On Mon, Oct 26, 2020 at 01:47:24PM +0100, Peter Zijlstra wrote:
> On Mon, Oct 19, 2020 at 09:43:18PM -0400, Joel Fernandes (Google) wrote:
>
> > @@ -4723,6 +4714,14 @@ pick_next_task(struct rq *rq, struct task_struct
> > *prev, struct rq_flags *rf)
> > update_rq_clock(rq_i);
> > }
> >
> > + /* Reset the snapshot if core is no longer in force-idle. */
> > + if (!fi_before) {
> > + for_each_cpu(i, smt_mask) {
> > + struct rq *rq_i = cpu_rq(i);
> > + rq_i->cfs.min_vruntime_fi = rq_i->cfs.min_vruntime;
> > + }
> > + }
>
> So this is the thing that drags vruntime_fi along when (both?) siblings
> are active, right? But should we not do that after pick? Consider 2
> tasks a weight 1 and a weight 10 task, one for each sibling. By syncing
> the vruntime before picking, the cfs_prio_less() loop will not be able
> to distinguish between these two, since they'll both have effectively
> the same lag.
Just to set some terms, by lag you mean "the delta between task->vruntime and
cfs_rq->min_vruntime_fi" right ?
Assuming this is what you mean,
Say there's 2 task T1 and T2 on 2 different CPUs.
T1 has weight=10 vruntime=200
T2 has weight=1 vruntime=1000
Say T1's cfs_rq R1 has min_vruntime of 100
T2's cfs_rq R2 has min_vruntime of 200
First time we force idle, we do the "sync" in my patch (which is what I think
you can call snapshotting of min_vruntime). Assuming we do the sync _before_
picking:
This causes R1's ->min_vruntime_fi to be 100.
R2's ->min_vruntime_fi to be 200.
So during picking, cfs_prio_less() will see R1's "lag" as (200-100) = 100.
And R2's "lag" as (1000-200) = 800.
So the lags are different and I didn't get what you mean by "have effectively
the same lag".
Could you let me know what I am missing?
Also your patch is great and I feel it does the same thing as my patch except
for doing the min_vruntime snapshot up the hierarchy (which is great!). BTW,
do you really need force_idle_seq?
It seems to me, since you have fi_before, you can just set another variable
on the stack if the fi status changed during picking, and pass that along to
se_fi_update() or is there another case where the force_idle_seq is needed?
Thanks!
- Joel
> If however, you syn after pick, then the weight 1 task will have accreud
> far more runtime than the weight 10 task, and consequently the weight 10
> task will have preference when a decision will have to be made.
>
> (also, if this were the right place, the whole thing should've been part
> of the for_each_cpu() loop right before this)
>
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 56bea0decda1..9cae08c3fca1 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -10686,6 +10686,46 @@ static inline void task_tick_core(struct rq *rq,
> > struct task_struct *curr)
> > __entity_slice_used(&curr->se, MIN_NR_TASKS_DURING_FORCEIDLE))
> > resched_curr(rq);
> > }
> > +
> > +bool cfs_prio_less(struct task_struct *a, struct task_struct *b)
> > +{
> > + bool samecpu = task_cpu(a) == task_cpu(b);
> > + struct sched_entity *sea = &a->se;
> > + struct sched_entity *seb = &b->se;
> > + struct cfs_rq *cfs_rqa;
> > + struct cfs_rq *cfs_rqb;
> > + s64 delta;
> > +
> > + 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;
>
> This seems unfortunate, I think we can do better.
>
> > +
> > + cfs_rqa = sea->cfs_rq;
> > + cfs_rqb = seb->cfs_rq;
> > +
> > + /* normalize vruntime WRT their rq's base */
> > + delta = (s64)(sea->vruntime - seb->vruntime) +
> > + (s64)(cfs_rqb->min_vruntime_fi - cfs_rqa->min_vruntime_fi);
> > +out:
> > + return delta > 0;
> > +}
>
>
> How's something like this?
>
> - after each pick, such that the pick itself sees the divergence (see
> above); either:
>
> - pull the vruntime_fi forward, when !fi
> - freeze the vruntime_fi, when newly fi (A)
>
> - either way, update vruntime_fi for each cfs_rq in the active
> hierachy.
>
> - when comparing, and fi, update the vruntime_fi hierachy until we
> encounter a mark from (A), per doing it during the pick, but before
> runtime, this guaranteees it hasn't moved since (A).
>
> XXX, still buggered on SMT>2, imagine having {ta, tb, fi, i} on an SMT4,
> then when comparing any two tasks that do not involve the fi, we should
> (probably) have pulled them fwd -- but we can't actually pull them,
> because then the fi thing would break, mooo.
