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Message-ID: <CABWYdi1kiu1g1mAq6DpQWczg78tMzaVFnytNMemZATFHqYSqYw@mail.gmail.com> Date: Thu, 19 Oct 2023 15:35:01 -0700 From: Ivan Babrou <ivan@...udflare.com> To: Linux Kernel Network Developers <netdev@...r.kernel.org> Cc: kernel-team <kernel-team@...udflare.com>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, linux-kernel <linux-kernel@...r.kernel.org> Subject: wait_for_unix_gc can cause CPU overload for well behaved programs Hello, We have observed this issue twice (2019 and 2023): a well behaved service that doesn't pass any file descriptors around starts to spend a ton of CPU time in wait_for_unix_gc. The cause of this is that the unix send path unconditionally calls wait_for_unix_gc, which is a global garbage collection. If any misbehaved program exists on a system, it can force extra work for well behaved programs. This behavior is not new: 9915672d4127 ("af_unix: limit unix_tot_inflight") is from 2010. I managed to come up with a repro for this behavior: * https://gist.github.com/bobrik/82e5722261920c9f23d9402b88a0bb27 It also includes a flamegraph illustrating the issue. It's all in one program for convenience, but in reality the offender not picking up SCM_RIGHTS messages and the suffering program just minding its own business are separate. It is also non-trivial to find the offender when this happens as it can be completely idle while wrecking havoc for the rest of the system. I don't think it's fair to penalize every unix_stream_sendmsg like this. The 16k threshold also doesn't feel very flexible, surely computers are bigger these days and can handle more.
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