Re: [PATCH v2] nvme-tcp: pin io_cpu to submitter cpu

On 23/08/2026 20:43, Saravanan D wrote:
> On Sun, 23 Aug 2026 00:20:21 +0300 Sagi Grimberg <[email protected]> wrote:
>> Can you share a concrete example?
>> 1. lscpu
>> 2. grep -r ''  /sys/block/nvmeXnY/mq/*/cpu_list
>> Also, what would be the VMs cpusets on such a server?
> Very grateful for your review and suggestions.
>
> Concrete example from one of these hosts, where there are more cpus than
> the io queues NVMe-oF targets offer.
>
> lscpu (trimmed): AMD EPYC 9654, 2 sockets, 2 threads/core, 384 CPUs
>    NUMA node0 CPUs: 0-95,192-287
>    NUMA node1 CPUs: 96-191,288-383
>
> Each nvme-tcp controller exposes 128 io queues (queue_count 129) against
> 384 CPUs, so blk_mq_map_queues() folds three CPUs into every hctx. cpu_list for the namespace's path device, around one tenant's boundary:
>
>    hctx67:  6, 7, 198
>    hctx68:  8, 9, 200
>    hctx69:  10, 11, 202
>    hctx75:  22, 23, 214
>    hctx76:  24, 25, 216
>    hctx113: 199, 201, 203
>    hctx114: 205, 207, 209
>    hctx115: 211, 213, 215
>
> Two VMs are live on this same shared controller:
>    VM A cpuset: 8-23,200-215
>    VM B cpuset: 104-191,296-383
>
> VM A owns 200-215, so hctx113's group {199,201,203} has 201 and 203 inside
> VM A and 199 outside it. The connect time pick can select 199 as that
> queue's io_cpu, so VM A's I/O through hctx113 runs its socket work on 199,
> outside its cpuset for its entire lifetime. Adoption will pull it back
> to 201 or 203 when VM A submits I/O.
>
>> In an ideal setting, the nvme controller would support $NR_CPUS worth of
>> io queues [...] map cpu-queue with perfect alignment.
> Right, and that is the case we cannot reach here. The controller tops out
> at 128 io queues while the host has 384 CPUs, so the three to one folding
> is unavoidable and some groups end up straddling a tenant boundary.

Yes, most if not all nvme-of subsystems won't let the host open so many 
queues.

>
>> I think that for controllers with low queue count this approach would not
>> work very well. Plus the cpu<->queue mapping by the block layer is not
>> guaranteed to align with how workloads/VMs are scheduled on the host...
> Agreed on both. A queue whose cpu group straddles two busy tenants is only
> partially addressed, since adoption then selects which tenant hosts that
> shared queue rather than removing the sharing. It fixes the common case
> where cpu group sits within one VM's cpuset and the case where only one
> tenant on the queue is active, and it is strictly better than the connect
> time pick, which chooses that queue's io_cpu before any I/O and can land on
> any member. Solving the common case also lets the control plane, which owns
> the VM cpusets, affinitize each VM's virtio-blk submitter threads away from
> CPUs whose mapped queue would land socket work on a neighbor.

I just think that the adoption approach has downsides in many other 
workloads.
I wouldn't categorize your use-case as the common use-case. Not to say 
that it
shouldn't be solved.

>
>> To me the problem statement sounds to me like you are looking to get a
>> "private" controller (set of IO queues) for each VM. [...]
>> nvme connect -t tcp -a <traddr> --cpu-groups=0-63,64-127,128-191,192-255
> A private controller or a cpu-groups partition fits workloads where the CPU
> partition is known and stable at connect time. Our production constraint
> does not afford us that luxury. The nvme-tcp controllers are host wide and
> shared, the tenant VMs are namespaces under one multipath subsystem, and the
> control plane creates, destroys and bin packs VMs continuously on these core
> dense compute hosts. A cpu-groups mask fixed at connect would be stale as
> soon as the tenant mix changes, and re-partitioning would mean reconnecting
> a controller that many live tenants depend on. We also cannot give each
> tenant its own controller, because the overhead on the host adds up and we
> run the risk of exceeding the target's connection limits. Adopting the
> submitting CPU needs no per tenant configuration and follows the tenant
> churn on its own, which is why it fits a long lived shared connection whose
> tenant set is not known at connect time.
>
> Next the fleet operators who care about VM steal time already steer NIC IRQs,
> NAPI to a dedicated housekeeping CPU pool outside the tenant cpusets, so the
> common network processing overhead Nilay raised does not run on tenant CPUs.
> With that in place the remaining contributor that still lands on a
> tenant's CPU is nvme_tcp_io_work, which this patch pins to the submitting
> cpu and keeps within the tenant's cpuset.

I agree with you that queue io_cpu selection at connection time is too 
limiting.
Perhaps we can have each nvme-tcp queue expose a sysfs entry to set the 
io_cpu?

This way, its up to a control-plane to mangle with the queue io_cpu 
setting at
any point it would like.

Something like:
echo $CPU > /sys/class/nvme/nvme0/tcp_queues/$QUEUE/io_cpu

I am just trying to think of a way that will allow some control plane to 
set it exactly
as it wants, without the guessing part...