linux-kernel - Re: [PATCH v6 3/8] x86/resctrl: Prepare for different scope for control/monitor operations

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Date:   Fri, 29 Sep 2023 12:15:05 -0700
From:   Tony Luck <tony.luck@...el.com>
To:     Peter Newman <peternewman@...gle.com>
Cc:     Fenghua Yu <fenghua.yu@...el.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Jonathan Corbet <corbet@....net>,
        Shuah Khan <skhan@...uxfoundation.org>, x86@...nel.org,
        Shaopeng Tan <tan.shaopeng@...itsu.com>,
        James Morse <james.morse@....com>,
        Jamie Iles <quic_jiles@...cinc.com>,
        Babu Moger <babu.moger@....com>,
        Randy Dunlap <rdunlap@...radead.org>,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        patches@...ts.linux.dev
Subject: Re: [PATCH v6 3/8] x86/resctrl: Prepare for different scope for
 control/monitor operations

On Fri, Sep 29, 2023 at 03:10:18PM +0200, Peter Newman wrote:
> Hi Tony,
> 
> On Thu, Sep 28, 2023 at 9:14 PM Tony Luck <tony.luck@...el.com> wrote:
> >
> > @@ -352,7 +355,7 @@ struct rdt_domain *get_domain_from_cpu(int cpu, struct rdt_resource *r)
> >  {
> >         struct rdt_domain *d;
> >
> > -       list_for_each_entry(d, &r->domains, hdr.list) {
> > +       list_for_each_entry(d, &r->ctrl_domains, hdr.list) {
> 
> If someone were to call get_domain_from_cpu() looking for a
> mon_domain, I don't think they'd be happy with the result.

Indeed not. Type checking in "C" doesn't seem adequate to address this
(when using "container_of()" which blindly trusts the user provided the
right tyep/fieldname). I'm using the rdt_domain_hdr.type field to
provide necessary checks.

> 
> This problem seems adequately addressed by the next patch where a type
> mismatch on the return value would result.
> 
> In any case, perhaps the name could be updated to set expectations better.
> 
> 
> > @@ -549,44 +552,101 @@ static void domain_add_cpu(int cpu, struct rdt_resource *r)
> >
> >         rdt_domain_reconfigure_cdp(r);
> >
> > -       if (r->alloc_capable && domain_setup_ctrlval(r, d)) {
> > +       if (domain_setup_ctrlval(r, d)) {
> >                 domain_free(hw_dom);
> >                 return;
> >         }
> >
> > -       if (r->mon_capable && arch_domain_mbm_alloc(r->num_rmid, hw_dom)) {
> > +       list_add_tail(&d->hdr.list, add_pos);
> > +
> > +       err = resctrl_online_ctrl_domain(r, d);
> > +       if (err) {
> > +               list_del(&d->hdr.list);
> >                 domain_free(hw_dom);
> > +       }
> > +}
> > +
> > +static void domain_add_cpu_mon(int cpu, struct rdt_resource *r)
> > +{
> > +       int id = get_domain_id_from_scope(cpu, r->mon_scope);
> > +       struct list_head *add_pos = NULL;
> > +       struct rdt_hw_domain *hw_mondom;
> 
> It's still hw_dom in domain_add_cpu_ctrl(), so why hw_mondom here?

No good reason. I'll change it to "hw_dom".

> 
> 
> > @@ -3711,16 +3711,16 @@ static void domain_destroy_mon_state(struct rdt_domain *d)
> >         kfree(d->mbm_local);
> >  }
> >
> > -void resctrl_offline_domain(struct rdt_resource *r, struct rdt_domain *d)
> > +void resctrl_offline_ctrl_domain(struct rdt_resource *r, struct rdt_domain *d)
> >  {
> >         lockdep_assert_held(&rdtgroup_mutex);
> >
> >         if (supports_mba_mbps() && r->rid == RDT_RESOURCE_MBA)
> >                 mba_sc_domain_destroy(r, d);
> > +}
> >
> > -       if (!r->mon_capable)
> > -               return;
> > -
> > +void resctrl_offline_mon_domain(struct rdt_resource *r, struct rdt_domain *d)
> > +{
> >         /*
> >          * If resctrl is mounted, remove all the
> >          * per domain monitor data directories.
> 
> We did a lockdep_assert_held(&rdtgroup_mutex) for both types before.
> Should we continue to do so here?

Yes. Added it.

> 
> 
> > --
> > 2.41.0
> >
> 
> In the resctrl2 prototype I complained that resctrl_resource was
> awkwardly disjoint in its support for control and monitoring
> groups[1]. In this patch, you seem to have already done most of the
> hard work in separating the control and monitoring functionality, so
> taking the next step and using a different structure to represent
> control and monitoring resources would further improve the code by
> statically typing monitoring and control resources, which would be
> less error-prone than run-time checks on the alloc_capable and
> mon_capable fields, which seem easy to forget.
> 
> I don't think this is necessary to complete SNC support, but it would
> give me confidence that there isn't a misplaced {alloc,mon}_capable
> check resulting in the wrong domain list being traversed. I will
> probably do this myself later if you don't.

Simple change. It's split between previous patch to add the field
and current patch to initialize and check.

> 
> Thanks!
> -Peter
> 
> [1] https://lore.kernel.org/all/CALPaoCj_oa=nATvOO_uysYvu+PdTQtd0pvssvm9_M1+fP-Z8JA@mail.gmail.com/

Thanks

-Tony