8 files changed, 535 insertions, 41 deletions

diff --git a/Documentation/DocBook/kernel-locking.tmpl b/Documentation/DocBook/kernel-locking.tmpl
index 2e9d6b41f034..435413ca40dc 100644
--- a/Documentation/DocBook/kernel-locking.tmpl
+++ b/Documentation/DocBook/kernel-locking.tmpl
@@ -241,7 +241,7 @@
    </para>
    <para>
      The third type is a semaphore
-     (<filename class="headerfile">include/asm/semaphore.h</filename>): it
+     (<filename class="headerfile">include/linux/semaphore.h</filename>): it
      can have more than one holder at any time (the number decided at
      initialization time), although it is most commonly used as a
      single-holder lock (a mutex).  If you can't get a semaphore, your
@@ -290,7 +290,7 @@
      <para>
        If you have a data structure which is only ever accessed from
        user context, then you can use a simple semaphore
-       (<filename>linux/asm/semaphore.h</filename>) to protect it.  This 
+       (<filename>linux/linux/semaphore.h</filename>) to protect it.  This
        is the most trivial case: you initialize the semaphore to the number 
        of resources available (usually 1), and call
        <function>down_interruptible()</function> to grab the semaphore, and 
@@ -1656,7 +1656,7 @@ the amount of locking which needs to be done.
  #include &lt;linux/slab.h&gt;
  #include &lt;linux/string.h&gt;
 +#include &lt;linux/rcupdate.h&gt;
- #include &lt;asm/semaphore.h&gt;
+ #include &lt;linux/semaphore.h&gt;
  #include &lt;asm/errno.h&gt;
 
  struct object
diff --git a/Documentation/cpusets.txt b/Documentation/cpusets.txt
index ad2bb3b3acc1..aa854b9b18cd 100644
--- a/Documentation/cpusets.txt
+++ b/Documentation/cpusets.txt
@@ -8,6 +8,7 @@ Portions Copyright (c) 2004-2006 Silicon Graphics, Inc.
 Modified by Paul Jackson <pj@sgi.com>
 Modified by Christoph Lameter <clameter@sgi.com>
 Modified by Paul Menage <menage@google.com>
+Modified by Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
 
 CONTENTS:
 =========
@@ -20,7 +21,8 @@ CONTENTS:
   1.5 What is memory_pressure ?
   1.6 What is memory spread ?
   1.7 What is sched_load_balance ?
-  1.8 How do I use cpusets ?
+  1.8 What is sched_relax_domain_level ?
+  1.9 How do I use cpusets ?
 2. Usage Examples and Syntax
   2.1 Basic Usage
   2.2 Adding/removing cpus
@@ -497,7 +499,73 @@ the cpuset code to update these sched domains, it compares the new
 partition requested with the current, and updates its sched domains,
 removing the old and adding the new, for each change.
 
-1.8 How do I use cpusets ?
+
+1.8 What is sched_relax_domain_level ?
+--------------------------------------
+
+In sched domain, the scheduler migrates tasks in 2 ways; periodic load
+balance on tick, and at time of some schedule events.
+
+When a task is woken up, scheduler try to move the task on idle CPU.
+For example, if a task A running on CPU X activates another task B
+on the same CPU X, and if CPU Y is X's sibling and performing idle,
+then scheduler migrate task B to CPU Y so that task B can start on
+CPU Y without waiting task A on CPU X.
+
+And if a CPU run out of tasks in its runqueue, the CPU try to pull
+extra tasks from other busy CPUs to help them before it is going to
+be idle.
+
+Of course it takes some searching cost to find movable tasks and/or
+idle CPUs, the scheduler might not search all CPUs in the domain
+everytime.  In fact, in some architectures, the searching ranges on
+events are limited in the same socket or node where the CPU locates,
+while the load balance on tick searchs all.
+
+For example, assume CPU Z is relatively far from CPU X.  Even if CPU Z
+is idle while CPU X and the siblings are busy, scheduler can't migrate
+woken task B from X to Z since it is out of its searching range.
+As the result, task B on CPU X need to wait task A or wait load balance
+on the next tick.  For some applications in special situation, waiting
+1 tick may be too long.
+
+The 'sched_relax_domain_level' file allows you to request changing
+this searching range as you like.  This file takes int value which
+indicates size of searching range in levels ideally as follows,
+otherwise initial value -1 that indicates the cpuset has no request.
+
+  -1  : no request. use system default or follow request of others.
+   0  : no search.
+   1  : search siblings (hyperthreads in a core).
+   2  : search cores in a package.
+   3  : search cpus in a node [= system wide on non-NUMA system]
+ ( 4  : search nodes in a chunk of node [on NUMA system] )
+ ( 5~ : search system wide [on NUMA system])
+
+This file is per-cpuset and affect the sched domain where the cpuset
+belongs to.  Therefore if the flag 'sched_load_balance' of a cpuset
+is disabled, then 'sched_relax_domain_level' have no effect since
+there is no sched domain belonging the cpuset.
+
+If multiple cpusets are overlapping and hence they form a single sched
+domain, the largest value among those is used.  Be careful, if one
+requests 0 and others are -1 then 0 is used.
+
+Note that modifying this file will have both good and bad effects,
+and whether it is acceptable or not will be depend on your situation.
+Don't modify this file if you are not sure.
+
+If your situation is:
+ - The migration costs between each cpu can be assumed considerably
+   small(for you) due to your special application's behavior or
+   special hardware support for CPU cache etc.
+ - The searching cost doesn't have impact(for you) or you can make
+   the searching cost enough small by managing cpuset to compact etc.
+ - The latency is required even it sacrifices cache hit rate etc.
+then increasing 'sched_relax_domain_level' would benefit you.
+
+
+1.9 How do I use cpusets ?
 --------------------------
 
