linux-kernel - [PATCH v18 06/17] x86/resctrl: Introduce snc_nodes_per_l3

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Date: Wed, 15 May 2024 15:23:14 -0700
From: Tony Luck <tony.luck@...el.com>
To: Fenghua Yu <fenghua.yu@...el.com>,
	Reinette Chatre <reinette.chatre@...el.com>,
	Maciej Wieczor-Retman <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,
	linux-kernel@...r.kernel.org,
	patches@...ts.linux.dev,
	Tony Luck <tony.luck@...el.com>
Subject: [PATCH v18 06/17] x86/resctrl: Introduce snc_nodes_per_l3_cache

Intel Sub-NUMA Cluster (SNC) is a feature that subdivides the CPU cores
and memory controllers on a socket into two or more groups. These are
presented to the operating system as NUMA nodes.

This may enable some workloads to have slightly lower latency to memory
as the memory controller(s) in an SNC node are electrically closer to the
CPU cores on that SNC node. This cost may be offset by lower bandwidth
since the memory accesses for each core can only be interleaved between
the memory controllers on the same SNC node.

Resctrl monitoring on an Intel system depends upon attaching RMIDs to tasks
to track L3 cache occupancy and memory bandwidth. There is an MSR that
controls how the RMIDs are shared between SNC nodes.

The default mode divides them numerically. E.g. when there are two SNC
nodes on a socket the lower number half of the RMIDs are given to the
first node, the remainder to the second node. This would be difficult
to use with the Linux resctrl interface as specific RMID values assigned
to resctrl groups are not visible to users.

The other mode divides the RMIDs and renumbers the ones on the second
SNC node to start from zero.

Even with this renumbering SNC mode requires several changes in resctrl
behavior for correct operation.

Add a global integer "snc_nodes_per_l3_cache" that shows how many
SNC nodes share each L3 cache. When "snc_nodes_per_l3_cache" is "1",
SNC mode is either not implemented, or not enabled.

Update all places to take appropriate action when SNC mode is enabled:
1) The number of logical RMIDs per L3 cache available for use is the
   number of physical RMIDs divided by the number of SNC nodes.
2) Likewise the "mon_scale" value must be divided by the number of SNC
   nodes.
3) Disable the "-o mba_MBps" mount option in SNC mode
   because the monitoring is being done per SNC node, while the
   bandwidth allocation is still done at the L3 cache scope.
   Trying to use this feedback loop might result in contradictory
   changes to the throttling level coming from each of the SNC
   node bandwidth measurements.

Signed-off-by: Tony Luck <tony.luck@...el.com>
---
 arch/x86/kernel/cpu/resctrl/internal.h | 2 ++
 arch/x86/kernel/cpu/resctrl/core.c     | 6 ++++++
 arch/x86/kernel/cpu/resctrl/monitor.c  | 4 ++--
 arch/x86/kernel/cpu/resctrl/rdtgroup.c | 3 ++-
 4 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/cpu/resctrl/internal.h b/arch/x86/kernel/cpu/resctrl/internal.h
index 135190e0711c..49440f194253 100644
--- a/arch/x86/kernel/cpu/resctrl/internal.h
+++ b/arch/x86/kernel/cpu/resctrl/internal.h
@@ -484,6 +484,8 @@ extern struct rdt_hw_resource rdt_resources_all[];
 extern struct rdtgroup rdtgroup_default;
 extern struct dentry *debugfs_resctrl;
 
+extern unsigned int snc_nodes_per_l3_cache;
+
 enum resctrl_res_level {
 	RDT_RESOURCE_L3,
 	RDT_RESOURCE_L2,
diff --git a/arch/x86/kernel/cpu/resctrl/core.c b/arch/x86/kernel/cpu/resctrl/core.c
index 395bac851f6e..bfa9d3a429fd 100644
--- a/arch/x86/kernel/cpu/resctrl/core.c
+++ b/arch/x86/kernel/cpu/resctrl/core.c
@@ -331,6 +331,12 @@ static u32 delay_bw_map(unsigned long bw, struct rdt_resource *r)
 	return r->default_ctrl;
 }
 
+/*
+ * Number of SNC nodes that share each L3 cache.  Default is 1 for
+ * systems that do not support SNC, or have SNC disabled.
+ */
+unsigned int snc_nodes_per_l3_cache = 1;
+
 static void mba_wrmsr_intel(struct msr_param *m)
 {
 	struct rdt_hw_ctrl_domain *hw_dom = resctrl_to_arch_ctrl_dom(m->dom);
diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
index 89d7e6fcbaa1..0f66825a1ac9 100644
--- a/arch/x86/kernel/cpu/resctrl/monitor.c
+++ b/arch/x86/kernel/cpu/resctrl/monitor.c
@@ -1022,8 +1022,8 @@ int __init rdt_get_mon_l3_config(struct rdt_resource *r)
 	int ret;
 
 	resctrl_rmid_realloc_limit = boot_cpu_data.x86_cache_size * 1024;
-	hw_res->mon_scale = boot_cpu_data.x86_cache_occ_scale;
-	r->num_rmid = boot_cpu_data.x86_cache_max_rmid + 1;
+	hw_res->mon_scale = boot_cpu_data.x86_cache_occ_scale / snc_nodes_per_l3_cache;
+	r->num_rmid = (boot_cpu_data.x86_cache_max_rmid + 1) / snc_nodes_per_l3_cache;
 	hw_res->mbm_width = MBM_CNTR_WIDTH_BASE;
 
 	if (mbm_offset > 0 && mbm_offset <= MBM_CNTR_WIDTH_OFFSET_MAX)
diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
index cc31ede1a1e7..0923492a8bd0 100644
--- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
+++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
@@ -2346,7 +2346,8 @@ static bool supports_mba_mbps(void)
 	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() &&
+		snc_nodes_per_l3_cache == 1);
 }
 
 /*
-- 
2.44.0