Received-SPF: pass (google.com: domain of linux-kernel+bounces-104861-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:4601:e00::3 as permitted sender) client-ip=2604:1380:4601:e00::3;
Date: Fri, 15 Mar 2024 12:55:00 -0700
From: Yan Zhai <yan@cloudflare.com>
To: netdev@vger.kernel.org
Cc: "David S. Miller" <davem@davemloft.net>,
	Eric Dumazet <edumazet@google.com>,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	Jiri Pirko <jiri@resnulli.us>, Simon Horman <horms@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Lorenzo Bianconi <lorenzo@kernel.org>,
	Coco Li <lixiaoyan@google.com>, Wei Wang <weiwan@google.com>,
	Alexander Duyck <alexanderduyck@fb.com>,
	Hannes Frederic Sowa <hannes@stressinduktion.org>,
	linux-kernel@vger.kernel.org, rcu@vger.kernel.org,
	bpf@vger.kernel.org, kernel-team@cloudflare.com,
	Joel Fernandes <joel@joelfernandes.org>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= <toke@redhat.com>,
	Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	Steven Rostedt <rostedt@goodmis.org>, mark.rutland@arm.com,
	Jesper Dangaard Brouer <hawk@kernel.org>
Subject: [PATCH v4 net 0/3] Report RCU QS for busy network kthreads
Message-ID: <cover.1710525524.git.yan@cloudflare.com>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline

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