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SubjectRe: [PATCH v2] sched/core: forced idle accounting
Hi Josh,

On Mon, Oct 18, 2021 at 01:34:28PM -0700, Josh Don 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).
>
> Forced idle time is attributed to the thread responsible for causing the
> forced idle. A few details:
> - Forced idle time is displayed via /proc/PID/sched. It also requires
> that schedstats is enabled.
> - Forced idle is only accounted when a sibling hyperthread is held
> idle despite the presence of runnable tasks. No time is charged if
> a sibling is idle but has no runnable tasks.

This is clear to me now. Last time I am not get this. Thanks.

> - Tasks with 0 cookie are never charged forced idle.
> - For SMT > 2, we scale the amount of forced idle charged based on the
> number of forced idle siblings. Additionally, we split the time up and
> evenly charge it to all running tasks, as each is equally responsible
> for the forced idle.
>
> Signed-off-by: Josh Don <joshdon@google.com>
> ---
> v2:
> - simplify the SMT>2 case by tracking the forced idle count in pick()
> - added an accounting edge for sched_core_flip()
>
> include/linux/sched.h | 4 ++
> kernel/sched/core.c | 82 +++++++++++++++++++++++++++++----------
> kernel/sched/core_sched.c | 71 ++++++++++++++++++++++++++++++++-
> kernel/sched/debug.c | 4 ++
> kernel/sched/fair.c | 2 +-
> kernel/sched/sched.h | 20 +++++++++-
> 6 files changed, 159 insertions(+), 24 deletions(-)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 96abf9bd3864..ab6d401f367e 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -523,7 +523,11 @@ struct sched_statistics {
> u64 nr_wakeups_affine_attempts;
> u64 nr_wakeups_passive;
> u64 nr_wakeups_idle;
> +
> +#ifdef CONFIG_SCHED_CORE
> + u64 core_forceidle_sum;
> #endif
> +#endif /* CONFIG_SCHEDSTATS */
> } ____cacheline_aligned;
>
> struct sched_entity {
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index ada028e579b0..ff49d259bf92 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -144,7 +144,7 @@ static inline bool __sched_core_less(struct task_struct *a, struct task_struct *
> return false;
>
> /* flip prio, so high prio is leftmost */
> - if (prio_less(b, a, task_rq(a)->core->core_forceidle))
> + if (prio_less(b, a, !!task_rq(a)->core->core_forceidle_count))
> return true;
>
> return false;
> @@ -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_count && rq->curr == rq->idle)
> + resched_curr(rq);
> }
>
> /*
> @@ -280,6 +288,8 @@ static void __sched_core_flip(bool enabled)
> for_each_cpu(t, smt_mask)
> cpu_rq(t)->core_enabled = enabled;
>
> + cpu_rq(cpu)->core->core_forceidle_start = 0;
> +
> sched_core_unlock(cpu, &flags);
>
> cpumask_andnot(&sched_core_mask, &sched_core_mask, smt_mask);
> @@ -364,7 +374,8 @@ 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 +2031,7 @@ 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 +5267,7 @@ 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 +5680,7 @@ 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;
> @@ -5720,10 +5733,18 @@ 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 (rq->core->core_forceidle_count) {
> + if (!core_clock_updated) {
> + update_rq_clock(rq->core);
> + core_clock_updated = true;
> + }
> + sched_core_account_forceidle(rq);
> + /* reset after accounting force idle */
> + rq->core->core_forceidle_start = 0;
> + rq->core->core_forceidle_count = 0;
> + rq->core->core_forceidle_occupation = 0;
> need_sync = true;
> fi_before = true;
> - rq->core->core_forceidle = false;
> }
>
> /*
> @@ -5765,7 +5786,12 @@ 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)
> + /*
> + * Current cpu always has its clock updated on entrance to
> + * pick_next_task(). If the current cpu is not the core,
> + * the core may also have been updated above.
> + */

I intercept one sentence from last version's input to make this comment
comprehensive. Hope it will.

/*
* Current cpu always has its clock updated on entrance to
* pick_next_task(). If the current cpu is not the core,
* the core may also have been updated above. We have to be
* careful to avoid double update of core rq's clock.
*/

