Re: [PATCH v2] x86/resctrl: Clear the stale staged config after the configuration is completed

[Shawn Wang]

Hi Reinette,

On 10/25/2022 12:45 AM, Reinette Chatre wrote:
> Hi Shawn,
>
> On 10/23/2022 7:31 PM, Shawn Wang wrote:
> > On 10/22/2022 2:05 AM, Reinette Chatre wrote:
> >
> > ...
> >
> > > > It may not be enough to just clear staged_config[] when
> > > > resctrl_arch_update_domains() exits. I think the fix needs to make
> > > > sure staged_config[] can be cleared where it is set.
> > > >
> > > > The modification of staged_config[] comes from two paths:
> > > >
> > > > Path 1:
> > > > rdtgroup_schemata_write() {
> > > >       ...
> > > >       rdtgroup_parse_resource()     // set staged_config[]
> > > >       ...
> > > >       resctrl_arch_update_domains()     // clear staged_config[]
> > > >       ...
> > > > }
> > > >
> > > > Path 2:
> > > > rdtgroup_init_alloc() {
> > > >       ...
> > > >       rdtgroup_init_mba()/rdtgroup_init_cat()    // set staged_config[]
> > > >       ...
> > > >       resctrl_arch_update_domains()        // clear staged_config[]
> > > >       ...
> > > > }
> > > >
> > > > If we clear staged_config[] in resctrl_arch_update_domains(), goto
> > > > statement for error handling between setting staged_config[] and
> > > > calling resctrl_arch_update_domains() will be ignored. This can still
> > > > remain the stale staged_config[].
> > > ah - indeed. Thank you for catching that.
> > >
> > > > I think maybe it is better to put the clearing work where
> > > > rdtgroup_schemata_write() and rdtgroup_init_alloc() exit.
> > >
> > > It may be more robust to let rdtgroup_init_alloc() follow
> > > how rdtgroup_schemata_write() already ensures that it is
> > > working with a clean state by clearing staged_config[] before
> > > placing its staged config within.
> >
> > I want to make sure, do you mean just ignore the stale value and
> > place the clearing work before staged_config[] is used? If so, maybe
> > the only thing the fix should do is to add memset() to
> > rdtgroup_init_alloc().>
>
> No, let us not leave stale data lying around.
>
> The idea is that the function calling resctrl_arch_update_domains() is
> responsible for initializing staged_config[] correctly and completely.
> To confirm, yes, the idea is to clear the staged_config[] in
> rdtgroup_init_alloc() before resctrl_arch_update_domains() is called
> to follow how it is currently done in rdtgroup_schemata_write().
>
> But, as you indicate, by itself this would leave stale data lying around.
>
> The solution that you suggested earlier, to put the clearing work where
> rdtgroup_schemata_write() and rdtgroup_init_alloc() exit, is most logical.
> That makes the code symmetrical in that staged_config[] is cleared
> where it is initialized and no stale data is left lying around. What was
> not clear to me is how this would look in the end. Were you planning to
> keep the staged_config[] clearing within rdtgroup_schemata_write() but
> not do so in rdtgroup_init_alloc()? rdtgroup_schemata_write() and
> rdtgroup_init_alloc() has to follow the same pattern to reduce confusion.
>
> So, to be more robust, how about:
>
> /* Clear staged_config[] to make sure working from a clean slate */
> resctrl_arch_update_domains()
> /* Clear staged_config[] to not leave stale data lying around */

Thank you for your explanation, and it makes sense to me. But this will
require 4 memset() loops, how about putting the clearing work in
a separate function in rdtgroup.c, like rdt_last_cmd_clear():

void staged_configs_clear(void) {
	struct resctrl_schema *s;
	struct rdt_domain *dom;

	lockdep_assert_held(&rdtgroup_mutex);

	list_for_each_entry(s, &resctrl_schema_all, list) {
		list_for_each_entry(dom, &s->res->domains, list)
			memset(dom->staged_config, 0, sizeof(dom->staged_config));
	}
}

Thanks,

Shawn