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Message-ID: <20250910155110-mutt-send-email-mst@kernel.org> Date: Wed, 10 Sep 2025 16:10:56 -0400 From: "Michael S. Tsirkin" <mst@...hat.com> To: Jon Kohler <jon@...anix.com> Cc: "patchwork-bot+netdevbpf@...nel.org" <patchwork-bot+netdevbpf@...nel.org>, "jasowang@...hat.com" <jasowang@...hat.com>, "eperezma@...hat.com" <eperezma@...hat.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "virtualization@...ts.linux.dev" <virtualization@...ts.linux.dev>, "netdev@...r.kernel.org" <netdev@...r.kernel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org> Subject: Re: vhost_iotlb_miss tight loop lockup - RE vhost/net: Defer TX queue re-enable until after sendmsg On Wed, Sep 10, 2025 at 06:58:18PM +0000, Jon Kohler wrote: > > > > On May 5, 2025, at 9:40 PM, patchwork-bot+netdevbpf@...nel.org wrote: > > > > Hello: > > > > This patch was applied to netdev/net-next.git (main) > > by Jakub Kicinski <kuba@...nel.org>: > > Hey all, > Writing to fire up a flare and point out a problem that we’re seeing > with this patch internally, specifically when we enable iommu on the > virtio-net device. > > With this patch applied on 6.12.y-based bare metal instance and then > starting a 6.12.y based guest with iommu enabled, we see lockups > within the guest in short order, as well as vmm (qemu) stuck in a tight > loop responding to iommu misses from vhost net loop. > > We've bisected this in our internal tree, and for sure it is this > patch that is alledgedly causing the problem, so I wanted to point out > there is some sort of issue here. > > Working on trying to figure this out, but if jumps off the page to > anyone, happy to take advice! > > Flamegraph: > https://gist.github.com/JonKohler/0e83c014230ab59ddc950f10441335f1#file-iotlb-lockup-svg > > Guest dmesg errors like so: > [ 66.081694] virtio_net virtio0 eth0: NETDEV WATCHDOG: CPU: 1: transmit queue 0 timed out 5500 ms > [ 68.145155] virtio_net virtio0 eth0: TX timeout on queue: 0, sq: output.0, vq: 0x1, name: output.0, 7560000 usecs ago > [ 112.907012] virtio_net virtio0 eth0: NETDEV WATCHDOG: CPU: 1: transmit queue 0 timed out 5568 ms > [ 124.117540] virtio_net virtio0 eth0: TX timeout on queue: 0, sq: output.0, vq: 0x1, name: output.0, 16776000 usecs ago > [ 124.118050] virtio_net virtio0 eth0: NETDEV WATCHDOG: CPU: 1: transmit queue 0 timed out 16776 ms > [ 124.118447] virtio_net virtio0 eth0: TX timeout on queue: 0, sq: output.0, vq: 0x1, name: output.0, 16776000 usecs ago > > Host level top output > 3992758 qemu 20 0 16.6g 52168 26704 R 99.9 0.0 21:23.72 qemu-kvm <<< this is the qemu main thread > 3992769 qemu 20 0 16.6g 52168 26704 R 58.8 0.0 13:33.44 vhost-3992758 <<< this is the vhost-net kthread > > For qemu-kvm main thread: > Samples: 13K of event 'cycles:P', 4000 Hz, Event count (approx.): 5131922583 lost: 0/0 drop: 0/0 > Children Self Shared Object Symbol > - 87.41% 0.30% [kernel] [k] entry_SYSCALL_64_after_hwframe > - 87.11% entry_SYSCALL_64_after_hwframe > - do_syscall_64 > - 44.79% ksys_write > - 43.74% vfs_write > - 40.96% vhost_chr_write_iter > - 38.22% vhost_process_iotlb_msg > - 13.72% vhost_iotlb_add_range_ctx > - 7.43% vhost_iotlb_map_free > - 4.37% vhost_iotlb_itree_remove > rb_next > 1.78% __rb_erase_color > 0.73% kfree > 1.15% __rb_insert_augmented > 0.68% __kmalloc_cache_noprof > - 10.73% vhost_vq_work_queue > - 7.65% try_to_wake_up > - 2.55% ttwu_queue_wakelist > - 1.72% __smp_call_single_queue > 1.36% call_function_single_prep_ipi > - 1.32% __task_rq_lock > - _raw_spin_lock > native_queued_spin_lock_slowpath > - 1.30% select_task_rq > - select_task_rq_fair > - 0.88% wake_affine > available_idle_cpu > 2.06% llist_add_batch > - 4.05% __mutex_lock.constprop.0 > 2.14% mutex_spin_on_owner > 0.72% osq_lock > 3.00% mutex_lock > - 1.72% kfree > - 1.16% __slab_free > slab_update_freelist.constprop.0.isra.0 > 1.37% _raw_spin_lock > 1.08% mutex_unlock > 1.98% _copy_from_iter > - 1.86% rw_verify_area > - security_file_permission > - 1.13% file_has_perm > 0.69% avc_has_perm > 0.63% fdget_pos > - 27.86% syscall_exit_to_user_mode > - syscall_exit_to_user_mode_prepare > - 25.96% __audit_syscall_exit > - 25.03% __audit_filter_op > 6.66% audit_filter_rules.constprop.0 > 1.27% audit_reset_context.part.0.constprop.0 > - 10.86% ksys_read > - 9.37% vfs_read > - 6.67% vhost_chr_read_iter > 1.48% _copy_to_iter > 1.36% _raw_spin_lock > - 1.30% __wake_up > 0.81% _raw_spin_lock_irqsave > - 1.25% vhost_enqueue_msg > _raw_spin_lock > - 1.83% rw_verify_area > - security_file_permission > - 1.03% file_has_perm > 0.64% avc_has_perm > 0.65% fdget_pos > 0.57% fput > - 2.56% syscall_trace_enter > - 1.25% __seccomp_filter > seccomp_run_filters > 0.54% __audit_syscall_entry > > vhost-net thread > Samples: 20K of event 'cycles:P', 4000 Hz, Event count (approx.): 7796456297 lost: 0/0 drop: 0/0 > Children Self Shared Object Symbol > - 100.00% 3.38% [kernel] [k] vhost_task_fn > 38.26% 0xffffffff930bb8c0 > - 3.36% 0 > ret_from_fork_asm > ret_from_fork > - 1.16% vhost_task_fn > - 2.35% vhost_run_work_list > - 1.67% handle_tx > - 7.09% __mutex_lock.constprop.0 > 6.64% mutex_spin_on_owner > - 0.84% vq_meta_prefetch > - 3.22% iotlb_access_ok > 2.50% vhost_iotlb_itree_first > 0.80% mutex_lock > - 0.75% handle_tx_copy > 0.86% llist_reverse_order > > > > > On Wed, 30 Apr 2025 19:04:28 -0700 you wrote: > >> In handle_tx_copy, TX batching processes packets below ~PAGE_SIZE and > >> batches up to 64 messages before calling sock->sendmsg. > >> > >> Currently, when there are no more messages on the ring to dequeue, > >> handle_tx_copy re-enables kicks on the ring *before* firing off the > >> batch sendmsg. However, sock->sendmsg incurs a non-zero delay, > >> especially if it needs to wake up a thread (e.g., another vhost worker). > >> > >> [...] > > > > Here is the summary with links: > > - [net-next,v3] vhost/net: Defer TX queue re-enable until after sendmsg > > https://urldefense.proofpoint.com/v2/url?u=https-3A__git.kernel.org_netdev_net-2Dnext_c_8c2e6b26ffe2&d=DwIDaQ&c=s883GpUCOChKOHiocYtGcg&r=NGPRGGo37mQiSXgHKm5rCQ&m=0XoR6N9VbkaJ_wBENy8Z28uDdqjCe4HRNCyV-8o4etqXeEJOqoFFGjeGGP5sQcmt&s=-X8si_rU8pXKNyWNNzBqx5Fmv-ut9w2gS5E6coMDApM&e= > > > > You are awesome, thank you! > > -- > > Deet-doot-dot, I am a bot. > > https://urldefense.proofpoint.com/v2/url?u=https-3A__korg.docs.kernel.org_patchwork_pwbot.html&d=DwIDaQ&c=s883GpUCOChKOHiocYtGcg&r=NGPRGGo37mQiSXgHKm5rCQ&m=0XoR6N9VbkaJ_wBENy8Z28uDdqjCe4HRNCyV-8o4etqXeEJOqoFFGjeGGP5sQcmt&s=sydedZsBCMSJM9_Ldw6Al-BplvM7FokLwV_80bJpGnM&e= > > > > > Well it seems that if get_tx_bufs failed with -EAGAIN then we previously bailed out, but now we will redo poll and so on, forever. No? -- MST
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