 In order to minimize the impact of cpusets on critical kernel
diff --git a/Documentation/feature-removal-schedule.txt b/Documentation/feature-removal-schedule.txt
index af0e9393bf68..b45ea28abc99 100644
--- a/Documentation/feature-removal-schedule.txt
+++ b/Documentation/feature-removal-schedule.txt
@@ -282,6 +282,13 @@ Why:	Not used in-tree. The current out-of-tree users used it to
         out-of-tree driver.
 Who:    Thomas Gleixner <tglx@linutronix.de>
 
+----------------------------
+
+What:   usedac i386 kernel parameter
+When:   2.6.27
+Why:    replaced by allowdac and no dac combination
+Who:    Glauber Costa <gcosta@redhat.com>
+
 ---------------------------
 
 What:   /sys/o2cb symlink
@@ -291,3 +298,11 @@ Why:	/sys/fs/o2cb is the proper location for this information - /sys/o2cb
         ocfs2-tools. 2 years should be sufficient time to phase in new versions
         which know to look in /sys/fs/o2cb.
 Who:    ocfs2-devel@oss.oracle.com
+
+---------------------------
+
+What:   asm/semaphore.h
+When:   2.6.26
+Why:    Implementation became generic; users should now include
+        linux/semaphore.h instead.
+Who:    Matthew Wilcox <willy@linux.intel.com>
diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 4b0f1ae31a4c..f4839606988b 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -1280,8 +1280,16 @@ and is between 256 and 4096 characters. It is defined in the file
         noexec          [IA-64]
 
         noexec          [X86-32,X86-64]
+                        On X86-32 available only on PAE configured kernels.
                         noexec=on: enable non-executable mappings (default)
-                        noexec=off: disable nn-executable mappings
+                        noexec=off: disable non-executable mappings
+
+        noexec32        [X86-64]
+                        This affects only 32-bit executables.
+                        noexec32=on: enable non-executable mappings (default)
+                                read doesn't imply executable mappings
+                        noexec32=off: disable non-executable mappings
+                                read implies executable mappings
 
