linux-kernel - Re: [PATCH v3 17/38] x86/resctrl: Stop using the for_each_*_rdt

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Message-ID: <f629e958-39c0-48cf-870f-8f76cfa7b815@intel.com>
Date: Mon, 1 Jul 2024 14:10:17 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: James Morse <james.morse@....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>, D Scott Phillips OS
	<scott@...amperecomputing.com>, <carl@...amperecomputing.com>,
	<lcherian@...vell.com>, <bobo.shaobowang@...wei.com>,
	<tan.shaopeng@...itsu.com>, <baolin.wang@...ux.alibaba.com>, Jamie Iles
	<quic_jiles@...cinc.com>, Xin Hao <xhao@...ux.alibaba.com>,
	<peternewman@...gle.com>, <dfustini@...libre.com>, <amitsinght@...vell.com>,
	David Hildenbrand <david@...hat.com>, Rex Nie <rex.nie@...uarmicro.com>, Dave
 Martin <dave.martin@....com>, Shaopeng Tan <tan.shaopeng@...fujitsu.com>
Subject: Re: [PATCH v3 17/38] x86/resctrl: Stop using the
 for_each_*_rdt_resource() walkers

Hi James,

On 7/1/24 11:16 AM, James Morse wrote:
> Hi Reinette,
> 
> On 28/06/2024 17:48, Reinette Chatre wrote:
>> On 6/14/24 8:00 AM, James Morse wrote:
>>> The for_each_*_rdt_resource() helpers walk the architecture's array
>>> of structures, using the resctrl visible part as an iterator. These
>>> became over-complex when the structures were split into a
>>> filesystem and architecture-specific struct. This approach avoided
>>> the need to touch every call site.
>>>
>>> Once the filesystem parts of resctrl are moved to /fs/, both the
>>> architecture's resource array, and the definition of those structures
>>> is no longer accessible. To support resctrl, each architecture would
>>> have to provide equally complex macros.
>>>
>>> Change the resctrl code that uses these to walk through the resource_level
>>> enum and check the mon/alloc capable flags instead. Instances in core.c,
>>> and resctrl_arch_reset_resources() remain part of x86's architecture
>>> specific code.
> 
>>> diff --git a/arch/x86/kernel/cpu/resctrl/pseudo_lock.c
>>> b/arch/x86/kernel/cpu/resctrl/pseudo_lock.c
>>> index aacf236dfe3b..ad20822bb64e 100644
>>> --- a/arch/x86/kernel/cpu/resctrl/pseudo_lock.c
>>> +++ b/arch/x86/kernel/cpu/resctrl/pseudo_lock.c
>>> @@ -854,7 +855,11 @@ bool rdtgroup_pseudo_locked_in_hierarchy(struct rdt_domain *d)
>>>         * First determine which cpus have pseudo-locked regions
>>>         * associated with them.
>>>         */
>>> -    for_each_alloc_capable_rdt_resource(r) {
>>> +    for (i = 0; i < RDT_NUM_RESOURCES; i++) {
>>> +        r = resctrl_arch_get_resource(i);
>>> +        if (!r->alloc_capable)
>>> +            continue;
>>> +
>>
>> This looks like enough duplicate boilerplate for a new macro. For simplicity the
>> macro could require two arguments with enum resctrl_res_level also provided?
> 
> I was hoping to escape from these clever macros! If you think this is too much:
> - we'd need to come up with another name, as the arch code keeps the existing definition.
> - to avoid touching every caller, it needs doing without an explicit iterator variable.
> 
> I guess the cleanest thing is to redefine the existing macros to use
> resctrl_arch_get_resource(). Putting this in include/linxu/resctrl.h at least avoids each
> architecture needing to define these, or forcing it to use an array.
> 
> The result is slightly more readable than the current version:
> | #define for_each_rdt_resource(_r)                              \
> |        for (_r = resctrl_arch_get_resource(0);                 \
> |             _r->rid < RDT_NUM_RESOURCES;                       \
> |             _r = resctrl_arch_get_resource(_r->rid + 1))
> 
> This leans heavily on resctrl_arch_get_resource() not being able to return NULL, and
> having to return a dummy resource that is neither alloc nor mon capable. We may need to
> revisit that if it becomes a burden for the arch code.

Replacing the repetitive four lines of code with a single line seems good to me.

resctrl_arch_get_resource() being able to return NULL is introduced in this series but
I am not seeing any handling of a possible NULL value. Not being able to return NULL thus
already seems a requirement?

Reinette