linux-kernel - Re: [PATCH] sched/topology: Remove redundant variable and fix incorrect type in build_sched

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Date:   Fri, 4 Mar 2022 14:49:23 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     K Prateek Nayak <kprateek.nayak@....com>,
        aubrey.li@...ux.intel.com, efault@....de, gautham.shenoy@....com,
        linux-kernel@...r.kernel.org, mgorman@...hsingularity.net,
        mingo@...nel.org, song.bao.hua@...ilicon.com,
        srikar@...ux.vnet.ibm.com, vincent.guittot@...aro.org
Subject: Re: [PATCH] sched/topology: Remove redundant variable and fix
 incorrect type in build_sched_domains

On Fri, Mar 04, 2022 at 12:56:44PM +0000, Valentin Schneider wrote:
> On 18/02/22 21:57, K Prateek Nayak wrote:
> > While investigating the sparse warning reported by the LKP bot [1],
> > observed that we have a redundant variable "top" in the function
> > build_sched_domains that was introduced in the recent commit
> > e496132ebedd ("sched/fair: Adjust the allowed NUMA imbalance when
> > SD_NUMA spans multiple LLCs")
> >
> > The existing variable "sd" suffices which allows us to remove the
> > redundant variable "top" while annotating the other variable "top_p"
> > with the "__rcu" annotation to silence the sparse warning.
> >
> > [1] https://lore.kernel.org/lkml/202202170853.9vofgC3O-lkp@intel.com/
> >
> > Fixes: e496132ebedd ("sched/fair: Adjust the allowed NUMA imbalance when SD_NUMA spans multiple LLCs")
> > Reported-by: kernel test robot <lkp@...el.com>
> > Signed-off-by: K Prateek Nayak <kprateek.nayak@....com>
> > ---
> >  kernel/sched/topology.c | 8 +++-----
> >  1 file changed, 3 insertions(+), 5 deletions(-)
> >
> > diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> > index e6cd55951304..4b757fb8f661 100644
> > --- a/kernel/sched/topology.c
> > +++ b/kernel/sched/topology.c
> > @@ -2255,7 +2255,7 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
> >
> >                       if (!(sd->flags & SD_SHARE_PKG_RESOURCES) && child &&
> >                           (child->flags & SD_SHARE_PKG_RESOURCES)) {
> > -				struct sched_domain *top, *top_p;
> > +				struct sched_domain __rcu *top_p;
> >                               unsigned int nr_llcs;
> >
> >                               /*
> > @@ -2280,11 +2280,9 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
> >                               sd->imb_numa_nr = imb;
> >
> >                               /* Set span based on the first NUMA domain. */
> > -				top = sd;
> > -				top_p = top->parent;
> > +				top_p = sd->parent;
> >                               while (top_p && !(top_p->flags & SD_NUMA)) {
> > -					top = top->parent;
> > -					top_p = top->parent;
> > +					top_p = top_p->parent;
> >                               }
> >                               imb_span = top_p ? top_p->span_weight : sd->span_weight;
> 
> 
> So sd starts as the first domain without SD_SHARE_PKG_RESOURCES (but its
> direct child has it), and the loop searches upward for a NUMA domain. This
> indeed shouldn't require two variables.
> 
> I was going to say that while you're at it, you could init top_p to sd
> rather than sd->parent in case someone runs a non-default
> sched_domain_topology[], but NODE should have neither
> SD_SHARE_PKG_RESOURCES nor SD_NUMA, so that doesn't seem required.
> 
> Reviewed-by: Valentin Schneider <valentin.schneider@....com>

Thanks!