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SubjectRe: [PATCH] sched/core: forced idle accounting
On Thu, Oct 7, 2021 at 5:08 PM Josh Don <joshdon@google.com> wrote:
>
> Adds accounting for "forced idle" time, which is time where a cookie'd
> task forces its SMT sibling to idle, despite the presence of runnable
> tasks.
>
> Forced idle time is one means to measure the cost of enabling core
> scheduling (ie. the capacity lost due to the need to force idle).
>
> Signed-off-by: Josh Don <joshdon@google.com>
> ---
> include/linux/sched.h | 1 +
> kernel/sched/core.c | 46 ++++++++++++++++-----
> kernel/sched/core_sched.c | 85 ++++++++++++++++++++++++++++++++++++++-
> kernel/sched/debug.c | 3 ++
> kernel/sched/sched.h | 11 ++++-
> 5 files changed, 135 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 2aa30e8e1440..d114ba350802 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -784,6 +784,7 @@ struct task_struct {
> struct rb_node core_node;
> unsigned long core_cookie;
> unsigned int core_occupation;
> + u64 core_forceidle_sum;
> #endif
>
> #ifdef CONFIG_CGROUP_SCHED
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index ada028e579b0..baa4f48cacff 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -181,15 +181,23 @@ void sched_core_enqueue(struct rq *rq, struct task_struct *p)
> rb_add(&p->core_node, &rq->core_tree, rb_sched_core_less);
> }
>
> -void sched_core_dequeue(struct rq *rq, struct task_struct *p)
> +void sched_core_dequeue(struct rq *rq, struct task_struct *p, int flags)
> {
> rq->core->core_task_seq++;
>
> - if (!sched_core_enqueued(p))
> - return;
> + if (sched_core_enqueued(p)) {
> + rb_erase(&p->core_node, &rq->core_tree);
> + RB_CLEAR_NODE(&p->core_node);
> + }
>
> - rb_erase(&p->core_node, &rq->core_tree);
> - RB_CLEAR_NODE(&p->core_node);
> + /*
> + * Migrating the last task off the cpu, with the cpu in forced idle
> + * state. Reschedule to create an accounting edge for forced idle,
> + * and re-examine whether the core is still in forced idle state.
> + */
> + if (!(flags & DEQUEUE_SAVE) && rq->nr_running == 1 &&
> + rq->core->core_forceidle && rq->curr == rq->idle)
> + resched_curr(rq);

Resched_curr is probably an unwanted side effect of dequeue. Maybe we
could extract the check and resched_curr out into a function, and call
the function outside of sched_core_dequeue(). In that way, the
interface of dequeue doesn't need to change.

> }
>
> /*
> @@ -364,7 +372,8 @@ void sched_core_put(void)
> #else /* !CONFIG_SCHED_CORE */
>
> static inline void sched_core_enqueue(struct rq *rq, struct task_struct *p) { }
> -static inline void sched_core_dequeue(struct rq *rq, struct task_struct *p) { }
> +static inline void
> +sched_core_dequeue(struct rq *rq, struct task_struct *p, int flags) { }
>
> #endif /* CONFIG_SCHED_CORE */
>
> @@ -2020,7 +2029,7 @@ static inline void enqueue_task(struct rq *rq, struct task_struct *p, int flags)
> static inline void dequeue_task(struct rq *rq, struct task_struct *p, int flags)
> {
> if (sched_core_enabled(rq))
> - sched_core_dequeue(rq, p);
> + sched_core_dequeue(rq, p, flags);
>
> if (!(flags & DEQUEUE_NOCLOCK))
> update_rq_clock(rq);
> @@ -5256,6 +5265,7 @@ void scheduler_tick(void)
> if (sched_feat(LATENCY_WARN))
> resched_latency = cpu_resched_latency(rq);
> calc_global_load_tick(rq);
> + sched_core_tick(rq);
>
> rq_unlock(rq, &rf);
>
> @@ -5668,6 +5678,7 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> struct task_struct *next, *p, *max = NULL;
> const struct cpumask *smt_mask;
> bool fi_before = false;
> + bool core_clock_updated = (rq == rq->core);
> unsigned long cookie;
> int i, cpu, occ = 0;
> struct rq *rq_i;
> @@ -5721,9 +5732,15 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> /* reset state */
> rq->core->core_cookie = 0UL;
> if (rq->core->core_forceidle) {
> + if (!core_clock_updated) {
> + update_rq_clock(rq->core);
> + core_clock_updated = true;
> + }
> + sched_core_account_forceidle(rq);
> + rq->core->core_forceidle = false;
> + rq->core->core_forceidle_start = 0;
> need_sync = true;
> fi_before = true;
> - rq->core->core_forceidle = false;
> }
>
> /*
> @@ -5765,7 +5782,7 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> for_each_cpu_wrap(i, smt_mask, cpu) {
> rq_i = cpu_rq(i);
>
> - if (i != cpu)
> + if (i != cpu && (rq_i != rq->core || !core_clock_updated))
> update_rq_clock(rq_i);

