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

From: Reinette Chatre
Date: Thu Jun 20 2024 - 17:21:43 EST


Hi Tony,

On 6/10/24 11:35 AM, Tony Luck 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.

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");
...
}


Reinette