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    SubjectRe: [PATCH] eventfd: Enlarge recursion limit to allow vhost to work
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    Date
    Can this be considered for this moment?
    This is actually v2 of
    "[PATCH 1/2] eventfd: Make wake counter work for single fd instead of all".

    Thanks,
    Zhe

    On 4/10/20 7:47 PM, 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>
    > ---
    > fs/eventfd.c | 3 ++-
    > include/linux/eventfd.h | 3 +++
    > 2 files changed, 5 insertions(+), 1 deletion(-)
    >
    > diff --git a/fs/eventfd.c b/fs/eventfd.c
    > index 78e41c7c3d05..8b9bd6fb08cd 100644
    > --- a/fs/eventfd.c
    > +++ b/fs/eventfd.c
    > @@ -70,7 +70,8 @@ __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n)
    > * it returns true, the eventfd_signal() call should be deferred to a
    > * safe context.
    > */
    > - if (WARN_ON_ONCE(this_cpu_read(eventfd_wake_count)))
    > + if (WARN_ON_ONCE(this_cpu_read(eventfd_wake_count) >
    > + EFD_WAKE_COUNT_MAX))
    > return 0;
    >
    > spin_lock_irqsave(&ctx->wqh.lock, flags);
    > diff --git a/include/linux/eventfd.h b/include/linux/eventfd.h
    > index dc4fd8a6644d..e7684d768e3f 100644
    > --- a/include/linux/eventfd.h
    > +++ b/include/linux/eventfd.h
    > @@ -29,6 +29,9 @@
    > #define EFD_SHARED_FCNTL_FLAGS (O_CLOEXEC | O_NONBLOCK)
    > #define EFD_FLAGS_SET (EFD_SHARED_FCNTL_FLAGS | EFD_SEMAPHORE)
    >
    > +/* This is the maximum recursion depth we find so far */
    > +#define EFD_WAKE_COUNT_MAX 1
    > +
    > struct eventfd_ctx;
    > struct file;
    >

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