Re: [PATCH 1/2] eventfd: Make wake counter work for single fd instead of all

[He Zhe]

On 4/9/20 11:44 PM, Jens Axboe wrote:
> On 4/9/20 3:37 AM, He Zhe wrote:
>>
>> On 4/8/20 4:06 AM, Jens Axboe wrote:
>>> On 4/7/20 3:59 AM, zhe.he@xxxxxxxxxxxxx wrote:
>>>> From: He Zhe <zhe.he@xxxxxxxxxxxxx>
>>>>
>>>> commit b5e683d5cab8 ("eventfd: track eventfd_signal() recursion depth")
>>>> introduces a percpu counter that tracks the percpu recursion depth and
>>>> warn if it greater than one, to avoid potential deadlock and stack
>>>> overflow.
>>>>
>>>> However sometimes different eventfds may be used in parallel.
>>>> Specifically, when high network load goes through kvm and vhost, working
>>>> as below, it would trigger the following call trace.
>>>>
>>>> -  100.00%
>>>>    - 66.51%
>>>>         ret_from_fork
>>>>         kthread
>>>>       - vhost_worker
>>>>          - 33.47% handle_tx_kick
>>>>               handle_tx
>>>>               handle_tx_copy
>>>>               vhost_tx_batch.isra.0
>>>>               vhost_add_used_and_signal_n
>>>>               eventfd_signal
>>>>          - 33.05% handle_rx_net
>>>>               handle_rx
>>>>               vhost_add_used_and_signal_n
>>>>               eventfd_signal
>>>>    - 33.49%
>>>>         ioctl
>>>>         entry_SYSCALL_64_after_hwframe
>>>>         do_syscall_64
>>>>         __x64_sys_ioctl
>>>>         ksys_ioctl
>>>>         do_vfs_ioctl
>>>>         kvm_vcpu_ioctl
>>>>         kvm_arch_vcpu_ioctl_run
>>>>         vmx_handle_exit
>>>>         handle_ept_misconfig
>>>>         kvm_io_bus_write
>>>>         __kvm_io_bus_write
>>>>         eventfd_signal
>>>>
>>>> 001: WARNING: CPU: 1 PID: 1503 at fs/eventfd.c:73 eventfd_signal+0x85/0xa0
>>>> ---- snip ----
>>>> 001: Call Trace:
>>>> 001:  vhost_signal+0x15e/0x1b0 [vhost]
>>>> 001:  vhost_add_used_and_signal_n+0x2b/0x40 [vhost]
>>>> 001:  handle_rx+0xb9/0x900 [vhost_net]
>>>> 001:  handle_rx_net+0x15/0x20 [vhost_net]
>>>> 001:  vhost_worker+0xbe/0x120 [vhost]
>>>> 001:  kthread+0x106/0x140
>>>> 001:  ? log_used.part.0+0x20/0x20 [vhost]
>>>> 001:  ? kthread_park+0x90/0x90
>>>> 001:  ret_from_fork+0x35/0x40
>>>> 001: ---[ end trace 0000000000000003 ]---
>>>>
>>>> This patch moves the percpu counter into eventfd control structure and
>>>> does the clean-ups, so that eventfd can still be protected from deadlock
>>>> while allowing different ones to work in parallel.
>>>>
>>>> As to potential stack overflow, we might want to figure out a better
>>>> solution in the future to warn when the stack is about to overflow so it
>>>> can be better utilized, rather than break the working flow when just the
>>>> second one comes.
>>> This doesn't work for the infinite recursion case, the state has to be
>>> global, or per thread.
>> Thanks, but I'm not very clear about why the counter has to be global
>> or per thread.
>>
>> If the recursion happens on the same eventfd, the attempt to re-grab
>> the same ctx->wqh.lock would be blocked by the fd-specific counter in
>> this patch.
>>
>> If the recursion happens with a chain of different eventfds, that
>> might lead to a stack overflow issue. The issue should be handled but
>> it seems unnecessary to stop the just the second ring(when the counter
>> is going to be 2) of the chain.
>>
>> Specifically in the vhost case, it runs very likely with heavy network
>> load which generates loads of eventfd_signal. Delaying the
>> eventfd_signal to worker threads will still end up violating the
>> global counter later and failing as above.
>>
>> So we might want to take care of the potential overflow later,
>> hopefully with a measurement that can tell us if it's about to
>> overflow.
> The worry is different eventfds, recursion on a single one could be
> detected by keeping state in the ctx itself. And yeah, I agree that one
> level isn't very deep, but wakeup chains can be deep and we can't allow
> a whole lot more. I'm sure folks would be open to increasing it, if some
> worst case kind of data was collected to prove it's fine to go deeper.

OK, thanks. v2 will be sent.

Zhe

>