         nofxsr          [BUGS=X86-32] Disables x86 floating point extended
                         register save and restore. The kernel will only save
diff --git a/Documentation/prctl/disable-tsc-ctxt-sw-stress-test.c b/Documentation/prctl/disable-tsc-ctxt-sw-stress-test.c
new file mode 100644
index 000000000000..f8e8e95e81fd
--- /dev/null
+++ b/Documentation/prctl/disable-tsc-ctxt-sw-stress-test.c
@@ -0,0 +1,96 @@
+/*
+ * Tests for prctl(PR_GET_TSC, ...) / prctl(PR_SET_TSC, ...)
+ *
+ * Tests if the control register is updated correctly
+ * at context switches
+ *
+ * Warning: this test will cause a very high load for a few seconds
+ *
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <signal.h>
+#include <inttypes.h>
+#include <wait.h>
+
+
+#include <sys/prctl.h>
+#include <linux/prctl.h>
+
+/* Get/set the process' ability to use the timestamp counter instruction */
+#ifndef PR_GET_TSC
+#define PR_GET_TSC 25
+#define PR_SET_TSC 26
+# define PR_TSC_ENABLE          1   /* allow the use of the timestamp counter */
+# define PR_TSC_SIGSEGV         2   /* throw a SIGSEGV instead of reading the TSC */
+#endif
+
+uint64_t rdtsc() {
+uint32_t lo, hi;
+/* We cannot use "=A", since this would use %rax on x86_64 */
+__asm__ __volatile__ ("rdtsc" : "=a" (lo), "=d" (hi));
+return (uint64_t)hi << 32 | lo;
+}
+
+void sigsegv_expect(int sig)
+{
+        /* */
+}
+
+void segvtask(void)
+{
+        if (prctl(PR_SET_TSC, PR_TSC_SIGSEGV) < 0)
+        {
+                perror("prctl");
+                exit(0);
+        }
+        signal(SIGSEGV, sigsegv_expect);
+        alarm(10);
+        rdtsc();
+        fprintf(stderr, "FATAL ERROR, rdtsc() succeeded while disabled\n");
+        exit(0);
+}
+
+
+void sigsegv_fail(int sig)
+{
+        fprintf(stderr, "FATAL ERROR, rdtsc() failed while enabled\n");
+        exit(0);
+}
+
+void rdtsctask(void)
+{
+        if (prctl(PR_SET_TSC, PR_TSC_ENABLE) < 0)
+        {
+                perror("prctl");
+                exit(0);
+        }
+        signal(SIGSEGV, sigsegv_fail);
+        alarm(10);
+        for(;;) rdtsc();
+}
+
+
+int main(int argc, char **argv)
+{
+        int n_tasks = 100, i;
+
+        fprintf(stderr, "[No further output means we're allright]\n");
+
+        for (i=0; i<n_tasks; i++)
+                if (fork() == 0)
+                {
+                        if (i & 1)
+                                segvtask();
+                        else
+                                rdtsctask();
+                }
+
+        for (i=0; i<n_tasks; i++)
+                wait(NULL);
+
+        exit(0);
+}
+
diff --git a/Documentation/prctl/disable-tsc-on-off-stress-test.c b/Documentation/prctl/disable-tsc-on-off-stress-test.c
new file mode 100644
index 000000000000..1fcd91445375
--- /dev/null
+++ b/Documentation/prctl/disable-tsc-on-off-stress-test.c
@@ -0,0 +1,95 @@
+/*
+ * Tests for prctl(PR_GET_TSC, ...) / prctl(PR_SET_TSC, ...)
+ *
+ * Tests if the control register is updated correctly
+ * when set with prctl()
+ *
+ * Warning: this test will cause a very high load for a few seconds
+ *
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <signal.h>
+#include <inttypes.h>
+#include <wait.h>
+
+
+#include <sys/prctl.h>
+#include <linux/prctl.h>
+
+/* Get/set the process' ability to use the timestamp counter instruction */
+#ifndef PR_GET_TSC
+#define PR_GET_TSC 25
+#define PR_SET_TSC 26
+# define PR_TSC_ENABLE          1   /* allow the use of the timestamp counter */
+# define PR_TSC_SIGSEGV         2   /* throw a SIGSEGV instead of reading the TSC */
+#endif
+
+/* snippet from wikipedia :-) */
+
+uint64_t rdtsc() {
+uint32_t lo, hi;
+/* We cannot use "=A", since this would use %rax on x86_64 */
+__asm__ __volatile__ ("rdtsc" : "=a" (lo), "=d" (hi));
+return (uint64_t)hi << 32 | lo;
+}
+
+int should_segv = 0;
+
+void sigsegv_cb(int sig)
+{
+        if (!should_segv)
+        {
+                fprintf(stderr, "FATAL ERROR, rdtsc() failed while enabled\n");
+                exit(0);
+        }
+        if (prctl(PR_SET_TSC, PR_TSC_ENABLE) < 0)
+        {
+                perror("prctl");
