linux-kernel - Re: [PATCH v3 07/21] x86/resctrl: Create mba_sc configuration in the rdt

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Date:   Mon, 4 Apr 2022 17:35:45 +0100
From:   James Morse <james.morse@....com>
To:     Reinette Chatre <reinette.chatre@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Cc:     Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Babu Moger <Babu.Moger@....com>,
        shameerali.kolothum.thodi@...wei.com,
        Jamie Iles <jamie@...iainc.com>,
        D Scott Phillips OS <scott@...amperecomputing.com>,
        lcherian@...vell.com, bobo.shaobowang@...wei.com,
        tan.shaopeng@...itsu.com
Subject: Re: [PATCH v3 07/21] x86/resctrl: Create mba_sc configuration in the
 rdt_domain

Hi Reinette,

On 4/1/22 23:54, Reinette Chatre wrote:
> On 3/30/2022 9:43 AM, James Morse wrote:
>> On 16/03/2022 21:50, Reinette Chatre wrote:
>>> On 2/17/2022 10:20 AM, James Morse wrote:
>>>> To support resctrl's MBA software controller, the architecture must provide
>>>> a second configuration array to hold the mbps_val[] from user-space.
>>>>
>>>> This complicates the interface between the architecture specific code and
>>>> the filesystem portions of resctrl that will move to /fs/, to allow
>>>> multiple architectures to support resctrl.
>>>>
>>>> Make the filesystem parts of resctrl create an array for the mba_sc
>>>> values when is_mba_sc() is set to true. The software controller
>>>> can be changed to use this, allowing the architecture code to only
>>>> consider the values configured in hardware.

[...]

>>> Considering that no domain belonging to RDT_RESOURCE_MBA will have this array this
>>> always ends up being a null pointer de-reference.
>>
>> Ugh. I'm not sure how I managed to miss that. Thanks for debugging it!
>>
>> That loop was added to reset the array when the filesystem is mounted, as it may hold
>> stale values from a previous mount of the filesystem. Its currently done by
>> reset_all_ctrls(), but that function should really belong to the architecture code.
>>
>> Because mbm_handle_overflow() always passes a domain from the L3 to update_mba_bw(), I
>> think the cleanest thing to do is move the reset to a helper that always operates on the
>> L3 array. (and leave some breadcrumbs in the comments).

> I think this points to more than a need to reset the correct array on mount/unmount ... or
> perhaps I am not understanding this correctly?
> 
> As the analysis above shows the mbps_val array only exists for rdt_domains associated
> with RDT_RESOURCE_L3 but yet mbps_val will contain the MB value provided by user space
> associated with RDT_RESOURCE_MBA.

I've finally got my head round what is going on here: I've muddled up whether mon_capable
is a resource or system property. mba_sc depends on the L3 being mon_capable, but the
configuration should be associated with MBA (wherever that is).
(basically ignore my previous reply!)

The creation of the mbps_val[] should depend on supports_mba_mbps(), which uses
is_mbm_enabled() to check whether the L3 is mon_capable. I'll check the rid too
to make it clear its only MBA that has this.
The call to allocate the domain in resctrl_online_domain() should be above the mon_capable
check. (which is still needed to avoid the work guarded by is_mbm_enabled() and friends
running for each domain).


Thanks,

James

-----------------------%<-----------------------
diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
index e3c90f33baf2..ad0411eb2147 100644
--- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
+++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
@@ -3345,6 +3345,14 @@ int resctrl_online_domain(struct rdt_resource *r, struct rdt_domain *d)
  
         lockdep_assert_held(&rdtgroup_mutex);
  
+       if (is_mbm_enabled() && r->rid == RDT_RESOURCE_MBA) {
+               err = mba_sc_domain_allocate(r, d);
+               if (err) {
+                       domain_destroy_mon_state(d);
+                       return err;
+               }
+       }
+
         if (!r->mon_capable)
                 return 0;
  
@@ -3352,12 +3360,6 @@ int resctrl_online_domain(struct rdt_resource *r, struct rdt_domain *d)
         if (err)
                 return err;
  
-       err = mba_sc_domain_allocate(r, d);
-       if (err) {
-               domain_destroy_mon_state(d);
-               return err;
-       }
-
         if (is_mbm_enabled()) {
                 INIT_DELAYED_WORK(&d->mbm_over, mbm_handle_overflow);
                 mbm_setup_overflow_handler(d, MBM_OVERFLOW_INTERVAL);
-----------------------%<-----------------------