[PATCH v4 net 0/3] Report RCU QS for busy network kthreads

[Yan Zhai <yan@cloudflare.com>]

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.

V3: https://lore.kernel.org/lkml/20240314145459.7b3aedf1@kernel.org/t/
V2: https://lore.kernel.org/bpf/ZeFPz4D121TgvCje@debian.debian/
V1: https://lore.kernel.org/lkml/Zd4DXTyCf17lcTfq@debian.debian/#t

changes since v3:
 * fixed kernel-doc errors

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 | 24 ++++++++++++++++++++++++
 kernel/bpf/cpumap.c      |  3 +++
 net/core/dev.c           |  3 +++
 3 files changed, 30 insertions(+)

-- 
2.30.2