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documentation: Add real-time requirements from CPU-bound workloads
authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Sat, 12 Dec 2015 00:18:22 +0000 (16:18 -0800)
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Thu, 31 Mar 2016 20:33:19 +0000 (13:33 -0700)
This commit records RCU's responsibility to avoid degrading latencies
of CPUs running tight loops within properly configured workloads,
both in kernel and in userspace.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Documentation/RCU/Design/Requirements/Requirements.html patch | blob | blame | history
Documentation/RCU/Design/Requirements/Requirements.htmlx patch | blob | blame | history

diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html
index a725f9900ec8962a43dfb888f71f1a2b05efabf0..3004baa71bcc3ae79a04e210977dd4aba3e7f0ea 100644 (file)
--- a/Documentation/RCU/Design/Requirements/Requirements.html
+++ b/Documentation/RCU/Design/Requirements/Requirements.html
@@ -1,5 +1,5 @@
 <!-- DO NOT HAND EDIT. -->
-<!-- Instead, edit Documentation/RCU/Design/Requirements/Requirements.htmlx and run 'sh htmlqqz.sh Documentation/RCU/Design/Requirements/Requirements' -->
+<!-- Instead, edit Requirements.htmlx and run 'sh htmlqqz.sh Requirements' -->
 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
         "http://www.w3.org/TR/html4/loose.dtd">
         <html>
@@ -2170,6 +2170,14 @@ up to and including systems with 4096 CPUs.
 This real-time requirement motivated the grace-period kthread, which
 also simplified handling of a number of race conditions.
 
+<p>
+RCU must avoid degrading real-time response for CPU-bound threads, whether
+executing in usermode (which is one use case for
+<tt>CONFIG_NO_HZ_FULL=y</tt>) or in the kernel.
+That said, CPU-bound loops in the kernel must execute
+<tt>cond_resched_rcu_qs()</tt> at least once per few tens of milliseconds
+in order to avoid receiving an IPI from RCU.
+
 <p>
 Finally, RCU's status as a synchronization primitive means that
 any RCU failure can result in arbitrary memory corruption that can be
diff --git a/Documentation/RCU/Design/Requirements/Requirements.htmlx b/Documentation/RCU/Design/Requirements/Requirements.htmlx
index 3a97ba490c42b03d606c5fe36e5afe1a9b213e28..61caffc86823d2f5fe335f08e8cd98e424c67382 100644 (file)
--- a/Documentation/RCU/Design/Requirements/Requirements.htmlx
+++ b/Documentation/RCU/Design/Requirements/Requirements.htmlx
@@ -2337,6 +2337,14 @@ up to and including systems with 4096 CPUs.
 This real-time requirement motivated the grace-period kthread, which
 also simplified handling of a number of race conditions.
 
+<p>
+RCU must avoid degrading real-time response for CPU-bound threads, whether
+executing in usermode (which is one use case for
+<tt>CONFIG_NO_HZ_FULL=y</tt>) or in the kernel.
+That said, CPU-bound loops in the kernel must execute
+<tt>cond_resched_rcu_qs()</tt> at least once per few tens of milliseconds
+in order to avoid receiving an IPI from RCU.
+
 <p>
 Finally, RCU's status as a synchronization primitive means that
 any RCU failure can result in arbitrary memory corruption that can be