linux-kernel - Re: [PATCH v5 17/40] x86/resctrl: Rewrite and move the for_each_*_rdt

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Message-ID: <f4bd5866-ba8d-46de-a073-2ad44ac8306e@arm.com>
Date: Fri, 7 Feb 2025 15:44:00 +0000
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,
 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 v5 17/40] x86/resctrl: Rewrite and move the
 for_each_*_rdt_resource() walkers

Hi Reinette,

On 23/10/2024 22:51, Reinette Chatre wrote:
> On 10/4/24 11:03 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, and was done before there was a
>> helper to retrieve a resource by rid.
>>
>> 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.
>>
>> Rewrite the macro to make use of resctrl_arch_get_resource(), and
>> move these to the core header so existing x86 arch code continues
>> to use them.
> 
> The last part is not clear, why does it need to be moved to core
> header for x86 to use it?

If it moves to fs/resctrl/internal.h, this works for the filesystem code, but not x86.
If stays in arch/x86/kernel/cpu/resctrl/internal.h, then the existing filesystem users break.

Moving it to include/linux/resctrl.h means both filesystem and arch code can include it.
I'll add the header file path to the commit message.


>> diff --git a/include/linux/resctrl.h b/include/linux/resctrl.h
>> index 8894aed3c593..f75f0409ae09 100644
>> --- a/include/linux/resctrl.h
>> +++ b/include/linux/resctrl.h
>> @@ -26,6 +26,24 @@ int proc_resctrl_show(struct seq_file *m,
>>  /* max value for struct rdt_domain's mbps_val */
>>  #define MBA_MAX_MBPS   U32_MAX
>>  
>> +/* Walk all possible resources, with variants for only controls or monitors. */
>> +#define for_each_rdt_resource(_r)						\
>> +	for ((_r) = resctrl_arch_get_resource(0);				\
>> +	     (_r)->rid < RDT_NUM_RESOURCES - 1;					\

> I do not think this reaches all resources ... should this perhaps be:
> 	(_r) && (_r)->rid < RDT_NUM_RESOURCES

Good catch - I wrongly assumed I was off-by-one when this blew up because the increment is
executed before this expression, and the existing 'RDT_NUM_RESOURCES - 1' reinforced that.
Adding the "(_r) &&" was the correct fix.


Thanks!

James