Re: [PATCH 5.4 0/2] Fix epoll issue in 5.4 kernels

[Benjamin Segall]

Rishabh Bhatnagar <risbhat@xxxxxxxxxx> writes:

> Hi Greg
> After upgrading to 5.4.211 we were started seeing some nodes getting
> stuck in our Kubernetes cluster. All nodes are running this kernel
> version. After taking a closer look it seems that runc was command getting
> stuck. Looking at the stack it appears the thread is stuck in epoll wait for
> sometime. 
> [<0>] do_syscall_64+0x48/0xf0
> [<0>] entry_SYSCALL_64_after_hwframe+0x5c/0xc1
> [<0>] ep_poll+0x48d/0x4e0
> [<0>] do_epoll_wait+0xab/0xc0
> [<0>] __x64_sys_epoll_pwait+0x4d/0xa0
> [<0>] do_syscall_64+0x48/0xf0
> [<0>] entry_SYSCALL_64_after_hwframe+0x5c/0xc1
> [<0>] futex_wait_queue_me+0xb6/0x110
> [<0>] futex_wait+0xe2/0x260
> [<0>] do_futex+0x372/0x4f0
> [<0>] __x64_sys_futex+0x134/0x180
> [<0>] do_syscall_64+0x48/0xf0
> [<0>] entry_SYSCALL_64_after_hwframe+0x5c/0xc1
>
> I noticed there are other discussions going on as well
> regarding this.
> https://lore.kernel.org/all/Y1pY2n6E1Xa58MXv@xxxxxxxxx/
> Reverting the below patch does fix the issue:
> https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=linux-5.4.y&id=cf2db24ec4b8e9d399005ececd6f6336916ab6fc
> We don't see this issue in latest upstream kernel or even latest 5.10
> stable tree. Looking at the patches that went in for 5.10 stable there's
> one that stands out that seems to be missing in 5.4.
> 289caf5d8f6c61c6d2b7fd752a7f483cd153f182 (epoll: check for events when removing
> a timed out thread from the wait queue)
>
> Backporting this patch to 5.4 we don't see the hangups anymore. Looks like
> this patch fixes time out scenarios which might cause missed wake ups.
> The other patch in the patch series also fixes a race and is needed for
> the second patch to apply.

Yes, this definitely makes sense to me; the aggressive removal was only
valid because the rest of the epoll machinery did plenty of extra
checking. And I didn't as carefully check the backports when I saw the
-stable emails.