+                exit(0);
+        }
+        should_segv = 0;
+
+        rdtsc();
+}
+
+void task(void)
+{
+        signal(SIGSEGV, sigsegv_cb);
+        alarm(10);
+        for(;;)
+        {
+                rdtsc();
+                if (should_segv)
+                {
+                        fprintf(stderr, "FATAL ERROR, rdtsc() succeeded while disabled\n");
+                        exit(0);
+                }
+                if (prctl(PR_SET_TSC, PR_TSC_SIGSEGV) < 0)
+                {
+                        perror("prctl");
+                        exit(0);
+                }
+                should_segv = 1;
+        }
+}
+
+
+int main(int argc, char **argv)
+{
+        int n_tasks = 100, i;
+
+        fprintf(stderr, "[No further output means we're allright]\n");
+
+        for (i=0; i<n_tasks; i++)
+                if (fork() == 0)
+                        task();
+
+        for (i=0; i<n_tasks; i++)
+                wait(NULL);
+
+        exit(0);
+}
+
diff --git a/Documentation/prctl/disable-tsc-test.c b/Documentation/prctl/disable-tsc-test.c
new file mode 100644
index 000000000000..843c81eac235
--- /dev/null
+++ b/Documentation/prctl/disable-tsc-test.c
@@ -0,0 +1,94 @@
+/*
+ * Tests for prctl(PR_GET_TSC, ...) / prctl(PR_SET_TSC, ...)
+ *
+ * Basic test to test behaviour of PR_GET_TSC and PR_SET_TSC
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <signal.h>
+#include <inttypes.h>
+
+
+#include <sys/prctl.h>
+#include <linux/prctl.h>
+
+/* Get/set the process' ability to use the timestamp counter instruction */
+#ifndef PR_GET_TSC
+#define PR_GET_TSC 25
+#define PR_SET_TSC 26
+# define PR_TSC_ENABLE          1   /* allow the use of the timestamp counter */
+# define PR_TSC_SIGSEGV         2   /* throw a SIGSEGV instead of reading the TSC */
+#endif
+
+const char *tsc_names[] =
+{
+        [0] = "[not set]",
+        [PR_TSC_ENABLE] = "PR_TSC_ENABLE",
+        [PR_TSC_SIGSEGV] = "PR_TSC_SIGSEGV",
+};
+
+uint64_t rdtsc() {
+uint32_t lo, hi;
+/* We cannot use "=A", since this would use %rax on x86_64 */
+__asm__ __volatile__ ("rdtsc" : "=a" (lo), "=d" (hi));
+return (uint64_t)hi << 32 | lo;
+}
+
+void sigsegv_cb(int sig)
+{
+        int tsc_val = 0;
+
+        printf("[ SIG_SEGV ]\n");
+        printf("prctl(PR_GET_TSC, &tsc_val); ");
+        fflush(stdout);
+
+        if ( prctl(PR_GET_TSC, &tsc_val) == -1)
+                perror("prctl");
+
+        printf("tsc_val == %s\n", tsc_names[tsc_val]);
+        printf("prctl(PR_SET_TSC, PR_TSC_ENABLE)\n");
+        fflush(stdout);
+        if ( prctl(PR_SET_TSC, PR_TSC_ENABLE) == -1)
+                perror("prctl");
+
+        printf("rdtsc() == ");
+}
+
+int main(int argc, char **argv)
+{
+        int tsc_val = 0;
+
+        signal(SIGSEGV, sigsegv_cb);
+
+        printf("rdtsc() == %llu\n", (unsigned long long)rdtsc());
+        printf("prctl(PR_GET_TSC, &tsc_val); ");
+        fflush(stdout);
+
+        if ( prctl(PR_GET_TSC, &tsc_val) == -1)
+                perror("prctl");
+
+        printf("tsc_val == %s\n", tsc_names[tsc_val]);
+        printf("rdtsc() == %llu\n", (unsigned long long)rdtsc());
+        printf("prctl(PR_SET_TSC, PR_TSC_ENABLE)\n");
+        fflush(stdout);
+
+        if ( prctl(PR_SET_TSC, PR_TSC_ENABLE) == -1)
+                perror("prctl");
+
+        printf("rdtsc() == %llu\n", (unsigned long long)rdtsc());
+        printf("prctl(PR_SET_TSC, PR_TSC_SIGSEGV)\n");
+        fflush(stdout);
+
+        if ( prctl(PR_SET_TSC, PR_TSC_SIGSEGV) == -1)
+                perror("prctl");
+
+        printf("rdtsc() == ");
+        fflush(stdout);
+        printf("%llu\n", (unsigned long long)rdtsc());
+        fflush(stdout);
+
+        exit(EXIT_SUCCESS);
+}
+
diff --git a/Documentation/scheduler/sched-rt-group.txt b/Documentation/scheduler/sched-rt-group.txt
index 1c6332f4543c..14f901f639ee 100644
--- a/Documentation/scheduler/sched-rt-group.txt
+++ b/Documentation/scheduler/sched-rt-group.txt
@@ -1,59 +1,177 @@
+                                Real-Time group scheduling
+                                --------------------------
 
