On Wed, Oct 07, 2020 at 06:55:33PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 07, 2020 at 11:12:29PM +0800, Peng Liu wrote:
> > +/* Used for dl_bw check and update. */
> > +static u32 dl_generation;
>
> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index 28709f6b0975..53477e8b26b0 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -798,6 +798,13 @@ struct root_domain {
> > */
> > cpumask_var_t dlo_mask;
> > atomic_t dlo_count;
> > +
> > + /*
> > + * Indicate whether a root_domain's dl_bw has been checked or
> > + * updated. It's monotonously increasing, then wrap around.
> > + */
> > + u32 visit_gen;
> > +
> > struct dl_bw dl_bw;
> > struct cpudl cpudl;
> >
> > diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> > index dd7770226086..90f3e5558fa2 100644
> > --- a/kernel/sched/topology.c
> > +++ b/kernel/sched/topology.c
> > @@ -516,6 +516,7 @@ static int init_rootdomain(struct root_domain *rd)
> > init_irq_work(&rd->rto_push_work, rto_push_irq_work_func);
> > #endif
> >
> > + rd->visit_gen = 0;
> > init_dl_bw(&rd->dl_bw);
> > if (cpudl_init(&rd->cpudl) != 0)
> > goto free_rto_mask;
>
> I'm fairly sure I made the generation a u64, the above is susceptible to
> a false positive due to wrap-around.
>
> Increase the generation to -1, create a new root domain, then the next
> generation is 0 and we'll skip the new domain, even though it should be
> updated.
Ah... at first, I also thought that u32 is "big enough" given that
no one would frequently change the settings, 'wrap-around' shouldn't
be a concern.
So...OK, I will revert it back to u64. What a big circle! :)
Thanks for your time!
