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

[Reinette Chatre]

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@xxxxxxxxx>
> > > ---
> > >    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