Re: [PATCH] eventfd: Enlarge recursion limit to allow vhost to work

[Juri Lelli <juri.lelli@redhat.com>]

On 03/07/20 19:11, He Zhe wrote:
> 
> 
> On 7/3/20 4:12 PM, Juri Lelli wrote:
> > Hi,
> >
> > On 10/04/20 19:47, zhe.he@windriver.com wrote:
> >> From: He Zhe <zhe.he@windriver.com>
> >>
> >> commit b5e683d5cab8 ("eventfd: track eventfd_signal() recursion depth")
> >> introduces a percpu counter that tracks the percpu recursion depth and
> >> warn if it greater than zero, to avoid potential deadlock and stack
> >> overflow.
> >>
> >> However sometimes different eventfds may be used in parallel. Specifically,
> >> when heavy 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 enlarges the limit to 1 which is the maximum recursion depth we
> >> have found so far.
> >>
> >> Signed-off-by: He Zhe <zhe.he@windriver.com>
> >> ---
> > Not sure if this approch can fly, but I also encountered the same
> > warning (which further caused hangs during VM install) and this change
> > addresses that.
> >
> > I'd be interested in understanding what is the status of this problem/fix.
> 
> This is actually v2 of the patch and has not got any reply yet. Here is the v1. FYI.
> https://lore.kernel.org/lkml/1586257192-58369-1-git-send-email-zhe.he@windriver.com/

I see, thanks. Hope this gets reviewed soon! :-)

> > On a side note, by looking at the code, I noticed that (apart from
> > samples) all callers don't actually check eventfd_signal() return value
> > and I'm wondering why is that the case and if is it safe to do so.
> 
> Checking the return value right after sending the signal can tell us if the
> event counter has just overflowed, that is, exceeding ULLONG_MAX. I guess the
> authors of the callers listed in the commit log just don't worry about that,
> since they add only one to a dedicated eventfd.

OK. I was mostly wondering if returning early in case the WARN_ON_ONCE
fires would cause a missing wakeup for the eventfd_ctx wait queue.

Best,

Juri