+CONTENTS
+========
 
-Real-Time group scheduling.
+1. Overview
+  1.1 The problem
+  1.2 The solution
+2. The interface
+  2.1 System-wide settings
+  2.2 Default behaviour
+  2.3 Basis for grouping tasks
+3. Future plans
 
-The problem space:
 
-In order to schedule multiple groups of realtime tasks each group must
-be assigned a fixed portion of the CPU time available. Without a minimum
-guarantee a realtime group can obviously fall short. A fuzzy upper limit
-is of no use since it cannot be relied upon. Which leaves us with just
-the single fixed portion.
+1. Overview
+===========
 
-CPU time is divided by means of specifying how much time can be spent
-running in a given period. Say a frame fixed realtime renderer must
-deliver 25 frames a second, which yields a period of 0.04s. Now say
-it will also have to play some music and respond to input, leaving it
-with around 80% for the graphics. We can then give this group a runtime
-of 0.8 * 0.04s = 0.032s.
 
-This way the graphics group will have a 0.04s period with a 0.032s runtime
-limit.
+1.1 The problem
+---------------
 
-Now if the audio thread needs to refill the DMA buffer every 0.005s, but
-needs only about 3% CPU time to do so, it can do with a 0.03 * 0.005s
-= 0.00015s.
+Realtime scheduling is all about determinism, a group has to be able to rely on
+the amount of bandwidth (eg. CPU time) being constant. In order to schedule
+multiple groups of realtime tasks, each group must be assigned a fixed portion
+of the CPU time available.  Without a minimum guarantee a realtime group can
+obviously fall short. A fuzzy upper limit is of no use since it cannot be
+relied upon. Which leaves us with just the single fixed portion.
 
+1.2 The solution
+----------------
 
-The Interface:
+CPU time is divided by means of specifying how much time can be spent running
+in a given period. We allocate this "run time" for each realtime group which
+the other realtime groups will not be permitted to use.
 
-system wide:
+Any time not allocated to a realtime group will be used to run normal priority
+tasks (SCHED_OTHER). Any allocated run time not used will also be picked up by
+SCHED_OTHER.
 
-/proc/sys/kernel/sched_rt_period_ms
-/proc/sys/kernel/sched_rt_runtime_us
+Let's consider an example: a frame fixed realtime renderer must deliver 25
+frames a second, which yields a period of 0.04s per frame. Now say it will also
+have to play some music and respond to input, leaving it with around 80% CPU
+time dedicated for the graphics. We can then give this group a run time of 0.8
+* 0.04s = 0.032s.
 
-CONFIG_FAIR_USER_SCHED
+This way the graphics group will have a 0.04s period with a 0.032s run time
+limit. Now if the audio thread needs to refill the DMA buffer every 0.005s, but
+needs only about 3% CPU time to do so, it can do with a 0.03 * 0.005s =
+0.00015s. So this group can be scheduled with a period of 0.005s and a run time
+of 0.00015s.
 
-/sys/kernel/uids/<uid>/cpu_rt_runtime_us
+The remaining CPU time will be used for user input and other tass. Because
+realtime tasks have explicitly allocated the CPU time they need to perform
+their tasks, buffer underruns in the graphocs or audio can be eliminated.
 
-or
+NOTE: the above example is not fully implemented as of yet (2.6.25). We still
+lack an EDF scheduler to make non-uniform periods usable.
 
-CONFIG_FAIR_CGROUP_SCHED
 
-/cgroup/<cgroup>/cpu.rt_runtime_us
+2. The Interface
+================
 
-[ time is specified in us because the interface is s32; this gives an
-  operating range of ~35m to 1us ]
 
-The period takes values in [ 1, INT_MAX ], runtime in [ -1, INT_MAX - 1 ].
+2.1 System wide settings
+------------------------
 
-A runtime of -1 specifies runtime == period, ie. no limit.
+The system wide settings are configured under the /proc virtual file system:
 
-New groups get the period from /proc/sys/kernel/sched_rt_period_us and
-a runtime of 0.
+/proc/sys/kernel/sched_rt_period_us:
+  The scheduling period that is equivalent to 100% CPU bandwidth
 
