Subject: Re: [PATCH v3 net 0/3] Report RCU QS for busy network kthreads

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Wed, 13 Mar 2024 22:53:57 +0100

Yan Zhai <[email-masked]> writes:

> This changeset fixes a common problem for busy networking kthreads.
> These threads, e.g. NAPI threads, typically will do:
>
> * polling a batch of packets
> * if there are more work, call cond_resched to allow scheduling
> * continue to poll more packets when rx queue is not empty
>
> We observed this being a problem in production, since it can block RCU
> tasks from making progress under heavy load. Investigation indicates
> that just calling cond_resched is insufficient for RCU tasks to reach
> quiescent states. This at least affects NAPI threads, napi_busy_loop, and
> also cpumap kthread for now.
>
> By reporting RCU QSes in these kthreads periodically before
> cond_resched, the blocked RCU waiters can correctly progress. Instead of
> just reporting QS for RCU tasks, these code share the same concern as
> noted in the commit d28139c4e967 ("rcu: Apply RCU-bh QSes to RCU-sched
> and RCU-preempt when safe"). So report a consolidated QS for safety.
>
> It is worth noting that, although this problem is reproducible in
> napi_busy_loop, it only shows up when setting the polling interval to as
> high as 2ms, which is far larger than recommended 50us-100us in the
> documentation. So napi_busy_loop is left untouched.
>
> V2: https://lore.kernel.org/bpf/[email-masked]/
> V1: https://lore.kernel.org/lkml/[email-masked]/#t
>
> changes since v2:
>  * created a helper in rcu header to abstract the behavior
>  * fixed cpumap kthread in addition
>
> changes since v1:
>  * disable preemption first as Paul McKenney suggested
>
> Yan Zhai (3):
>   rcu: add a helper to report consolidated flavor QS
>   net: report RCU QS on threaded NAPI repolling
>   bpf: report RCU QS in cpumap kthread
>
>  include/linux/rcupdate.h | 23 +++++++++++++++++++++++
>  kernel/bpf/cpumap.c      |  2 ++
>  net/core/dev.c           |  3 +++
>  3 files changed, 28 insertions(+)

For the series:

Reviewed-by: Toke Høiland-Jørgensen <[email-masked]>