Do you mean (rq_i != rq->core && !core_clock_updated)? I thought
rq->core has core_clock updated always.

>
> p = rq_i->core_pick = pick_task(rq_i);
> @@ -5804,6 +5821,9 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> }
> }
>
> + if (rq->core->core_forceidle && cookie)
> + rq->core->core_forceidle_start = rq_clock(rq->core);
> +
> rq->core->core_pick_seq = rq->core->core_task_seq;
> next = rq->core_pick;
> rq->core_sched_seq = rq->core->core_pick_seq;
> @@ -6051,6 +6071,13 @@ static void sched_core_cpu_deactivate(unsigned int cpu)
> core_rq->core_forceidle = rq->core_forceidle;
> core_rq->core_forceidle_seq = rq->core_forceidle_seq;
>
> + /*
> + * Accounting edge for forced idle is handled in pick_next_task().
> + * Don't need another one here, since the hotplug thread shouldn't
> + * have a cookie.
> + */
> + core_rq->core_forceidle_start = 0;
> +
> /* install new leader */
> for_each_cpu(t, smt_mask) {
> rq = cpu_rq(t);
> @@ -9424,6 +9451,7 @@ void __init sched_init(void)
> rq->core_enabled = 0;
> rq->core_tree = RB_ROOT;
> rq->core_forceidle = false;
> + rq->core_forceidle_start = 0;
>
> rq->core_cookie = 0UL;
> #endif
> diff --git a/kernel/sched/core_sched.c b/kernel/sched/core_sched.c
> index 48ac72696012..aae4ac2ac7ec 100644
> --- a/kernel/sched/core_sched.c
> +++ b/kernel/sched/core_sched.c
> @@ -73,7 +73,7 @@ static unsigned long sched_core_update_cookie(struct task_struct *p,
>
> enqueued = sched_core_enqueued(p);
> if (enqueued)
> - sched_core_dequeue(rq, p);
> + sched_core_dequeue(rq, p, DEQUEUE_SAVE);
>
> old_cookie = p->core_cookie;
> p->core_cookie = cookie;
> @@ -85,6 +85,10 @@ static unsigned long sched_core_update_cookie(struct task_struct *p,
> * If task is currently running, it may not be compatible anymore after
> * the cookie change, so enter the scheduler on its CPU to schedule it
> * away.
> + *
> + * Note that it is possible that as a result of this cookie change, the
> + * core has now entered/left forced idle state. Defer accounting to the
> + * next scheduling edge, rather than always forcing a reschedule here.
> */
> if (task_running(rq, p))
> resched_curr(rq);
> @@ -109,6 +113,7 @@ void sched_core_fork(struct task_struct *p)
> {
> RB_CLEAR_NODE(&p->core_node);
> p->core_cookie = sched_core_clone_cookie(current);
> + p->core_forceidle_sum = 0;
> }
>
> void sched_core_free(struct task_struct *p)
> @@ -228,3 +233,81 @@ int sched_core_share_pid(unsigned int cmd, pid_t pid, enum pid_type type,
> return err;
> }
>
> +/* REQUIRES: rq->core's clock recently updated. */
> +void sched_core_account_forceidle(struct rq *rq)
> +{
> + const struct cpumask *smt_mask = cpu_smt_mask(cpu_of(rq));
> + unsigned int smt_count;
> + u64 delta, now = rq_clock(rq->core);
> + struct rq *rq_i;
> + struct task_struct *p;
> + int i;
> +
> + lockdep_assert_rq_held(rq);
> +
> + WARN_ON_ONCE(!rq->core->core_forceidle);
> +
> + if (rq->core->core_forceidle_start == 0)
> + return;
> +
> + delta = now - rq->core->core_forceidle_start;
> + if (unlikely((s64)delta <= 0))
> + return;
> +
> + rq->core->core_forceidle_start = now;
> +
> + /*
