On 9/1/2026 5:32 AM, Michael Kelley wrote:
From: Naman Jain <[email protected]> Sent: Sunday, August 9, 2026
11:22 PM
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
This is not quite accurate. The condition is ncpus/2 < ngroups < ncpus.
If ngroups is <= ncpus/2, then all groups have at least 2 CPUs. If ngroups
is <= ncpus/3, then all groups have at least 3 CPUs, and so on. Rotating which
groups get the "extra" CPUs can be somewhat helpful even when all groups
have at least 2 CPUs. But as the minimum number of CPUs per group
increases, avoiding excessive stacking depends on how well the irqchip driver
does spreading when picking the effective affinity CPU from the multiple CPUs
assigned to the group (per our discussion elsewhere in this thread).
You're right. I should simplify this to something like this “When the
allocation leaves some groups with only a single CPU, those IRQs are
forced onto the same CPUs and are stacked up.”
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:
And FWIW, this example meets the stricter condition that I describe above ....
True.
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 &&
Explicitly testing extra_grps for zero isn't necessary. If it is
zero, the "less than" test below will always be false anyway.
That is true, but the intention was to improve readability of this
block. Unless you really feel that this should be removed, I would
prefer to retain this.
+ (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.
s/assigned/assigned to/
+ *
+ * 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
Having to recompute "cap" three times in the code below is fairly clumsy.
Is there any reason that cluster_groups[].ncpus and .ngroups need to be a
union? If they were separate fields, then "cap" would be immediately
available when you need it. I looked back through the commit history
and LKML discussion for when the union was originally added, and I
didn't find any explanation for why it is a union. A union saves a bit
of memory, but the overall amount of memory here is small, so the
implementation doesn't need to be particularly stingy.
I'll remove the union, and simplify this "cap" re-computation.
+ * 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;
+ }
We had a separate discussion about how your new code here should
go with alloc_groups_to_nodes() so that it is also applied at the NUMA
node level. You said you had it working. Question: Did you combine the
above "for" loop with the "for" loop in alloc_groups_to_nodes()? It
seems unnecessarily complex to do group allocations, including extras,
using the "for" loop in alloc_groups_to_nodes(), and then follow that
with another "for" loop here to figure out which nodes got extras and
strip them out. A single loop should be able to set all the nodes to
their base value and count the extras.
I just did a prototype and had not combined it yet. I'll try to combine
them into a single function now.
+
+ /*
+ * 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 */
Just so I'm clear, you could have "remaining extras" because a stride > 1
in the previous loop could miss some clusters/nodes that have available
space, depending on how the arithmetic works out. The code below
does essentially the same thing, but with a stride of 1 so that every
node is checked. And you might have to make multiple passes in
case some nodes have space for two or more extras. But you know
there's enough space available somewhere.
Is my understanding correct?
Yes, this is correct.
+ 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;
Is the control variable "placed" necessary? If all the extra groups are
guaranteed to fit somewhere, then total_extra should go to zero and
the checks on total_extra > 0 will break out of the loops. Thinking about
it from the other direction, if the "for" loop ever completed without
incrementing "placed", wouldn't that mean total_extra is still
non-zero, and you have an extra group that isn't assigned to a
cluster/node?
Michael
It's not required. I should remove it.
Regards,
Naman
