Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: suspicious RCU due to "Prefer using an idle CPU as a migration target instead of comparing tasks" | Date | Thu, 27 Feb 2020 17:30:40 +0000 |
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On Thu, Feb 27 2020, Mel Gorman wrote: > Thanks for reporting this! > > The proposed fix would be a lot of rcu locks and unlocks. While they are > cheap, they're not free and it's a fairly standard pattern to acquire > the rcu lock when scanning CPUs during a domain search (load balancing, > nohz balance, idle balance etc). While in this context the lock is only > needed for SMT, I do not think it's worthwhile fine-graining this or > conditionally acquiring the rcu lock so will we keep it simple? > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 11cdba201425..d34ac4ea5cee 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -1592,6 +1592,7 @@ static void update_numa_stats(struct task_numa_env *env, > memset(ns, 0, sizeof(*ns)); > ns->idle_cpu = -1; > > + rcu_read_lock(); > for_each_cpu(cpu, cpumask_of_node(nid)) { > struct rq *rq = cpu_rq(cpu); > > @@ -1611,6 +1612,7 @@ static void update_numa_stats(struct task_numa_env *env, > idle_core = numa_idle_core(idle_core, cpu); > } > } > + rcu_read_unlock(); > > ns->weight = cpumask_weight(cpumask_of_node(nid)); >
That's closer to what I was trying to suggest (i.e. broaden the section rather than reduce it).
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