[PATCH v2] lib/group_cpus: rotate extra groups to avoid IRQ stacking

[PATCH v2] lib/group_cpus: rotate extra groups to avoid IRQ stacking

[Naman Jain]

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 <longli@microsoft.com>
Signed-off-by: Long Li <longli@microsoft.com>
Signed-off-by: Naman Jain <namjain@linux.microsoft.com>
---
Changes since v1
(https://lore.kernel.org/all/20260324075352.2326972-1-namjain@linux.microsoft.com/):
- 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

Re: [PATCH v2] lib/group_cpus: rotate extra groups to avoid IRQ stacking

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] Integer overflow in intra-cluster group rotation breaks CPU allocation permutation, causing empty CPU masks.
--

--- Patch [1]: [PATCH v2] lib/group_cpus: rotate extra groups to avoid IRQ stacking ---
commit 3ea99a586f68e414d616ef9d2a0f13f390a05e7d
Author: Naman Jain <namjain@linux.microsoft.com>

lib/group_cpus: rotate extra groups to avoid IRQ stacking

group_cpus_evenly() computes how a device's queue interrupts are spread
across CPUs. This commit introduces a per-caller rotation counter to
shift which groups get extra CPUs. The rotation ensures that single-CPU
groups from multiple identical devices do not stack on the same CPUs,
improving throughput and reducing tail latency.

> 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
[ ... ]
> @@ -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++;

[Severity: Medium]
Can this addition overflow and break the CPU allocation?

The commit message and comments state that wrapping of the unsigned
counter is harmless. However, if spread_offset is very close to UINT_MAX,
v + spread_offset can wrap around mid-loop.

Since 2^32 is generally not a multiple of nv->ngroups, the overflow causes
the modulo result to skip some values and duplicate others.

This means the condition (v + spread_offset) % nv->ngroups < extra_grps
might evaluate to true more times than there are extra groups.

If cpus_per_grp is incremented too many times, the loop will consume more
CPUs than available, prematurely exhausting nmsk and leaving subsequent
masks empty.

Would it be safer to apply the modulo before the addition, such as:
(v + (spread_offset % nv->ngroups)) % nv->ngroups ?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260810062144.2108758-1-namjain@linux.microsoft.com?part=1

Re: [PATCH v2] lib/group_cpus: rotate extra groups to avoid IRQ stacking

[Naman Jain]

On 8/10/2026 11:51 AM, Naman Jain wrote:
> 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 <longli@microsoft.com>
> Signed-off-by: Long Li <longli@microsoft.com>
> Signed-off-by: Naman Jain <namjain@linux.microsoft.com>

Sashiko pointed to a minor issue in this patch, which can be addressed 
in the next version. It was not seen when I ran Sashiko locally.
I would also want to add CC: stable tag and stable list in the next version.

But I will wait for any reviews on this patch before sending the next 
version.

Regards,
Naman

Link: 
https://sashiko.dev/#/patchset/20260810062144.2108758-1-namjain%40linux.microsoft.com