From: Jens Axboe <firstname.lastname@example.org>
To: He Zhe <email@example.com>,
Subject: Re: [PATCH 1/2] eventfd: Make wake counter work for single fd instead of all
Date: Thu, 9 Apr 2020 08:44:30 -0700 [thread overview]
Message-ID: <firstname.lastname@example.org> (raw)
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, email@example.com wrote:
>>> From: He Zhe <firstname.lastname@example.org>
>>> 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
>>> 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%
>>> - vhost_worker
>>> - 33.47% handle_tx_kick
>>> - 33.05% handle_rx_net
>>> - 33.49%
>>> 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
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.
next prev parent reply other threads:[~2020-04-09 15:44 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-07 10:59 [PATCH 1/2] eventfd: Make wake counter work for single fd instead of all zhe.he
2020-04-07 10:59 ` [PATCH 2/2] aio: Update calling to eventfd_signal_count with righ parameter zhe.he
2020-04-07 20:06 ` [PATCH 1/2] eventfd: Make wake counter work for single fd instead of all Jens Axboe
2020-04-09 10:37 ` He Zhe
2020-04-09 15:44 ` Jens Axboe [this message]
2020-04-10 11:46 ` He Zhe
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