linux-kernel - [RFC] doc: Improve rcu_dynticks::dynticks documentation

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Message-Id: <20180624193451.250855-1-joel@joelfernandes.org>
Date:   Sun, 24 Jun 2018 12:34:51 -0700
From:   Joel Fernandes <joel@...lfernandes.org>
To:     linux-kernel@...r.kernel.org
Cc:     "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Jonathan Corbet <corbet@....net>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-doc@...r.kernel.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Steven Rostedt <rostedt@...dmis.org>, byungchul.park@....com
Subject: [RFC] doc: Improve rcu_dynticks::dynticks documentation

From: "Joel Fernandes (Google)" <joel@...lfernandes.org>

The very useful RCU Data-Structures describes that the dynticks counter
of the rcu_dynticks data structure is incremented when we transitions to
or from dynticks-idle mode. However it doesn't mention that it is also
incremented due to transitions to and from user mode which for dynticks
purposes is an extended quiescent state.

I found this with tracing calls to rcu_dynticks_eqs_enter which can also
happen from rcu_user_enter. Lets add this information to the
Data-Structures document.

Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
---
 .../RCU/Design/Data-Structures/Data-Structures.html       | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/Documentation/RCU/Design/Data-Structures/Data-Structures.html b/Documentation/RCU/Design/Data-Structures/Data-Structures.html
index f5120a00f511..50be87e59937 100644
--- a/Documentation/RCU/Design/Data-Structures/Data-Structures.html
+++ b/Documentation/RCU/Design/Data-Structures/Data-Structures.html
@@ -1227,9 +1227,11 @@ to overflow the counter, this approach corrects the
 CPU enters the idle loop from process context.
 
 </p><p>The <tt>-&gt;dynticks</tt> field counts the corresponding
-CPU's transitions to and from dyntick-idle mode, so that this counter
-has an even value when the CPU is in dyntick-idle mode and an odd
-value otherwise.
+CPU's transitions to and from either dyntick-idle or user mode, so
+that this counter has an even value when the CPU is in dyntick-idle
+mode or user mode and an odd value otherwise. The transitions to/from
+user mode need to be counted for user mode adaptive-ticks support
+(see timers/NO_HZ.txt).
 
 </p><p>The <tt>-&gt;rcu_need_heavy_qs</tt> field is used
 to record the fact that the RCU core code would really like to
-- 
2.18.0.rc2.346.g013aa6912e-goog