group_cpus_evenly() computes how a device's queue interrupts are spread
across CPUs. It backs managed-interrupt affinity (kernel/irq/affinity.c)
and block-multiqueue mappings (block/blk-mq-cpumap.c), and is invoked
independently by every device that uses them - NVMe, NICs, storage HBAs,
and virtio devices. Its output is deterministic, i.e. for a given
topology, two similar devices produce an identical group-to-CPU mapping.
When ngroups < ncpus, some groups end up with only a single CPU. An
interrupt whose mask has one CPU can only run there, making that CPU a
"hot" handler. Because the mapping is deterministic, identical devices
compute the same layout and stack all their single-CPU IRQs onto the
very same CPUs, leaving the rest of the system idle.
This is easy to hit in practice. On an Azure L96as_v4 VM (96 vCPUs, 2
NUMA nodes of 48 CPUs, 6 NVMe disks with 62 I/O queues each),
group_cpus_evenly() splits each disk's 62 queues into 31 per node over
48 CPUs. 48 does not divide evenly by 31:
per NUMA node: 48 CPUs / 31 queues
17 groups get 2 CPUs (cover 34 CPUs)
14 groups get 1 CPU (cover 14 CPUs) <- single-CPU "hot" queues
That is 14 hot queues per node, 28 per disk. All 6 disks land them on
the same 28 CPUs, so 168 hot interrupts pile onto 28 of 96 CPUs while
two-thirds of the system handles none:
Before (per-CPU, disks whose IRQs it services):
CPU 0: 3 disks ... CPU 34: 6 disks (all six)
CPU 1: 3 disks ... CPU 47: 6 disks (all six)
Summary: 28 CPUs (34-47, 82-95) served all 6 disks and the other 68
served only 3. Those 28 CPUs cap throughput and inflate tail
latency while most of the system is idle.
Fix this by introducing a per-caller rotation via a static atomic
counter (group_spread_cnt). Each call to group_cpus_evenly() takes a
unique spread_offset, applied to the two decisions that were previously
deterministic:
1) Cluster-level rotation in __try_group_cluster_cpus(): after
alloc_groups_to_nodes() distributes groups proportionally across
clusters, integer rounding leaves some clusters with one extra
group. The extras are redistributed starting from a rotated
position, with a stride of ncluster/total_extra to minimize overlap
between consecutive callers. A multi-pass fallback ensures all
extras are placed even when some clusters are at capacity.
2) Intra-cluster rotation in assign_cpus_to_groups(): the sequential
extra assignment is replaced with a modular expression,
(v + spread_offset) % nv->ngroups < extra_grps
rotating which groups within a cluster receive the extra CPU.
Nothing else about the layout changes - same queue count, same NUMA
weighting, same full CPU coverage and locality. Each caller simply
starts its mapping from a different point, and each individual call
still produces a valid, fair distribution. Across callers, different
CPUs absorb the single-CPU group IRQ load:
After (same setup, with the rotation):
CPU 0: 4 disks CPU 2: 4 disks CPU 47: 4 disks
CPU 1: 4 disks CPU 3: 4 disks ...
Summary: no CPU serves more than 4 disks, and all 96 CPUs are used.
The total interrupt work is unchanged - every CPU still handles one
queue per disk; only the placement of the single-CPU hot queues moves.
This benefits every managed-IRQ, blk-mq, and virtio-vdpa / virtio-fs
device with no driver changes.
Because the offset comes from a global counter advanced once per call,
the mapping now depends on call (device probe) order. A given device's
exact layout can differ from one boot to the next, and a later recompute
(e.g. a blk-mq remap) may pick a different layout. Every such layout is
still valid, fair, and proportional - only the choice among equally good
mappings varies.
On a 96-vCPU Hyper-V VM running 4K random-read fio across 6 NVMe disks,
worst-disk degradation versus average dropped from 11% to 5%, and the
previously penalized disks gained 12% IOPS at 10% lower latency.
Fixes: 89802ca36c96 ("lib/group_cpus: make group CPU cluster aware")
Co-developed-by: Long Li <[email protected]>
Signed-off-by: Long Li <[email protected]>
Signed-off-by: Naman Jain <[email protected]>
---
Changes since v1
(https://lore.kernel.org/all/[email protected]/):
- Cluster base is now a per-cluster proportional floor
(ngroups * cap / ncpus) instead of the global per-cluster minimum,
so proportional weighting is preserved on asymmetric (e.g.
big.LITTLE) cluster topologies. (Sashiko review)
- Document that the rotation offset is call/probe-order dependent: a
device's exact layout may vary across boots and recomputes (each
layout is still valid, fair, and proportional).
