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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Message-ID: <SJ1PR11MB608342C2230D448CF27EAC26FCC82@SJ1PR11MB6083.namprd11.prod.outlook.com>
Date: Thu, 20 Jun 2024 22:07:26 +0000
From: "Luck, Tony" <tony.luck@...el.com>
To: "Chatre, Reinette" <reinette.chatre@...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

> > 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);
	...
}

-Tony