linux-kernel - Re: [PATCH v20 07/18] x86/resctrl: Block use of mba_MBps mount option on Sub-NUMA Cluster (SNC) systems

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Date: Thu, 20 Jun 2024 18:56:56 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: "Luck, Tony" <tony.luck@...el.com>, "Yu, Fenghua" <fenghua.yu@...el.com>,
	"Wieczor-Retman, Maciej" <maciej.wieczor-retman@...el.com>, Peter Newman
	<peternewman@...gle.com>, James Morse <james.morse@....com>, Babu Moger
	<babu.moger@....com>, Drew Fustini <dfustini@...libre.com>, Dave Martin
	<Dave.Martin@....com>
CC: "x86@...nel.org" <x86@...nel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "patches@...ts.linux.dev"
	<patches@...ts.linux.dev>
Subject: Re: [PATCH v20 07/18] x86/resctrl: Block use of mba_MBps mount option
 on Sub-NUMA Cluster (SNC) systems

Hi Tony,

On 6/20/24 3:07 PM, Luck, Tony wrote:
>>> When SNC is enabled there is a mismatch between the MBA control function
>>> which operates at L3 cache scope and the MBM monitor functions which
>>> measure memory bandwidth on each SNC node.
>>>
>>> Block use of the mba_MBps when scopes for MBA/MBM do not match.
>>>
>>> Signed-off-by: Tony Luck <tony.luck@...el.com>
>>> ---
>>>    arch/x86/kernel/cpu/resctrl/rdtgroup.c | 4 +++-
>>>    1 file changed, 3 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>> index eb3bbfa96d5a..a0a43dbe011b 100644
>>> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>> @@ -2339,10 +2339,12 @@ static void mba_sc_domain_destroy(struct rdt_resource *r,
>>>     */
>>>    static bool supports_mba_mbps(void)
>>>    {
>>> +   struct rdt_resource *rmbm = &rdt_resources_all[RDT_RESOURCE_L3].r_resctrl;
>>>      struct rdt_resource *r = &rdt_resources_all[RDT_RESOURCE_MBA].r_resctrl;
>>>
>>>      return (is_mbm_local_enabled() &&
>>> -           r->alloc_capable && is_mba_linear());
>>> +           r->alloc_capable && is_mba_linear() &&
>>> +           r->ctrl_scope == rmbm->mon_scope);
>>>    }
>>>
>>>    /*
>>
>> The function comments of supports_mba_mbps() needs an update to accompany
>> this new requirement.
> 
> Will add comment on extra requirement.
> 
>> I also think that the "mba_MBps" mount option is now complicated enough to
>> warrant a clear error to user space when using it fails. invalfc() is
>> available for this and enables user space to get detailed log message
>> from a read() on an fd created by fsopen().
>>
>> Perhaps something like (please check line length and feel free to improve
>> since as is it may quite cryptic):
>>        rdt_parse_param(...)
>>        {
>>
>>
>>        ...
>>        case Opt_mba_mbps:
>>                if (!supports_mba_mbps())
>>                        return invalfc(fc, "mba_MBps requires both MBM and (linear scale) MBA at L3 scope");
>>        ...
>>        }
> 
> Line length is indeed a problem (108 characters). Usual line split methods barely help as the moving the
> string to the next line and aligning with the "(" only saves 4 characters.
> 
> How about this (suggestions for a shorter variable name - line is 97 characters)
> 
> static char mba_mbps_invalid[] = "mba_MBps requires both MBM and (linear scale) MBA at L3 scope";
> 
> rdt_parse_param(...)
> {
> 	...
> 	case Opt_mba_mbps:
> 		if (!supports_mba_mbps())
> 			return invalfc(fc, mba_mbps_invalid);
> 	...
> }

On 6/20/24 3:12 PM, Luck, Tony wrote:
>> static char mba_mbps_invalid[] = "mba_MBps requires both MBM and (linear scale) MBA at L3 scope";
> 
> checkpatch recommends "static const char ..." pushing this over 100 chars :-(
> 

How about something like below that reaches 96:

	case Opt_mba_mbps:
		if (!supports_mba_mbps())
			return invalfc(fc,
				       "mba_MBps requires both MBM and linear MBA at L3 scope");


Reinette