- Rewrite the commit message with a worked example and fio numbers.
lib/group_cpus.c | 149 +++++++++++++++++++++++++++++++++++++++++++----
1 file changed, 137 insertions(+), 12 deletions(-)
diff --git a/lib/group_cpus.c b/lib/group_cpus.c
index e6e18d7a49bba..8bed0f9d2110b 100644
--- a/lib/group_cpus.c
+++ b/lib/group_cpus.c
@@ -7,6 +7,7 @@
#include <linux/slab.h>
#include <linux/cpu.h>
#include <linux/sort.h>
+#include <linux/atomic.h>
#include <linux/group_cpus.h>
#ifdef CONFIG_SMP
@@ -255,12 +256,20 @@ static void alloc_nodes_groups(unsigned int numgrps,
alloc_groups_to_nodes(numgrps, numcpus, node_groups, nr_node_ids);
}
+/*
+ * Per-caller rotation counter for group_cpus_evenly().
+ * Wrapping is harmless: the offset is only used modulo small values
+ * (ncluster or nv->ngroups), so any unsigned value works.
+ */
+static atomic_t group_spread_cnt = ATOMIC_INIT(0);
+
static void assign_cpus_to_groups(unsigned int ncpus,
struct cpumask *nmsk,
struct node_groups *nv,
struct cpumask *masks,
unsigned int *curgrp,
- unsigned int last_grp)
+ unsigned int last_grp,
+ unsigned int spread_offset)
{
unsigned int v, cpus_per_grp, extra_grps;
/* Account for rounding errors */
@@ -270,11 +279,15 @@ static void assign_cpus_to_groups(unsigned int ncpus,
for (v = 0; v < nv->ngroups; v++, *curgrp += 1) {
cpus_per_grp = ncpus / nv->ngroups;
- /* Account for extra groups to compensate rounding errors */
- if (extra_grps) {
+ /*
+ * Rotate which groups get the extra CPU so that
+ * successive callers produce different mappings,
+ * avoiding IRQ stacking when multiple devices
+ * share the same CPU topology.
+ */
+ if (extra_grps &&
+ (v + spread_offset) % nv->ngroups < extra_grps)
cpus_per_grp++;
- --extra_grps;
- }
/*
* wrapping has to be considered given 'startgrp'
@@ -361,7 +374,8 @@ static bool __try_group_cluster_cpus(unsigned int ncpus,
struct cpumask *node_cpumask,
struct cpumask *masks,
unsigned int *curgrp,
- unsigned int last_grp)
+ unsigned int last_grp,
+ unsigned int spread_offset)
{
struct node_groups *cluster_groups;
const struct cpumask **clusters;
@@ -379,6 +393,111 @@ static bool __try_group_cluster_cpus(unsigned int ncpus,
if (ncluster == 0)
goto fail_no_clusters;
+ /*
+ * Rotate which clusters receive extra groups so that different
+ * callers of group_cpus_evenly() produce different group-to-CPU
+ * mappings. Without this, all devices get identical affinity
+ * masks, causing IRQ stacking on CPUs assigned single-CPU groups.
+ *
+ * alloc_groups_to_nodes() distributes ngroups proportionally, but
+ * integer rounding causes some clusters to get one more group
+ * than others. The assignment is deterministic, so every device
+ * gets the same mapping. Fix: compute a proportional floor for
+ * each cluster (ngroups * cap / ncpus), collect only the
+ * rounding-induced extras, then redistribute them starting from
+ * a rotated position. This preserves the proportional weighting
+ * across differently-sized clusters while rotating the rounding
+ * extras, keeping the rotation effective on both symmetric and
+ * asymmetric cluster topologies.
+ *
+ * Note: after alloc_groups_to_nodes(), cluster_groups[].ngroups
+ * holds the group count (the union no longer holds per-cluster CPU
+ * counts), so each cluster's CPU capacity (cap) is taken from its
+ * mask. The ncpus divisor is the function parameter, which equals
+ * the sum of the per-cluster caps.
+ */
+ if (ncluster > 1) {
+ unsigned int total_extra = 0;
+ unsigned int start, stride;
+
+ /*
+ * Compute a per-cluster proportional floor and collect
+ * only the rounding-induced extras for redistribution.
+ *
+ * Each cluster's floor is ngroups * cap / ncpus, which
+ * preserves its proportional share. Only the rounding
+ * remainders (typically one per cluster) are collected
+ * for rotated redistribution, keeping the rotation
+ * effective even on asymmetric topologies (e.g.
+ * big.LITTLE) where differently-sized clusters would
+ * otherwise absorb all extras deterministically.
+ */
+ for (i = 0; i < ncluster; i++) {
+ unsigned int cap, prop_floor, base;
+
+ cap = cpumask_weight_and(clusters[cluster_groups[i].id],
+ node_cpumask);
+ prop_floor = ngroups * cap / ncpus;
+
+ /*
+ * Use proportional floor as base. Ensure at
+ * least 1 group per cluster, and never exceed
+ * alloc_groups_to_nodes()'s original allocation
+ * (which may be less than prop_floor when small
+ * clusters consumed groups via max(1,...)).
