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Date: Fri, 15 Mar 2024 12:55:03 -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 1/3] rcu: add a helper to report consolidated flavor QS
Message-ID: <491d3af6c7d66dfb3b60b2f210f38e843dfe6ed2.1710525524.git.yan@cloudflare.com>
References: <cover.1710525524.git.yan@cloudflare.com>
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In-Reply-To: <cover.1710525524.git.yan@cloudflare.com>

There are several scenario in network processing that can run
extensively under heavy traffic. In such situation, RCU synchronization
might not observe desired quiescent states for indefinitely long period.
Create a helper to safely raise the desired RCU quiescent states for
such scenario.

Currently the frequency is locked at HZ/10, i.e. 100ms, which is
sufficient to address existing problems around RCU tasks. It's unclear
yet if there is any future scenario for it to be further tuned down.

Suggested-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Jesper Dangaard Brouer <hawk@kernel.org>
Signed-off-by: Yan Zhai <yan@cloudflare.com>
---
v3->v4: comment fixup

---
 include/linux/rcupdate.h | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 0746b1b0b663..da224706323e 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -247,6 +247,30 @@ do { \
 	cond_resched(); \
 } while (0)
 
+/**
+ * rcu_softirq_qs_periodic - Periodically report consolidated quiescent states
+ * @old_ts: last jiffies when QS was reported. Might be modified in the macro.
+ *
+ * This helper is for network processing in non-RT kernels, where there could
+ * be busy polling threads that block RCU synchronization indefinitely.  In
+ * such context, simply calling cond_resched is insufficient, so give it a
+ * stronger push to eliminate all potential blockage of all RCU types.
+ *
+ * NOTE: unless absolutely sure, this helper should in general be called
+ * outside of bh lock section to avoid reporting a surprising QS to updaters,
+ * who could be expecting RCU read critical section to end at local_bh_enable().
+ */
+#define rcu_softirq_qs_periodic(old_ts) \
+do { \
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT) && \
+	    time_after(jiffies, (old_ts) + HZ / 10)) { \
+		preempt_disable(); \
+		rcu_softirq_qs(); \
+		preempt_enable(); \
+		(old_ts) = jiffies; \
+	} \
+} while (0)
+
 /*
  * Infrastructure to implement the synchronize_() primitives in
  * TREE_RCU and rcu_barrier_() primitives in TINY_RCU.
-- 
2.30.2