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SubjectRe: [RFC PATCH 09/16] sched/fair: core wide cfs task priority comparison(Internet mail)
Date


> On Jul 1, 2020, at 5:32 AM, Vineeth Remanan Pillai <vpillai@digitalocean.com> wrote:
>
> From: Aaron Lu <aaron.lwe@gmail.com>
>
> This patch provides a vruntime based way to compare two cfs task's
> priority, be it on the same cpu or different threads of the same core.
>
> When the two tasks are on the same CPU, we just need to find a common
> cfs_rq both sched_entities are on and then do the comparison.
>
> When the two tasks are on differen threads of the same core, each thread
> will choose the next task to run the usual way and then the root level
> sched entities which the two tasks belong to will be used to decide
> which task runs next core wide.
>
> An illustration for the cross CPU case:
>
> cpu0 cpu1
> / | \ / | \
> se1 se2 se3 se4 se5 se6
> / \ / \
> se21 se22 se61 se62
> (A) /
> se621
> (B)
>
> Assume CPU0 and CPU1 are smt siblings and cpu0 has decided task A to
> run next and cpu1 has decided task B to run next. To compare priority
> of task A and B, we compare priority of se2 and se6. Whose vruntime is
> smaller, who wins.
>
> To make this work, the root level sched entities' vruntime of the two
> threads must be directly comparable. So one of the hyperthread's root
> cfs_rq's min_vruntime is chosen as the core wide one and all root level
> sched entities' vruntime is normalized against it.
>
> All sub cfs_rqs and sched entities are not interesting in cross cpu
> priority comparison as they will only participate in the usual cpu local
> schedule decision so no need to normalize their vruntimes.
>
> Signed-off-by: Aaron Lu <ziqian.lzq@antfin.com>
> ---
> kernel/sched/core.c | 23 +++----
> kernel/sched/fair.c | 142 ++++++++++++++++++++++++++++++++++++++++++-
> kernel/sched/sched.h | 3 +
> 3 files changed, 150 insertions(+), 18 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index f51e5c4798c8..4d6d6a678013 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -114,19 +114,8 @@ static inline bool prio_less(struct task_struct *a, struct task_struct *b)
> if (pa == -1) /* dl_prio() doesn't work because of stop_class above */
> return !dl_time_before(a->dl.deadline, b->dl.deadline);
>
> - if (pa == MAX_RT_PRIO + MAX_NICE) { /* fair */
> - u64 vruntime = b->se.vruntime;
> -
> - /*
> - * Normalize the vruntime if tasks are in different cpus.
> - */
> - if (task_cpu(a) != task_cpu(b)) {
> - vruntime -= task_cfs_rq(b)->min_vruntime;
> - vruntime += task_cfs_rq(a)->min_vruntime;
> - }
> -
> - return !((s64)(a->se.vruntime - vruntime) <= 0);
> - }
> + if (pa == MAX_RT_PRIO + MAX_NICE) /* fair */
> + return cfs_prio_less(a, b);
>
> return false;
> }
> @@ -229,8 +218,12 @@ static int __sched_core_stopper(void *data)
> bool enabled = !!(unsigned long)data;
> int cpu;
>
> - for_each_possible_cpu(cpu)
> - cpu_rq(cpu)->core_enabled = enabled;
> + for_each_possible_cpu(cpu) {
> + struct rq *rq = cpu_rq(cpu);
> + rq->core_enabled = enabled;
> + if (cpu_online(cpu) && rq->core != rq)
> + sched_core_adjust_sibling_vruntime(cpu, enabled);
> + }
>
> return 0;
> }
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 61d19e573443..d16939766361 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -462,11 +462,142 @@ find_matching_se(struct sched_entity **se, struct sched_entity **pse)
>
> #endif /* CONFIG_FAIR_GROUP_SCHED */
>
> +#ifdef CONFIG_SCHED_CORE
> +static inline struct cfs_rq *root_cfs_rq(struct cfs_rq *cfs_rq)
> +{
> + return &rq_of(cfs_rq)->cfs;
> +}
> +
> +static inline bool is_root_cfs_rq(struct cfs_rq *cfs_rq)
> +{
> + return cfs_rq == root_cfs_rq(cfs_rq);
> +}
> +
> +static inline struct cfs_rq *core_cfs_rq(struct cfs_rq *cfs_rq)
> +{
> + return &rq_of(cfs_rq)->core->cfs;
> +}
> +
> +static inline u64 cfs_rq_min_vruntime(struct cfs_rq *cfs_rq)
> +{
> + if (!sched_core_enabled(rq_of(cfs_rq)) || !is_root_cfs_rq(cfs_rq))
> + return cfs_rq->min_vruntime;
> +
> + return core_cfs_rq(cfs_rq)->min_vruntime;
> +}
> +
> +#ifndef CONFIG_64BIT
> +static inline u64 cfs_rq_min_vruntime_copy(struct cfs_rq *cfs_rq)
> +{
> + if (!sched_core_enabled(rq_of(cfs_rq)) || !is_root_cfs_rq(cfs_rq))
> + return cfs_rq->min_vruntime_copy;
> +
> + return core_cfs_rq(cfs_rq)->min_vruntime_copy;
> +}
> +#endif /* CONFIG_64BIT */
> +
> +bool cfs_prio_less(struct task_struct *a, struct task_struct *b)
> +{
> + struct sched_entity *sea = &a->se;
> + struct sched_entity *seb = &b->se;
> + bool samecpu = task_cpu(a) == task_cpu(b);
> + s64 delta;
> +
> +#ifdef CONFIG_FAIR_GROUP_SCHED
> + if (samecpu) {
> + /* vruntime is per cfs_rq */
> + while (!is_same_group(sea, seb)) {
> + int sea_depth = sea->depth;
> + int seb_depth = seb->depth;
> +
> + if (sea_depth >= seb_depth)
> + sea = parent_entity(sea);
> + if (sea_depth <= seb_depth)
> + seb = parent_entity(seb);
> + }
> +
> + delta = (s64)(sea->vruntime - seb->vruntime);
> + goto out;
> + }
> +
> + /* crosscpu: compare root level se's vruntime to decide priority */
> + while (sea->parent)
> + sea = sea->parent;
> + while (seb->parent)
> + seb = seb->parent;
> +#else
> + /*
> + * Use the min_vruntime of root cfs_rq if the tasks are
> + * enqueued in different cpus.
> + * */
> + if (!samecpu) {
> + delta = (s64)(task_rq(a)->cfs.min_vruntime -
> + task_rq(b)->cfs.min_vruntime);
> + goto out;
> + }
> +#endif /* CONFIG_FAIR_GROUP_SCHED */
> +
> + delta = (s64)(sea->vruntime - seb->vruntime);
> +
> +out:
> + return delta > 0;
> +}
> +
> +/*
> + * This function takes care of adjusting the min_vruntime of siblings of
> + * a core during coresched enable/disable.
> + * This is called in stop machine context so no need to take the rq lock.
Hi,

