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sched_ule(4): Reduce duplicate search for load.

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When sched_highest() called for some CPU group returns nothing, idle
thread calls it for the parent CPU group.  But the parent CPU group
also includes the CPU group we've just searched, and unless there is
a race going on, it is unlikely we find anything new this time.

Avoid the double search in case of parent group having only two sub-
groups (the most prominent case). Instead of escalating to the parent
group run the next search over the sibling subgroup and escalate two
levels up after if that fail too.  In case of more than two siblings
the difference is less significant, while searching the parent group
can result in better decision if we find several candidate CPUs.

On 2-socket 40-core Xeon system I am measuring ~25% reduction of CPU
time spent inside cpu_search_highest() in both SMT (2x20x2) and non-
SMT (2x20) cases.

MFC after:	1 month
This commit is contained in:
Alexander Motin 2021-08-01 22:07:51 -04:00
parent 665895db26
commit 2668bb2add
1 changed files with 52 additions and 11 deletions
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63
sys/kern/sched_ule.c
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@ -933,10 +933,10 @@ tdq_move(struct tdq *from, struct tdq *to)
static int
tdq_idled(struct tdq *tdq)
{
struct cpu_group *cg;
struct cpu_group *cg, *parent;
struct tdq *steal;
cpuset_t mask;
int cpu, switchcnt;
int cpu, switchcnt, goup;
if (smp_started == 0 || steal_idle == 0 || tdq->tdq_cg == NULL)
return (1);
@ -944,7 +944,7 @@ tdq_idled(struct tdq *tdq)
CPU_CLR(PCPU_GET(cpuid), &mask);
restart:
switchcnt = tdq->tdq_switchcnt + tdq->tdq_oldswitchcnt;
for (cg = tdq->tdq_cg; ; ) {
for (cg = tdq->tdq_cg, goup = 0; ; ) {
cpu = sched_highest(cg, &mask, steal_thresh);
/*
* We were assigned a thread but not preempted. Returning
@ -952,10 +952,29 @@ tdq_idled(struct tdq *tdq)
*/
if (tdq->tdq_load)
return (0);
/*
* We found no CPU to steal from in this group. Escalate to
* the parent and repeat. But if parent has only two children
* groups we can avoid searching this group again by searching
* the other one specifically and then escalating two levels.
*/
if (cpu == -1) {
cg = cg->cg_parent;
if (cg == NULL)
if (goup) {
cg = cg->cg_parent;
goup = 0;
}
parent = cg->cg_parent;
if (parent == NULL)
return (1);
if (parent->cg_children == 2) {
if (cg == &parent->cg_child[0])
cg = &parent->cg_child[1];
else
cg = &parent->cg_child[0];
goup = 1;
} else
cg = parent;
continue;
}
steal = TDQ_CPU(cpu);
@ -1868,10 +1887,10 @@ sched_lend_user_prio_cond(struct thread *td, u_char prio)
static void
tdq_trysteal(struct tdq *tdq)
{
struct cpu_group *cg;
struct cpu_group *cg, *parent;
struct tdq *steal;
cpuset_t mask;
int cpu, i;
int cpu, i, goup;
if (smp_started == 0 || trysteal_limit == 0 || tdq->tdq_cg == NULL)
return;
@ -1880,7 +1899,7 @@ tdq_trysteal(struct tdq *tdq)
/* We don't want to be preempted while we're iterating. */
spinlock_enter();
TDQ_UNLOCK(tdq);
for (i = 1, cg = tdq->tdq_cg; ; ) {
for (i = 1, cg = tdq->tdq_cg, goup = 0; ; ) {
cpu = sched_highest(cg, &mask, steal_thresh);
/*
* If a thread was added while interrupts were disabled don't
@ -1890,13 +1909,35 @@ tdq_trysteal(struct tdq *tdq)
TDQ_LOCK(tdq);
break;
}
/*
* We found no CPU to steal from in this group. Escalate to
* the parent and repeat. But if parent has only two children
* groups we can avoid searching this group again by searching
* the other one specifically and then escalating two levels.
*/
if (cpu == -1) {
i++;
cg = cg->cg_parent;
if (cg == NULL || i > trysteal_limit) {
if (goup) {
cg = cg->cg_parent;
goup = 0;
}
if (++i > trysteal_limit) {
TDQ_LOCK(tdq);
break;
}
parent = cg->cg_parent;
if (parent == NULL) {
TDQ_LOCK(tdq);
break;
}
if (parent->cg_children == 2) {
if (cg == &parent->cg_child[0])
cg = &parent->cg_child[1];
else
cg = &parent->cg_child[0];
goup = 1;
} else
cg = parent;
continue;
}
steal = TDQ_CPU(cpu);