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

Luck, Tony
2024-06-20T22:07:32Z

> > 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);
...
}

-Tony