linux-kernel - Re: [PATCH v5 14/40] x86/resctrl: Add a resctrl helper to reset all the resources

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Message-ID: <04d1d72d-dd03-494e-b1b1-04ecddb94f80@intel.com>
Date: Thu, 13 Feb 2025 15:52:03 -0800
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 v5 14/40] x86/resctrl: Add a resctrl helper to reset all
 the resources

Hi James,

On 2/7/25 7:43 AM, James Morse wrote:
> Hi Reinette,
> 
> On 23/10/2024 22:32, Reinette Chatre wrote:
>> On 10/4/24 11:03 AM, James Morse wrote:
>>> On umount(), resctrl resets each resource back to its default
>>> configuration. It only ever does this for all resources in one go.
>>>
>>> reset_all_ctrls() is architecture specific as it works with struct
>>> rdt_hw_resource.
>>>
>>> Add an architecture helper to reset all resources.
> 
>>> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>> index 61c8add103fe..a15198f90b29 100644
>>> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>> @@ -2883,6 +2883,14 @@ static int reset_all_ctrls(struct rdt_resource *r)
>>>  	return 0;
>>>  }
>>>  
>>> +void resctrl_arch_reset_resources(void)
>>> +{
>>> +	struct rdt_resource *r;
>>> +
>>> +	for_each_alloc_capable_rdt_resource(r)
>>> +		reset_all_ctrls(r);
>>> +}
> 
>> Wouldn't this require all archs to have a duplicate helper as above with
>> only the resctrl_all_ctrls() actually being arch specific? 
> 
> I was hoping to be able to save a few IPI by doing the per-core work once, instead of
> per-resource-per-core ... but its only done on umount, so I doubt anyone will complain!

This is very reasonable but that is not what the code does today and the
helper is added to today's code without providing insight into future optimizations. It
sounds as though MPAM was planning something differently (better) for which the helper in this
version would be appropriate and I expect that x86 could also benefit from that. I
understand that this is not a "fast" path but it raises the question of how optimizations
across archs should be handled. Ideally we should look across archs for ideal helpers
and not force one arch to adapt to what works for another. I am not advocating for a
change to what you submitted in v6 but instead would like to share that I think by not
having a discussion before switching to new helper there was a missed opportunity for
both archs.

Reinette