+ */
+ base = prop_floor > 0 ? prop_floor : 1;
+ if (base > cluster_groups[i].ngroups)
+ base = cluster_groups[i].ngroups;
+
+ total_extra += cluster_groups[i].ngroups - base;
+ cluster_groups[i].ngroups = base;
+ }
+
+ /*
+ * Redistribute rounding extras using a stride to scatter
+ * them across clusters. With stride = ncluster / extras,
+ * consecutive callers' extra sets overlap minimally
+ * (e.g. max 2 overlap for 6 callers with 24 clusters
+ * and 7 extras, vs 6 overlap with stride 1).
+ */
+ start = spread_offset % ncluster;
+ stride = (total_extra > 0 && total_extra < ncluster) ?
+ ncluster / total_extra : 1;
+
+ for (i = 0; i < ncluster && total_extra > 0; i++) {
+ unsigned int idx =
+ (start + i * stride) % ncluster;
+ unsigned int cap;
+
+ cap =
cpumask_weight_and(clusters[cluster_groups[idx].id],
+ node_cpumask);
+ if (cluster_groups[idx].ngroups < cap) {
+ cluster_groups[idx].ngroups++;
+ total_extra--;
+ }
+ }
+
+ /* Fallback: place remaining extras wherever they fit */
+ while (total_extra > 0) {
+ unsigned int placed = 0;
+
+ for (i = 0; i < ncluster && total_extra > 0; i++) {
+ unsigned int cap;
+
+ cap =
cpumask_weight_and(clusters[cluster_groups[i].id],
+ node_cpumask);
+ if (cluster_groups[i].ngroups < cap) {
+ cluster_groups[i].ngroups++;
+ total_extra--;
+ placed++;
+ }
+ }
+ if (!placed)
+ break;
+ }
+ }
+
for (i = 0; i < ncluster; i++) {
struct node_groups *nv = &cluster_groups[i];
@@ -389,7 +508,8 @@ static bool __try_group_cluster_cpus(unsigned int ncpus,
continue;
WARN_ON_ONCE(nv->ngroups > nc);
- assign_cpus_to_groups(nc, nmsk, nv, masks, curgrp, last_grp);
+ assign_cpus_to_groups(nc, nmsk, nv, masks, curgrp, last_grp,
+ spread_offset);
}
ret = true;
@@ -404,7 +524,8 @@ static bool __try_group_cluster_cpus(unsigned int ncpus,
static int __group_cpus_evenly(unsigned int startgrp, unsigned int numgrps,
cpumask_var_t *node_to_cpumask,
const struct cpumask *cpu_mask,
- struct cpumask *nmsk, struct cpumask *masks)
+ struct cpumask *nmsk, struct cpumask *masks,
+ unsigned int spread_offset)
{
unsigned int i, n, nodes, done = 0;
unsigned int last_grp = numgrps;
@@ -455,13 +576,14 @@ static int __group_cpus_evenly(unsigned int startgrp,
unsigned int numgrps,
WARN_ON_ONCE(nv->ngroups > ncpus);
if (__try_group_cluster_cpus(ncpus, nv->ngroups, nmsk,
- masks, &curgrp, last_grp)) {
+ masks, &curgrp, last_grp,
+ spread_offset)) {
done += nv->ngroups;
continue;
}
assign_cpus_to_groups(ncpus, nmsk, nv, masks, &curgrp,
- last_grp);
+ last_grp, spread_offset);
done += nv->ngroups;
}
kfree(node_groups);
@@ -488,6 +610,7 @@ static int __group_cpus_evenly(unsigned int startgrp,
unsigned int numgrps,
struct cpumask *group_cpus_evenly(unsigned int numgrps, unsigned int *nummasks)
{
unsigned int curgrp = 0, nr_present = 0, nr_others = 0;
+ unsigned int spread_offset;
cpumask_var_t *node_to_cpumask;
cpumask_var_t nmsk, npresmsk;
int ret = -ENOMEM;
@@ -510,6 +633,8 @@ struct cpumask *group_cpus_evenly(unsigned int numgrps,
unsigned int *nummasks)
if (!masks)
goto fail_node_to_cpumask;
+ spread_offset = (unsigned int)atomic_fetch_inc(&group_spread_cnt);
+
build_node_to_cpumask(node_to_cpumask);
/*
@@ -528,7 +653,7 @@ struct cpumask *group_cpus_evenly(unsigned int numgrps,
unsigned int *nummasks)
/* grouping present CPUs first */
ret = __group_cpus_evenly(curgrp, numgrps, node_to_cpumask,
- npresmsk, nmsk, masks);
+ npresmsk, nmsk, masks, spread_offset);
if (ret < 0)
goto fail_node_to_cpumask;
nr_present = ret;
@@ -545,7 +670,7 @@ struct cpumask *group_cpus_evenly(unsigned int numgrps,
unsigned int *nummasks)
curgrp = nr_present;
cpumask_andnot(npresmsk, cpu_possible_mask, npresmsk);
ret = __group_cpus_evenly(curgrp, numgrps, node_to_cpumask,
- npresmsk, nmsk, masks);
+ npresmsk, nmsk, masks, spread_offset);
if (ret >= 0)
nr_others = ret;
--
2.43.0