> + * For larger SMT configurations, we need to scale the charged
> + * forced idle amount since there can be more than one forced idle
> + * sibling and more than one running cookied task.
> + */
> + smt_count = cpumask_weight(smt_mask);
> + if (smt_count > 2) {
> + unsigned int nr_forced_idle = 0, nr_running = 0;
> +
> + for_each_cpu(i, smt_mask) {
> + rq_i = cpu_rq(i);
> + p = rq_i->core_pick ?: rq_i->curr;
> +
> + if (p != rq_i->idle)
> + nr_running++;
> + else if (rq_i->nr_running)
> + nr_forced_idle++;
> + }
> +
> + if (WARN_ON_ONCE(!nr_running)) {
> + /* can't be forced idle without a running task */
> + } else {
> + delta *= nr_forced_idle;
> + delta /= nr_running;
> + }
> + }
> +
> + for_each_cpu(i, smt_mask) {
> + rq_i = cpu_rq(i);
> + p = rq_i->core_pick ?: rq_i->curr;
> +
> + if (!p->core_cookie)
> + continue;
> +
> + p->core_forceidle_sum += delta;
> +
> + /* Optimize for common case. */
> + if (smt_count == 2)
> + break;
> + }
> +}
> +
> +void sched_core_tick(struct rq *rq)
> +{
> + if (!sched_core_enabled(rq))
> + return;
> +
> + if (!rq->core->core_forceidle)
> + return;
> +
> + if (rq != rq->core)
> + update_rq_clock(rq->core);
> + sched_core_account_forceidle(rq);
> +}
> +
> diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> index 26fac5e28bc0..0fe6a1bb8b60 100644
> --- a/kernel/sched/debug.c
> +++ b/kernel/sched/debug.c
> @@ -1045,6 +1045,9 @@ void proc_sched_show_task(struct task_struct *p, struct pid_namespace *ns,
> __PS("uclamp.max", p->uclamp_req[UCLAMP_MAX].value);
> __PS("effective uclamp.min", uclamp_eff_value(p, UCLAMP_MIN));
> __PS("effective uclamp.max", uclamp_eff_value(p, UCLAMP_MAX));
> +#endif
> +#ifdef CONFIG_SCHED_CORE
> + PN(core_forceidle_sum);
> #endif
> P(policy);
> P(prio);
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index a00fc7057d97..4678b85754f2 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1113,6 +1113,7 @@ struct rq {
> unsigned long core_cookie;
> unsigned char core_forceidle;
> unsigned int core_forceidle_seq;
> + u64 core_forceidle_start;
> #endif
> };
>
> @@ -1253,11 +1254,15 @@ static inline bool sched_core_enqueued(struct task_struct *p)
> }
>
> extern void sched_core_enqueue(struct rq *rq, struct task_struct *p);
> -extern void sched_core_dequeue(struct rq *rq, struct task_struct *p);
> +extern void sched_core_dequeue(struct rq *rq, struct task_struct *p, int flags);
>
> extern void sched_core_get(void);
> extern void sched_core_put(void);
>
> +extern void sched_core_account_forceidle(struct rq *rq);
> +
> +extern void sched_core_tick(struct rq *rq);
> +
> #else /* !CONFIG_SCHED_CORE */
>
> static inline bool sched_core_enabled(struct rq *rq)
> @@ -1300,6 +1305,10 @@ static inline bool sched_group_cookie_match(struct rq *rq,
> {
> return true;
> }
> +
> +static inline void sched_core_account_forceidle(struct rq *rq) {}
> +
> +static inline void sched_core_tick(struct rq *rq) {}
> #endif /* CONFIG_SCHED_CORE */
>
> static inline void lockdep_assert_rq_held(struct rq *rq)
> --
> 2.33.0.882.g93a45727a2-goog
>

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