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: Fri, 21 Jun 2024 08:24:57 -0700
From: Tony Luck <tony.luck@...el.com>
To: Reinette Chatre <reinette.chatre@...el.com>
Cc: "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>,
	"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

On Thu, Jun 20, 2024 at 06:56:56PM -0700, Reinette Chatre wrote:
> 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,

Alternative option. Move the messaging into supports_mba_mbps() and
split into shorter pieces for each reason. The other callers of
supports_mba_mbps() that are just re-checking status would pass
a NULL argument.

If this looks reasonable I can do it in two patches. First to add
invalfc() for the existing cases. Second to add the SNC change.

-Tony

---

static bool supports_mba_mbps(struct fs_context *fc)
{
			return invalfc(fc, mba_mbps_invalid);
	struct rdt_resource *rmbm = &rdt_resources_all[RDT_RESOURCE_L3].r_resctrl;
	struct rdt_resource *r = &rdt_resources_all[RDT_RESOURCE_MBA].r_resctrl;

	if (!is_mbm_local_enabled()) {
		if (fc)
			invalfc(fc, "mba_MBps requires local MBM");
		return false;
	}
	if (!r->alloc_capable) {
		if (fc)
			invalfc(fc, "mba_MBps requires MBA");
		return false;
	}
	if (!is_mba_linear()) {
		if (fc)
			invalfc(fc, "mba_MBps requires linear MBA");
		return false;
	}
	if (r->ctrl_scope != rmbm->mon_scope) {
		if (fc)
			invalfc(fc, "mba_MBps requires MBM/MBA at L3 scope");
		return false;
	}

	return true;
}

rdt_parse_param(...)
{
	...
	case Opt_mba_mbps:
		if (!supports_mba_mbps(fc))
			return -EINVAL;
	...
}