linux-kernel - vfio_pin_map_dma cause synchronize

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Date:   Mon, 2 Dec 2019 17:10:26 +0800
From:   "Longpeng (Mike)" <longpeng2@...wei.com>
To:     Alex Williamson <alex.williamson@...hat.com>, <pbonzini@...hat.com>
CC:     <qemu-devel@...gnu.org>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        "Longpeng(Mike)" <longpeng.mike@...il.com>,
        Gonglei <arei.gonglei@...wei.com>,
        Huangzhichao <huangzhichao@...wei.com>
Subject: vfio_pin_map_dma cause synchronize_sched wait too long

Hi guys,

Suppose there're two VMs: VM1 is bind to node-0 and calling vfio_pin_map_dma(),
VM2 is a migrate incoming VM which bind to node-1. We found the vm_start( QEMU
function) of VM2 will take too long occasionally, the reason is as follow.

- VM2 -
qemu: vm_start
        vm_start_notify
          virtio_vmstate_change
            virtio_pci_vmstate_change
              virtio_pci_start_ioeventfd
                virtio_device_start_ioeventfd_impl
                  event_notifier_init
                    eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC) <-- too long
kern: sys_eventfd2
        get_unused_fd_flags
          __alloc_fd
            expand_files
              expand_fdtable
                synchronize_sched <-- too long

- VM1 -
The VM1 is doing vfio_pin_map_dma at the same time.

The CPU must finish vfio_pin_map_dma and then rcu-sched grace period can be
elapsed, so synchronize_sched would wait for a long time.

Is there any solution to this ? Any suggestion would be greatly appreciated, thanks!

-- 
Regards,
Longpeng(Mike)