-Settings are constrained to:
+/proc/sys/kernel/sched_rt_runtime_us:
+  A global limit on how much time realtime scheduling may use.  Even without
+  CONFIG_RT_GROUP_SCHED enabled, this will limit time reserved to realtime
+  processes. With CONFIG_RT_GROUP_SCHED it signifies the total bandwidth
+  available to all realtime groups.
+
+  * Time is specified in us because the interface is s32. This gives an
+    operating range from 1us to about 35 minutes.
+  * sched_rt_period_us takes values from 1 to INT_MAX.
+  * sched_rt_runtime_us takes values from -1 to (INT_MAX - 1).
+  * A run time of -1 specifies runtime == period, ie. no limit.
+
+
+2.2 Default behaviour
+---------------------
+
+The default values for sched_rt_period_us (1000000 or 1s) and
+sched_rt_runtime_us (950000 or 0.95s).  This gives 0.05s to be used by
+SCHED_OTHER (non-RT tasks). These defaults were chosen so that a run-away
+realtime tasks will not lock up the machine but leave a little time to recover
+it.  By setting runtime to -1 you'd get the old behaviour back.
+
+By default all bandwidth is assigned to the root group and new groups get the
+period from /proc/sys/kernel/sched_rt_period_us and a run time of 0. If you
+want to assign bandwidth to another group, reduce the root group's bandwidth
+and assign some or all of the difference to another group.
+
+Realtime group scheduling means you have to assign a portion of total CPU
+bandwidth to the group before it will accept realtime tasks. Therefore you will
+not be able to run realtime tasks as any user other than root until you have
+done that, even if the user has the rights to run processes with realtime
+priority!
+
+
+2.3 Basis for grouping tasks
+----------------------------
+
+There are two compile-time settings for allocating CPU bandwidth. These are
+configured using the "Basis for grouping tasks" multiple choice menu under
+General setup > Group CPU Scheduler:
+
+a. CONFIG_USER_SCHED (aka "Basis for grouping tasks" =  "user id")
+
+This lets you use the virtual files under
+"/sys/kernel/uids/<uid>/cpu_rt_runtime_us" to control he CPU time reserved for
+each user .
+
+The other option is:
+
+.o CONFIG_CGROUP_SCHED (aka "Basis for grouping tasks" = "Control groups")
+
+This uses the /cgroup virtual file system and "/cgroup/<cgroup>/cpu.rt_runtime_us"
+to control the CPU time reserved for each control group instead.
+
+For more information on working with control groups, you should read
+Documentation/cgroups.txt as well.
+
+Group settings are checked against the following limits in order to keep the configuration
+schedulable:
 
    \Sum_{i} runtime_{i} / global_period <= global_runtime / global_period
 
-in order to keep the configuration schedulable.
+For now, this can be simplified to just the following (but see Future plans):
+
+   \Sum_{i} runtime_{i} <= global_runtime
+
+
+3. Future plans
+===============
+
+There is work in progress to make the scheduling period for each group
+("/sys/kernel/uids/<uid>/cpu_rt_period_us" or
+"/cgroup/<cgroup>/cpu.rt_period_us" respectively) configurable as well.
+
+The constraint on the period is that a subgroup must have a smaller or
+equal period to its parent. But realistically its not very useful _yet_
+as its prone to starvation without deadline scheduling.
+
+Consider two sibling groups A and B; both have 50% bandwidth, but A's
+period is twice the length of B's.
+
+* group A: period=100000us, runtime=10000us
+        - this runs for 0.01s once every 0.1s
+
+* group B: period= 50000us, runtime=10000us
+        - this runs for 0.01s twice every 0.1s (or once every 0.05 sec).
+
+This means that currently a while (1) loop in A will run for the full period of
+B and can starve B's tasks (assuming they are of lower priority) for a whole
+period.
+
+The next project will be SCHED_EDF (Earliest Deadline First scheduling) to bring
+full deadline scheduling to the linux kernel. Deadline scheduling the above
+groups and treating end of the period as a deadline will ensure that they both
+get their allocated time.
+
+Implementing SCHED_EDF might take a while to complete. Priority Inheritance is
+the biggest challenge as the current linux PI infrastructure is geared towards
+the limited static priority levels 0-139. With deadline scheduling you need to
+do deadline inheritance (since priority is inversely proportional to the
+deadline delta (deadline - now).
+
+This means the whole PI machinery will have to be reworked - and that is one of
+the most complex pieces of code we have.