Messages in this thread Patch in this message | | | Date | Mon, 23 Aug 2021 13:16:56 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] sched/core: fix pick_next_task 'max' tracking |
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On Tue, Aug 17, 2021 at 05:56:15PM -0700, Josh Don wrote: > For core-sched, pick_next_task will update the 'max' task if there is a > cookie mismatch (since in this case the new task must be of higher > priority than the current max). However, we fail to update 'max' if > we've found a task with a matching cookie and higher priority than > 'max'. > > This can result in extra iterations on SMT-X machines, where X > 2. > > As an example, on a machine with SMT=3, on core 0, SMT-0 might pick > the following, in order: > > - SMT-0: p1, with cookie A, and priority 10 (max = p1) > - SMT-1: p2, with cookie A, and priority 30 (max not updated here) > - SMT-2: p3, with cookie B, and priority 20 (max = p2) > > invalidate the other picks and retry > > Here, we should have instead updated 'max' when picking for SMT-1. Note > that this code would eventually have righted itself, since the retry > loop would re-pick p2, and update 'max' accordingly. However, this patch > avoids the extra round-trip.
Going with the observation Tao made; how about we rewrite the whole lot to not be mind-bending complicated :-)
How's this? It seems to build and pass the core-sched selftest thingy (so it must be perfect, right? :-)
--- kernel/sched/core.c | 147 ++++++++++++++------------------------------------- kernel/sched/sched.h | 1 + 2 files changed, 41 insertions(+), 107 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index ceae25ea8a0e..e896250c2fb8 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -5404,66 +5404,18 @@ static inline bool cookie_match(struct task_struct *a, struct task_struct *b) return a->core_cookie == b->core_cookie; } -// XXX fairness/fwd progress conditions -/* - * Returns - * - NULL if there is no runnable task for this class. - * - the highest priority task for this runqueue if it matches - * rq->core->core_cookie or its priority is greater than max. - * - Else returns idle_task. - */ -static struct task_struct * -pick_task(struct rq *rq, const struct sched_class *class, struct task_struct *max, bool in_fi) -{ - struct task_struct *class_pick, *cookie_pick; - unsigned long cookie = rq->core->core_cookie; - - class_pick = class->pick_task(rq); - if (!class_pick) - return NULL; - - if (!cookie) { - /* - * If class_pick is tagged, return it only if it has - * higher priority than max. - */ - if (max && class_pick->core_cookie && - prio_less(class_pick, max, in_fi)) - return idle_sched_class.pick_task(rq); - - return class_pick; - } - - /* - * If class_pick is idle or matches cookie, return early. - */ - if (cookie_equals(class_pick, cookie)) - return class_pick; - - cookie_pick = sched_core_find(rq, cookie); - - /* - * If class > max && class > cookie, it is the highest priority task on - * the core (so far) and it must be selected, otherwise we must go with - * the cookie pick in order to satisfy the constraint. - */ - if (prio_less(cookie_pick, class_pick, in_fi) && - (!max || prio_less(max, class_pick, in_fi))) - return class_pick; - - return cookie_pick; -} - extern void task_vruntime_update(struct rq *rq, struct task_struct *p, bool in_fi); static struct task_struct * pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf) { - struct task_struct *next, *max = NULL; + struct task_struct *next, *p, *max = NULL; const struct sched_class *class; const struct cpumask *smt_mask; bool fi_before = false; - int i, j, cpu, occ = 0; + unsigned long cookie; + int i, cpu, occ = 0; + struct rq *rq_i; bool need_sync; if (!sched_core_enabled(rq)) @@ -5554,76 +5506,57 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf) } } - for_each_cpu(i, smt_mask) { - struct rq *rq_i = cpu_rq(i); - + /* + * For each thread: do the regular task pick and find the max prio task + * amongst them. + * + * Tie-break prio towards the current CPU + */ + for_each_cpu_wrap(i, smt_mask, cpu) { + rq_i = cpu_rq(i); rq_i->core_pick = NULL; if (i != cpu) update_rq_clock(rq_i); + + for_each_class(class) { + p = rq_i->core_temp = class->pick_task(rq_i); + if (p) + break; + } + + if (!max || prio_less(max, p, fi_before)) + max = p; } + cookie = rq->core->core_cookie = max->core_cookie; + /* - * Try and select tasks for each sibling in descending sched_class - * order. + * For each thread: try and find a runnable task that matches @max or + * force idle. */ - for_each_class(class) { -again: - for_each_cpu_wrap(i, smt_mask, cpu) { - struct rq *rq_i = cpu_rq(i); - struct task_struct *p; - - if (rq_i->core_pick) - continue; + for_each_cpu(i, smt_mask) { + rq_i = cpu_rq(i); + p = rq_i->core_temp; - /* - * If this sibling doesn't yet have a suitable task to - * run; ask for the most eligible task, given the - * highest priority task already selected for this - * core. - */ - p = pick_task(rq_i, class, max, fi_before); + if (!cookie_equals(p, cookie)) { + p = NULL; + if (cookie) + p = sched_core_find(rq_i, cookie); if (!p) - continue; + p = idle_sched_class.pick_task(rq_i); + } - if (!is_task_rq_idle(p)) - occ++; + rq_i->core_pick = p; - rq_i->core_pick = p; - if (rq_i->idle == p && rq_i->nr_running) { + if (p == rq_i->idle) { + if (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 - * previous; and they're incompatible; we need to wipe - * the slate and start over. pick_task makes sure that - * p's priority is more than max if it doesn't match - * max's cookie. - * - * NOTE: this is a linear max-filter and is thus bounded - * in execution time. - */ - if (!max || !cookie_match(max, p)) { - struct task_struct *old_max = max; - - rq->core->core_cookie = p->core_cookie; - max = p; - - if (old_max) { - rq->core->core_forceidle = false; - for_each_cpu(j, smt_mask) { - if (j == i) - continue; - - cpu_rq(j)->core_pick = NULL; - } - occ = 1; - goto again; - } - } + } else { + occ++; } } @@ -5643,7 +5576,7 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf) * non-matching user state. */ for_each_cpu(i, smt_mask) { - struct rq *rq_i = cpu_rq(i); + rq_i = cpu_rq(i); /* * An online sibling might have gone offline before a task diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index d9f8d73a1d84..0760b460903a 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -1091,6 +1091,7 @@ struct rq { /* per rq */ struct rq *core; struct task_struct *core_pick; + struct task_struct *core_temp; unsigned int core_enabled; unsigned int core_sched_seq; struct rb_root core_tree;
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