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authorLinus Torvalds <torvalds@linux-foundation.org>2016-10-03 20:29:53 +0300
committerLinus Torvalds <torvalds@linux-foundation.org>2016-10-03 20:29:53 +0300
commit4b978934a440c1aafce986353001b03289eaa040 (patch)
treee8e0b54d128e16c35ccf77b724d1640df12d82ba /Documentation
parent72a9cdd083005900f15934e8568f1ac43a6bb755 (diff)
parent2d8fbcd13ea1d0be3a7ea5f20c3a5b44b592e79c (diff)
downloadlinux-4b978934a440c1aafce986353001b03289eaa040.tar.xz
Merge branch 'core-rcu-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
Pull RCU updates from Ingo Molnar: "The main changes in this cycle were: - Expedited grace-period changes, most notably avoiding having user threads drive expedited grace periods, using a workqueue instead. - Miscellaneous fixes, including a performance fix for lists that was sent with the lists modifications. - CPU hotplug updates, most notably providing exact CPU-online tracking for RCU. This will in turn allow removal of the checks supporting RCU's prior heuristic that was based on the assumption that CPUs would take no longer than one jiffy to come online. - Torture-test updates. - Documentation updates" * 'core-rcu-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (22 commits) list: Expand list_first_entry_or_null() torture: TOROUT_STRING(): Insert a space between flag and message rcuperf: Consistently insert space between flag and message rcutorture: Print out barrier error as document says torture: Add task state to writer-task stall printk()s torture: Convert torture_shutdown() to hrtimer rcutorture: Convert to hotplug state machine cpu/hotplug: Get rid of CPU_STARTING reference rcu: Provide exact CPU-online tracking for RCU rcu: Avoid redundant quiescent-state chasing rcu: Don't use modular infrastructure in non-modular code sched: Make wake_up_nohz_cpu() handle CPUs going offline rcu: Use rcu_gp_kthread_wake() to wake up grace period kthreads rcu: Use RCU's online-CPU state for expedited IPI retry rcu: Exclude RCU-offline CPUs from expedited grace periods rcu: Make expedited RCU CPU stall warnings respond to controls rcu: Stop disabling expedited RCU CPU stall warnings rcu: Drive expedited grace periods from workqueue rcu: Consolidate expedited grace period machinery documentation: Record reason for rcu_head two-byte alignment ...
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/RCU/Design/Requirements/Requirements.html22
-rw-r--r--Documentation/RCU/torture.txt15
2 files changed, 22 insertions, 15 deletions
diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html
index ece410f40436..a4d3838130e4 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.html
+++ b/Documentation/RCU/Design/Requirements/Requirements.html
@@ -2493,6 +2493,28 @@ or some future &ldquo;lazy&rdquo;
variant of <tt>call_rcu()</tt> that might one day be created for
energy-efficiency purposes.
+<p>
+That said, there are limits.
+RCU requires that the <tt>rcu_head</tt> structure be aligned to a
+two-byte boundary, and passing a misaligned <tt>rcu_head</tt>
+structure to one of the <tt>call_rcu()</tt> family of functions
+will result in a splat.
+It is therefore necessary to exercise caution when packing
+structures containing fields of type <tt>rcu_head</tt>.
+Why not a four-byte or even eight-byte alignment requirement?
+Because the m68k architecture provides only two-byte alignment,
+and thus acts as alignment's least common denominator.
+
+<p>
+The reason for reserving the bottom bit of pointers to
+<tt>rcu_head</tt> structures is to leave the door open to
+&ldquo;lazy&rdquo; callbacks whose invocations can safely be deferred.
+Deferring invocation could potentially have energy-efficiency
+benefits, but only if the rate of non-lazy callbacks decreases
+significantly for some important workload.
+In the meantime, reserving the bottom bit keeps this option open
+in case it one day becomes useful.
+
<h3><a name="Performance, Scalability, Response Time, and Reliability">
Performance, Scalability, Response Time, and Reliability</a></h3>
diff --git a/Documentation/RCU/torture.txt b/Documentation/RCU/torture.txt
index 118e7c176ce7..278f6a9383b6 100644
--- a/Documentation/RCU/torture.txt
+++ b/Documentation/RCU/torture.txt
@@ -10,21 +10,6 @@ status messages via printk(), which can be examined via the dmesg
command (perhaps grepping for "torture"). The test is started
when the module is loaded, and stops when the module is unloaded.
-CONFIG_RCU_TORTURE_TEST_RUNNABLE
-
-It is also possible to specify CONFIG_RCU_TORTURE_TEST=y, which will
-result in the tests being loaded into the base kernel. In this case,
-the CONFIG_RCU_TORTURE_TEST_RUNNABLE config option is used to specify
-whether the RCU torture tests are to be started immediately during
-boot or whether the /proc/sys/kernel/rcutorture_runnable file is used
-to enable them. This /proc file can be used to repeatedly pause and
-restart the tests, regardless of the initial state specified by the
-CONFIG_RCU_TORTURE_TEST_RUNNABLE config option.
-
-You will normally -not- want to start the RCU torture tests during boot
-(and thus the default is CONFIG_RCU_TORTURE_TEST_RUNNABLE=n), but doing
-this can sometimes be useful in finding boot-time bugs.
-
MODULE PARAMETERS