> >
> > @@ -67,16 +68,14 @@ static unsigned long sched_core_update_cookie(struct task_struct *p,
> >        */
> >       SCHED_WARN_ON((p->core_cookie || cookie) && !sched_core_enabled(rq));
> >
> > -     if (sched_core_enqueued(p))
> > +     enqueued = task_on_rq_queued(p);
> > +     if (enqueued)
> >               sched_core_dequeue(rq, p, DEQUEUE_SAVE);
> >
> >       old_cookie = p->core_cookie;
> >       p->core_cookie = cookie;
> >
> > -     /*
> > -      * Consider the cases: !prev_cookie and !cookie.
> > -      */
> > -     if (cookie && task_on_rq_queued(p))
> > +     if (enqueued)
> >               sched_core_enqueue(rq, p);
> >
> >       /*
> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index 3e8df6d31c1e..f5a0ee7fccae 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -1148,6 +1148,7 @@ struct rq {
> >       unsigned int            core_task_seq;
> >       unsigned int            core_pick_seq;
> >       unsigned long           core_cookie;
> > +     unsigned int            cookied_count;
> >       unsigned int            core_forceidle_count;
> >       unsigned int            core_forceidle_seq;
> >       unsigned int            core_forceidle_occupation;