From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from linux.microsoft.com (linux.microsoft.com [13.77.154.182]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 4D2B231E824; Wed, 2 Sep 2026 05:02:54 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=13.77.154.182 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788325376; cv=none; b=PkESLmQyLmkZEiuIP/p1V0frqMRV9dg/MR4RgkGlYv7K66Ecem+dEbQixwtYS3ERZWdOzhhu4Zya/A+Otc302DIQES2+jqQptpL+15eAcDScq23ejfgadxdY757Lw27fJxV2zHc4mhruIgJIaQs5nB5Uc4XykY+eIhAsVPbX0CA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788325376; c=relaxed/simple; bh=ZnwLyVk6Q7tVd0Acc2yLxF+iGTo+Lx7EJKXUsMixozo=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=eO57dOnFzS3YbG50X1v0+9QniUuad0cWBDb5qlIs6O9SRZfgEMoH1gX0Rw07YqtiEYUODqZqlO541vIdVI8TrhutwQWFFkgjgVL61uxW3sxqSiyiLqmvLV84IjAZULCQoVoyCyoghu6BFwNAAtJL5PKJCG7hTjbwX7I+E6X86a0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.microsoft.com; spf=pass smtp.mailfrom=linux.microsoft.com; dkim=pass (1024-bit key) header.d=linux.microsoft.com header.i=@linux.microsoft.com header.b=P/M3qmAJ; arc=none smtp.client-ip=13.77.154.182 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.microsoft.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.microsoft.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.microsoft.com header.i=@linux.microsoft.com header.b="P/M3qmAJ" Received: from [192.168.1.70] (unknown [4.194.122.144]) by linux.microsoft.com (Postfix) with ESMTPSA id DCEA520B7129; Tue, 1 Sep 2026 22:02:13 -0700 (PDT) DKIM-Filter: OpenDKIM Filter v2.11.0 linux.microsoft.com DCEA520B7129 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.microsoft.com; s=default; t=1788325337; bh=ENp71upqIOZykYWR/vy+lgBkrXVjharHWw75NHimAk0=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=P/M3qmAJ9nG2XdKKAD3+MUkmhjpAiy/wJbHsiMg8agH81rAHkhfV+Zs8/FLvWrZ2d wLF6vn7Ge2yx7/jVufgt7k8xbAwl9lgPT9dkOLEiwVV4giftoEkfov5g7ygzSJe2zI zRb5QOwoOZ33Vx1DSrBgqbk8bdbdVccOerDpxQD0= Message-ID: <19005077-4dde-4193-aafa-5b248a2131a6@linux.microsoft.com> Date: Wed, 2 Sep 2026 10:32:52 +0530 Precedence: bulk X-Mailing-List: linux-hyperv@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] lib/group_cpus: rotate extra groups to avoid IRQ stacking To: Michael Kelley , Andrew Morton , Thomas Gleixner , Ming Lei , Ming Lei Cc: Wangyang Guo , Tianyou Li , Tim Chen , Long Li , "linux-kernel@vger.kernel.org" , "linux-hyperv@vger.kernel.org" References: <20260810062144.2108758-1-namjain@linux.microsoft.com> Content-Language: en-US From: Naman Jain In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 9/1/2026 5:32 AM, Michael Kelley wrote: > From: Naman Jain 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 >> Signed-off-by: Long Li >> Signed-off-by: Naman Jain >> --- >> 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 >> #include >> #include >> +#include >> #include >> >> #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