> + if (i != cpu && (rq_i != rq->core || !core_clock_updated))
> update_rq_clock(rq_i);
>
> p = rq_i->core_pick = pick_task(rq_i);
> @@ -5795,7 +5821,7 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>
> if (p == rq_i->idle) {
> if (rq_i->nr_running) {
> - rq->core->core_forceidle = true;
> + rq->core->core_forceidle_count++;
> if (!fi_before)
> rq->core->core_forceidle_seq++;
> }
> @@ -5804,6 +5830,12 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> }
> }
>
> + if (rq->core->core_forceidle_count) {
> + if (cookie)
> + rq->core->core_forceidle_start = rq_clock(rq->core);
> + rq->core->core_forceidle_occupation = occ;
> + }
> +
> rq->core->core_pick_seq = rq->core->core_task_seq;
> next = rq->core_pick;
> rq->core_sched_seq = rq->core->core_pick_seq;
> @@ -5840,8 +5872,8 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> * 1 0 1
> * 1 1 0
> */
> - if (!(fi_before && rq->core->core_forceidle))
> - task_vruntime_update(rq_i, rq_i->core_pick, rq->core->core_forceidle);
> + if (!(fi_before && rq->core->core_forceidle_count))
> + task_vruntime_update(rq_i, rq_i->core_pick, !!rq->core->core_forceidle_count);
>
> rq_i->core_pick->core_occupation = occ;
>
> @@ -6045,11 +6077,19 @@ static void sched_core_cpu_deactivate(unsigned int cpu)
> goto unlock;
>
> /* copy the shared state to the new leader */
> - core_rq->core_task_seq = rq->core_task_seq;
> - core_rq->core_pick_seq = rq->core_pick_seq;
> - core_rq->core_cookie = rq->core_cookie;
> - core_rq->core_forceidle = rq->core_forceidle;
> - core_rq->core_forceidle_seq = rq->core_forceidle_seq;
> + core_rq->core_task_seq = rq->core_task_seq;
> + core_rq->core_pick_seq = rq->core_pick_seq;
> + core_rq->core_cookie = rq->core_cookie;
> + core_rq->core_forceidle_count = rq->core_forceidle_count;
> + core_rq->core_forceidle_seq = rq->core_forceidle_seq;
> + core_rq->core_forceidle_occupation = rq->core_forceidle_occupation;
> +
> + /*
> + * 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) {
> @@ -9423,7 +9463,9 @@ void __init sched_init(void)
> rq->core_pick = NULL;
> rq->core_enabled = 0;
> rq->core_tree = RB_ROOT;
> - rq->core_forceidle = false;
> + rq->core_forceidle_count = 0;
> + rq->core_forceidle_occupation = 0;
> + 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..6587c4c409d2 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);
> @@ -228,3 +232,68 @@ int sched_core_share_pid(unsigned int cmd, pid_t pid, enum pid_type type,
> return err;
> }
>
> +#ifdef CONFIG_SCHEDSTATS
> +
> +/* 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));
> + 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_count);
> +
> + 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;
> +
> + if (!schedstat_enabled())
> + return;
> +
> + if (WARN_ON_ONCE(!rq->core->core_forceidle_occupation)) {
> + /* can't be forced idle without a running task */
> + } else if (rq->core->core_forceidle_count > 1 ||
> + rq->core->core_forceidle_occupation > 1) {

This also except that smt count in last version.

> + /*
> + * 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.
> + */
> + delta *= rq->core->core_forceidle_count;
> + delta = div_u64(delta, rq->core->core_forceidle_occupation);
> + }
> +
> + for_each_cpu(i, smt_mask) {
> + rq_i = cpu_rq(i);
> + p = rq_i->core_pick ?: rq_i->curr;
> +
> + if (!p->core_cookie)
> + continue;
> +
> + __schedstat_add(p->stats.core_forceidle_sum, delta);
> + }
> +}
> +
> +void sched_core_tick(struct rq *rq)
> +{
> + if (!sched_core_enabled(rq))
> + return;
> +
> + if (!rq->core->core_forceidle_count)
> + return;
> +
> + if (rq != rq->core)
> + update_rq_clock(rq->core);
> + sched_core_account_forceidle(rq);
> +}
> +
> +#endif /* CONFIG_SCHEDSTATS */
> diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> index 7dcbaa31c5d9..aa29211de1bf 100644
> --- a/kernel/sched/debug.c
> +++ b/kernel/sched/debug.c
> @@ -1023,6 +1023,10 @@ void proc_sched_show_task(struct task_struct *p, struct pid_namespace *ns,
>
> __PN(avg_atom);
> __PN(avg_per_cpu);
> +
> +#ifdef CONFIG_SCHED_CORE
> + PN_SCHEDSTAT(core_forceidle_sum);
> +#endif
> }
>
> __P(nr_switches);
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 87db481e8a56..da3a6732ddac 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -11047,7 +11047,7 @@ static inline void task_tick_core(struct rq *rq, struct task_struct *curr)
> * MIN_NR_TASKS_DURING_FORCEIDLE - 1 tasks and use that to check
> * if we need to give up the CPU.
> */
> - if (rq->core->core_forceidle && rq->cfs.nr_running == 1 &&
> + if (rq->core->core_forceidle_count && rq->cfs.nr_running == 1 &&
> __entity_slice_used(&curr->se, MIN_NR_TASKS_DURING_FORCEIDLE))
> resched_curr(rq);
> }
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index a00fc7057d97..fb7c9e3f2787 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1111,8 +1111,10 @@ struct rq {
> unsigned int core_task_seq;
> unsigned int core_pick_seq;
> unsigned long core_cookie;
> - unsigned char core_forceidle;
> + unsigned int core_forceidle_count;
> unsigned int core_forceidle_seq;
> + unsigned int core_forceidle_occupation;
> + u64 core_forceidle_start;
> #endif
> };
>
> @@ -1253,7 +1255,7 @@ 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);
> @@ -1302,6 +1304,20 @@ static inline bool sched_group_cookie_match(struct rq *rq,
> }
> #endif /* CONFIG_SCHED_CORE */
>
> +#if defined(CONFIG_SCHED_CORE) && defined(CONFIG_SCHEDSTATS)
> +
> +extern void sched_core_account_forceidle(struct rq *rq);
> +
> +extern void sched_core_tick(struct rq *rq);
> +
> +#else
> +
> +static inline void sched_core_account_forceidle(struct rq *rq) {}
> +
> +static inline void sched_core_tick(struct rq *rq) {}
> +
> +#endif /* CONFIG_SCHED_CORE && CONFIG_SCHEDSTATS */
> +
> static inline void lockdep_assert_rq_held(struct rq *rq)
> {
> lockdep_assert_held(__rq_lockp(rq));
> --
> 2.33.0.1079.g6e70778dc9-goog
>


Thanks,
Tao

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