linux-kernel - Re: [PATCH v1 02/31] x86/resctrl: Add a helper to avoid reaching into the arch code resource list

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Date: Tue, 16 Apr 2024 15:44:29 -0500
From: "Moger, Babu" <babu.moger@....com>
To: Dave Martin <Dave.Martin@....com>
Cc: James Morse <james.morse@....com>, x86@...nel.org,
 linux-kernel@...r.kernel.org, Fenghua Yu <fenghua.yu@...el.com>,
 Reinette Chatre <reinette.chatre@...el.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, H Peter Anvin <hpa@...or.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>
Subject: Re: [PATCH v1 02/31] x86/resctrl: Add a helper to avoid reaching into
 the arch code resource list

Hi Dave,

On 4/16/24 11:15, Dave Martin wrote:
> On Mon, Apr 15, 2024 at 03:28:18PM -0500, Moger, Babu wrote:
>> Hi James/Dave,
>>
>> On 3/21/24 11:50, James Morse wrote:
>>> Resctrl occasionally wants to know something about a specific resource,
>>> in these cases it reaches into the arch code's rdt_resources_all[]
>>> array.
>>>
>>> Once the filesystem parts of resctrl are moved to /fs/, this means it
>>> will need visibility of the architecture specific struct
>>> resctrl_hw_resource definition, and the array of all resources.
>>> All architectures would also need a r_resctrl member in this struct.
>>>
>>> Instead, abstract this via a helper to allow architectures to do
>>> different things here. Move the level enum to the resctrl header and
>>> add a helper to retrieve the struct rdt_resource by 'rid'.
>>>
>>> resctrl_arch_get_resource() should not return NULL for any value in
>>> the enum, it may instead return a dummy resource that is
>>> !alloc_enabled && !mon_enabled.
>>
>> Nit.
>> You may want to drop the second half of the statement. We don't have a
>> dummy resource.
> 
> I guess not, but MPAM will, although I haven't fully understood the
> logic.  See:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/morse/linux.git/tree/drivers/platform/mpam/mpam_resctrl.c?h=mpam/snapshot/v6.7-rc2
> 
> (Search for "dummy".)
> 
> 
> In any case, the statement above is part of the definition of the new
> interface: the resctrl core code is going to explicitly need to cope
> with a dummy resource being returned, and the arch code is required
> to return a pointer to something and not NULL.
> 
> So I would say that it is appropriate (or, at the very least, harmless)
> to keep that statement here?

Ok. fine.
> 
>>
>>>
>>> Signed-off-by: James Morse <james.morse@....com>
>>> ---
>>>  arch/x86/kernel/cpu/resctrl/core.c        | 10 +++++++++-
>>>  arch/x86/kernel/cpu/resctrl/ctrlmondata.c |  2 +-
>>>  arch/x86/kernel/cpu/resctrl/internal.h    | 10 ----------
>>>  arch/x86/kernel/cpu/resctrl/monitor.c     |  8 ++++----
>>>  arch/x86/kernel/cpu/resctrl/rdtgroup.c    | 15 +++++++--------
>>>  include/linux/resctrl.h                   | 17 +++++++++++++++++
>>>  6 files changed, 38 insertions(+), 24 deletions(-)
>>>
> 
> [...]
> 
>>> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>> index 1767c1affa60..45372b6a6215 100644
>>> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> 
> [...]
> 
>>> @@ -2625,10 +2625,10 @@ static void schemata_list_destroy(void)
>>>  
>>>  static int rdt_get_tree(struct fs_context *fc)
>>>  {
>>> +	struct rdt_resource *l3 = resctrl_arch_get_resource(RDT_RESOURCE_L3);
>>
>> Its is probably best to keep the resource name as r here to be consistent
>> with other changes.
>>
>>>  	struct rdt_fs_context *ctx = rdt_fc2context(fc);
>>>  	unsigned long flags = RFTYPE_CTRL_BASE;
>>>  	struct rdt_domain *dom;
>>> -	struct rdt_resource *r;
>>>  	int ret;
>>>  
>>>  	cpus_read_lock();
>>> @@ -2701,8 +2701,7 @@ static int rdt_get_tree(struct fs_context *fc)
>>>  		resctrl_mounted = true;
>>>  
>>>  	if (is_mbm_enabled()) {
>>> -		r = &rdt_resources_all[RDT_RESOURCE_L3].r_resctrl;
>>> -		list_for_each_entry(dom, &r->domains, list)
>>> +		list_for_each_entry(dom, &l3->domains, list)
>>>  			mbm_setup_overflow_handler(dom, MBM_OVERFLOW_INTERVAL,
>>>  						   RESCTRL_PICK_ANY_CPU);
>>>  	}
>>> @@ -3878,7 +3877,7 @@ static int rdtgroup_show_options(struct seq_file *seq, struct kernfs_root *kf)
> 
> [...]
> 
>> Thanks
>> Babu Moger
> 
> [...]
> 
> Yes, this does look a bit odd.
> 
> This looks like a no-op change to me -- I think that
> resctrl_arch_get_resource() is supposed to be without side-effects,
> so I would have expected this to be a one-line change at the assignment
> to r, with no particular need for renaming r.
> 
> Does that make sense to you, or is there some complexity I'm not
> noticing here?

No other complexity..  Just keep the variable name as r.

struct rdt_resource *r = resctrl_arch_get_resource(RDT_RESOURCE_L3);

-- 
Thanks
Babu Moger