>
>
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -115,19 +115,8 @@ static inline bool prio_less(struct task
> 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;
> }
> @@ -4642,12 +4631,15 @@ pick_task(struct rq *rq, const struct sc
> return cookie_pick;
> }
>
> +extern void task_vruntime_update(struct rq *rq, struct task_struct *p);
> +
> static struct task_struct *
> pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> {
> struct task_struct *next, *max = NULL;
> const struct sched_class *class;
> const struct cpumask *smt_mask;
> + bool fi_before = false;
> bool need_sync;
> int i, j, cpu;
>
> @@ -4707,6 +4699,7 @@ pick_next_task(struct rq *rq, struct tas
> need_sync = !!rq->core->core_cookie;
> if (rq->core->core_forceidle) {
> need_sync = true;
> + fi_before = true;
> rq->core->core_forceidle = false;
> }
>
> @@ -4757,6 +4750,11 @@ pick_next_task(struct rq *rq, struct tas
> continue;
>
> rq_i->core_pick = p;
> + if (rq_i->idle == p && rq_i->nr_running) {
> + rq->core->core_forceidle = true;
> + if (!fi_before)
> + rq->core->core_forceidle_seq++;
> + }
>
> /*
> * If this new candidate is of higher priority than the
> @@ -4775,6 +4773,7 @@ pick_next_task(struct rq *rq, struct tas
> max = p;
>
> if (old_max) {
> + rq->core->core_forceidle = false;
> for_each_cpu(j, smt_mask) {
> if (j == i)
> continue;
> @@ -4823,10 +4822,8 @@ pick_next_task(struct rq *rq, struct tas
> if (!rq_i->core_pick)
> continue;
>
> - if (is_task_rq_idle(rq_i->core_pick) && rq_i->nr_running &&
> - !rq_i->core->core_forceidle) {
> - rq_i->core->core_forceidle = true;
> - }
> + if (!(fi_before && rq->core->core_forceidle))
> + task_vruntime_update(rq_i, rq_i->core_pick);
>
> if (i == cpu) {
> rq_i->core_pick = NULL;
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10686,6 +10686,67 @@ static inline void task_tick_core(struct
> __entity_slice_used(&curr->se, MIN_NR_TASKS_DURING_FORCEIDLE))
> resched_curr(rq);
> }
> +
> +static void se_fi_update(struct sched_entity *se, unsigned int fi_seq, bool
> forceidle)
> +{
> + for_each_sched_entity(se) {
> + struct cfs_rq *cfs_rq = cfs_rq_of(se);
> +
> + if (forceidle) {
> + if (cfs_rq->forceidle_seq == fi_seq)
> + break;
> + cfs_rq->forceidle_seq = fi_seq;
> + }
> +
> + cfs_rq->min_vruntime_fi = cfs_rq->min_vruntime;
> + }
> +}
> +
> +void task_vruntime_update(struct rq *rq, struct task_struct *p)
> +{
> + struct sched_entity *se = &p->se;
> +
> + if (p->sched_class != &fair_sched_class)
> + return;
> +
> + se_fi_update(se, rq->core->core_forceidle_seq,
> rq->core->core_forceidle);
> +}
> +
> +bool cfs_prio_less(struct task_struct *a, struct task_struct *b)
> +{
> + struct rq *rq = task_rq(a);
> + struct sched_entity *sea = &a->se;
> + struct sched_entity *seb = &b->se;
> + struct cfs_rq *cfs_rqa;
> + struct cfs_rq *cfs_rqb;
> + s64 delta;
> +
> + SCHED_WARN_ON(task_rq(b)->core != rq->core);
> +
> + while (sea->cfs_rq->tg != seb->cfs_rq->tg) {
> + 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);
> + }
> +
> + if (rq->core->core_forceidle) {
> + se_fi_update(sea, rq->core->core_forceidle_seq, true);
> + se_fi_update(seb, rq->core->core_forceidle_seq, true);
> + }
> +
> + cfs_rqa = sea->cfs_rq;
> + cfs_rqb = seb->cfs_rq;
> +
> + /* normalize vruntime WRT their rq's base */
> + delta = (s64)(sea->vruntime - seb->vruntime) +
> + (s64)(cfs_rqb->min_vruntime_fi - cfs_rqa->min_vruntime_fi);
> +
> + return delta > 0;
> +}
> #else
> static inline void task_tick_core(struct rq *rq, struct task_struct *curr) {}
> #endif
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -522,6 +522,11 @@ struct cfs_rq {
> unsigned int h_nr_running; /* SCHED_{NORMAL,BATCH,IDLE}
> */
> unsigned int idle_h_nr_running; /* SCHED_IDLE */
>
> +#ifdef CONFIG_SCHED_CORE
> + unsigned int forceidle_seq;
> + u64 min_vruntime_fi;
> +#endif
> +
> u64 exec_clock;
> u64 min_vruntime;
> #ifndef CONFIG_64BIT
> @@ -1061,7 +1066,8 @@ struct rq {
> unsigned int core_task_seq;
> unsigned int core_pick_seq;
> unsigned long core_cookie;
> - unsigned char core_forceidle;
> + unsigned int core_forceidle;
> + unsigned int core_forceidle_seq;
> #endif
> };
>
> @@ -1106,6 +1112,8 @@ static inline raw_spinlock_t *rq_lockp(s
> return &rq->__lock;
> }
>
> +bool cfs_prio_less(struct task_struct *a, struct task_struct *b);
> +
> #else /* !CONFIG_SCHED_CORE */
>
> static inline bool sched_core_enabled(struct rq *rq)
