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SubjectRe: [patch 4/5] sched: RCU sched domains

* Nick Piggin <nickpiggin@yahoo.com.au> wrote:

> 4/5

> One of the problems with the multilevel balance-on-fork/exec is that
> it needs to jump through hoops to satisfy sched-domain's locking
> semantics (that is, you may traverse your own domain when not
> preemptable, and you may traverse others' domains when holding their
> runqueue lock).
>
> balance-on-exec had to potentially migrate between more than one CPU
> before finding a final CPU to migrate to, and balance-on-fork needed
> to potentially take multiple runqueue locks.
>
> So bite the bullet and make sched-domains go completely RCU. This
> actually simplifies the code quite a bit.
>
> Signed-off-by: Nick Piggin <nickpiggin@yahoo.com.au>

i like it conceptually, so:

Acked-by: Ingo Molnar <mingo@elte.hu>

from now on, all domain-tree readonly uses have to be rcu_read_lock()-ed
(or otherwise have to be in a non-preemptible section). But there's a
bug in show_shedstats() which does a for_each_domain() from within a
preemptible section. (It was a bug with the current hotplug logic too i
think.)

At a minimum i think we need the fix+comment below.

Ingo

Signed-off-by: Ingo Molnar <mingo@elte.hu>

--- linux/kernel/sched.c.orig
+++ linux/kernel/sched.c
@@ -260,6 +260,10 @@ struct runqueue {

static DEFINE_PER_CPU(struct runqueue, runqueues);

+/*
+ * The domain tree (rq->sd) is RCU locked. I.e. it may only be accessed
+ * from within an rcu_read_lock() [or otherwise preempt-disabled] sections.
+ */
#define for_each_domain(cpu, domain) \
for (domain = cpu_rq(cpu)->sd; domain; domain = domain->parent)

@@ -338,6 +342,7 @@ static int show_schedstat(struct seq_fil

#ifdef CONFIG_SMP
/* domain-specific stats */
+ rcu_read_lock();
for_each_domain(cpu, sd) {
enum idle_type itype;
char mask_str[NR_CPUS];
@@ -361,6 +366,7 @@ static int show_schedstat(struct seq_fil
sd->sbe_pushed, sd->sbe_attempts,
sd->ttwu_wake_remote, sd->ttwu_move_affine, sd->ttwu_move_balance);
}
+ rcu_read_unlock();
#endif
}
return 0;
-
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