linux-kernel - Re: [PATCH v5 5/5] cpuset: Rebuild root domain deadline accounting information

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Message-ID: <20180925130750.GA25664@localhost.localdomain>
Date:   Tue, 25 Sep 2018 15:07:50 +0200
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, rostedt@...dmis.org,
        linux-kernel@...r.kernel.org, luca.abeni@...tannapisa.it,
        claudio@...dence.eu.com, tommaso.cucinotta@...tannapisa.it,
        bristot@...hat.com, mathieu.poirier@...aro.org, lizefan@...wei.com,
        cgroups@...r.kernel.org
Subject: Re: [PATCH v5 5/5] cpuset: Rebuild root domain deadline accounting
 information

On 25/09/18 14:32, Peter Zijlstra wrote:
> On Mon, Sep 03, 2018 at 04:28:01PM +0200, Juri Lelli wrote:
> > +/*
> > + * Called with cpuset_mutex held (rebuild_sched_domains())
> > + * Called with hotplug lock held (rebuild_sched_domains_locked())
> > + * Called with sched_domains_mutex held (partition_and_rebuild_domains())
> 
> Isn't that what we have lockdep_assert_held() for?

Indeed. I can put three of them inside the function, even though we have
a single path to here atm. Guess makes sense to protect any future change.

> > + */
> > +static void rebuild_root_domains(void)
> > +{
> > +	struct cpuset *cs = NULL;
> > +	struct cgroup_subsys_state *pos_css;
> > +
> > +	rcu_read_lock();
> > +
> > +	/*
> > +	 * Clear default root domain DL accounting, it will be computed again
> > +	 * if a task belongs to it.
> > +	 */
> > +	dl_clear_root_domain(&def_root_domain);
> > +
> > +	cpuset_for_each_descendant_pre(cs, pos_css, &top_cpuset) {
> > +
> > +		if (cpumask_empty(cs->effective_cpus)) {
> > +			pos_css = css_rightmost_descendant(pos_css);
> > +			continue;
> > +		}
> > +
> > +		css_get(&cs->css);
> > +
> > +		rcu_read_unlock();
> 
> That looks really dodgy, but I suppose the comment near
> css_next_descendant_pre() spells out that this is in fact OK.

Plus update_cpumasks_hier() seems to do something similar. Maybe I
should switch to use css_tryget_online() as well?

Thanks,

- Juri