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

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Message-ID: <03bb5543-84b4-3523-741f-6c0497f46500@linux.alibaba.com>
Date:   Wed, 26 Oct 2022 19:03:36 +0800
From:   Shawn Wang <shawnwang@...ux.alibaba.com>
To:     Reinette Chatre <reinette.chatre@...el.com>, fenghua.yu@...el.com
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        James Morse <james.morse@....com>, jamie@...iainc.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] x86/resctrl: Clear the stale staged config after the
 configuration is completed

Hi Reinette,

On 10/26/2022 3:34 AM, Reinette Chatre wrote:
> Hi Shawn,
> 
> On 10/25/2022 8:30 AM, Shawn Wang wrote:
>> 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():
> 
> Yes, thanks for avoiding duplicating code.
> 
>>
>> 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));
>>      }
>> }
>>
> 
> I understand that you are just copying what is currently done in
> rdtgroup_schemata_write() but for a separate function I think something
> like below would be more efficient:
> 
> 
> 	for_each_alloc_capable_rdt_resource(r) {
> 		list_for_each_entry(dom, &r->domains, list)
> 			memset(dom->staged_config, 0, sizeof(dom->staged_config));
> 	}
> 
> This would be more efficient since it would not clean the same memory area
> twice when CDP is enabled.
> 

Thank you, I didn't notice this function before. I will add it in a new version.

Shawn