IMHO, it seems that stop machine context cannot guarantee race free. The param *cpu* maybe not *this_cpu*, rq lock should be taken even in stop machine context, and irq should be disabled too, to avoid potential races with other contexts.

Thx.
Regards,
Jiang

> + *
> + * Coresched enable case
> + * Once Core scheduling is enabled, the root level sched entities
> + * of both siblings will use cfs_rq->min_vruntime as the common cfs_rq
> + * min_vruntime, so it's necessary to normalize vruntime of existing root
> + * level sched entities in sibling_cfs_rq.
> + *
> + * Update of sibling_cfs_rq's min_vruntime isn't necessary as we will be
> + * only using cfs_rq->min_vruntime during the entire run of core scheduling.
> + *
> + * Coresched disable case
> + * During the entire run of core scheduling, sibling_cfs_rq's min_vruntime
> + * is left unused and could lag far behind its still queued sched entities.
> + * Sync it to the up2date core wide one to avoid problems.
> + */
> +void sched_core_adjust_sibling_vruntime(int cpu, bool coresched_enabled)
> +{
> + struct cfs_rq *cfs = &cpu_rq(cpu)->cfs;
> + struct cfs_rq *core_cfs = &cpu_rq(cpu)->core->cfs;
> + if (coresched_enabled) {
> + struct sched_entity *se, *next;
> + s64 delta = core_cfs->min_vruntime - cfs->min_vruntime;
> + rbtree_postorder_for_each_entry_safe(se, next,
> + &cfs->tasks_timeline.rb_root,
> + run_node) {
> + se->vruntime += delta;
> + }
> + } else {
> + cfs->min_vruntime = core_cfs->min_vruntime;
> +#ifndef CONFIG_64BIT
> + smp_wmb();
> + cfs->min_vruntime_copy = core_cfs->min_vruntime;
> +#endif
> + }
> +}
> +
> +#else
> static inline u64 cfs_rq_min_vruntime(struct cfs_rq *cfs_rq)
> {
> return cfs_rq->min_vruntime;
> }
>
> +#ifndef CONFIG_64BIT
> +static inline u64 cfs_rq_min_vruntime_copy(struct cfs_rq *cfs_rq)
> +{
> + return cfs_rq->min_vruntime_copy;
> +}
> +#endif /* CONFIG_64BIT */
> +
> +#endif /* CONFIG_SCHED_CORE */
> +
> static __always_inline
> void account_cfs_rq_runtime(struct cfs_rq *cfs_rq, u64 delta_exec);
>
> @@ -522,8 +653,13 @@ static void update_min_vruntime(struct cfs_rq *cfs_rq)
> vruntime = min_vruntime(vruntime, se->vruntime);
> }
>
> +#ifdef CONFIG_SCHED_CORE
> + if (sched_core_enabled(rq_of(cfs_rq)) && is_root_cfs_rq(cfs_rq))
> + cfs_rq = core_cfs_rq(cfs_rq);
> +#endif
> +
> /* ensure we never gain time by being placed backwards. */
> - cfs_rq->min_vruntime = max_vruntime(cfs_rq_min_vruntime(cfs_rq), vruntime);
> + cfs_rq->min_vruntime = max_vruntime(cfs_rq->min_vruntime, vruntime);
> #ifndef CONFIG_64BIT
> smp_wmb();
> cfs_rq->min_vruntime_copy = cfs_rq->min_vruntime;
> @@ -6700,9 +6836,9 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
> u64 min_vruntime_copy;
>
> do {
> - min_vruntime_copy = cfs_rq->min_vruntime_copy;
> + min_vruntime_copy = cfs_rq_min_vruntime_copy(cfs_rq);
> smp_rmb();
> - min_vruntime = cfs_rq->min_vruntime;
> + min_vruntime = cfs_rq_min_vruntime(cfs_rq);
> } while (min_vruntime != min_vruntime_copy);
> #else
> min_vruntime = cfs_rq_min_vruntime(cfs_rq);
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 4a738093d731..293aa1ae0308 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1086,6 +1086,9 @@ static inline raw_spinlock_t *rq_lockp(struct rq *rq)
> return &rq->__lock;
> }
>
> +bool cfs_prio_less(struct task_struct *a, struct task_struct *b);
> +void sched_core_adjust_sibling_vruntime(int cpu, bool coresched_enabled);
> +
> #else /* !CONFIG_SCHED_CORE */
>
> static inline bool sched_core_enabled(struct rq *rq)
> --
> 2.17.1
>
>

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