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

Reinette Chatre
2024-06-21T01:57:03Z

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@intel.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