Message-Id: <1424819804-4082-8-git-send-email-vikas.shivappa@linux.intel.com>
Date:	Tue, 24 Feb 2015 15:16:44 -0800
From:	Vikas Shivappa <vikas.shivappa@...ux.intel.com>
To:	linux-kernel@...r.kernel.org
Cc:	vikas.shivappa@...el.com, vikas.shivappa@...ux.intel.com,
	matt.fleming@...el.com, hpa@...or.com, tglx@...utronix.de,
	mingo@...nel.org, tj@...nel.org, peterz@...radead.org,
	will.auld@...el.com, dave.hansen@...el.com, andi.kleen@...el.com,
	tony.luck@...el.com, kanaka.d.juvva@...el.com
Subject: [PATCH 7/7] x86/intel_rdt: Add CAT documentation and usage guide

This patch adds a description of Cache allocation technology, overview
of kernel implementation and usage of CAT cgroup interface.

Signed-off-by: Vikas Shivappa <vikas.shivappa@...ux.intel.com>
---
 Documentation/cgroups/rdt.txt | 184 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 184 insertions(+)
 create mode 100644 Documentation/cgroups/rdt.txt

diff --git a/Documentation/cgroups/rdt.txt b/Documentation/cgroups/rdt.txt
new file mode 100644
index 0000000..9f9e445
--- /dev/null
+++ b/Documentation/cgroups/rdt.txt
@@ -0,0 +1,184 @@
+        RDT
+        ---
+
+Copyright (C) 2014 Intel Corporation
+Written by vikas.shivappa@...ux.intel.com
+(based on contents and format from cpusets.txt)
+
+CONTENTS:
+=========
+
+1. Cache Allocation Technology
+  1.1 What is RDT and CAT ?
+  1.2 Why is CAT needed ?
+  1.3 CAT implementation overview
+  1.4 Assignment of CBM and CLOS
+  1.5 Scheduling and Context Switch
+2. Usage Examples and Syntax
+
+1. Cache Allocation Technology(CAT)
+===================================
+
+1.1 What is RDT and CAT
+-----------------------
+
+CAT is a part of Resource Director Technology(RDT) or Platform Shared 
+resource control which provides support to control Platform shared 
+resources like cache. Currently Cache is the only resource that is 
+supported in RDT.
+
+Cache Allocation Technology provides a way for the Software (OS/VMM)
+to restrict cache allocation to a defined 'subset' of cache which may 
+be overlapping with other 'subsets'.  This feature is used when
+allocating a line in cache ie when pulling new data into the cache.
+The programming of the h/w is done via programming  MSRs.
+
+The different cache subsets are identified by CLOS identifier (class 
+of service) and each CLOS has a CBM (cache bit mask).  The CBM is a 
+contiguous set of bits which defines the amount of cache resource that 
+is available for each 'subset'.
+
+1.2 Why is CAT needed
+---------------------
+
+The CAT  enables more cache resources to be made available for higher
+priority applications based on guidance from the execution
+environment.  
+
+The architecture also allows dynamically changing these subsets during
+runtime to further optimize the performance of the higher priority
+application with minimal degradation to the low priority app.
+Additionally, resources can be rebalanced for system throughput
+benefit.  (Refer to Section 17.15 in the Intel SDM)
+
+This technique may be useful in managing large computer systems which
+large LLC. Examples may be large servers running  instances of
+webservers or database servers. In such complex systems, these subsets
+can be used for more careful placing of the available cache
+resources.
+
+The CAT kernel patch would provide a basic kernel framework for users
+to be able to implement such cache subsets. 
+
+1.3 CAT implementation Overview
+-------------------------------
+
+Kernel implements a cgroup subsystem to support cache allocation 
+
+Each cgroup has a CLOSid <-> CBM(cache bit mask) mapping.
+A CLOS(Class of service) is represented by a CLOSid.CLOSid is internal 
+to the kernel and not exposed to user.  Each cgroup would have one CBM 
+and would just represent one cache 'subset'.
+
+The cgroup follows cgroup hierarchy ,mkdir and adding tasks to the 
+cgroup never fails.  When a child cgroup is created it inherits the 
+CLOSid and the CBM from its parent.  When a user changes the default 
+CBM for a cgroup, a new CLOSid may be allocated if the CBM was not 
+used before.  The changing of 'cbm' may fail with -ERRNOSPC once the 
+kernel runs out of maximum CLOSids it can support.
+User can create as many cgroups as he wants but having different CBMs 
+at the same time is restricted by the maximum number of CLOSids 
+(multiple cgroups can have the same CBM).
+Kernel maintains a CLOSid<->cbm mapping which keeps reference counter 
+for each cgroup using a CLOSid.
+
+The tasks in the cgroup would get to fill the LLC cache represented by 
+the cgroup's 'cbm' file.
+
+Root directory would have all available  bits set in 'cbm' file by 
+default.
+
+1.4 Assignment of CBM,CLOS
+--------------------------
+
+The 'cbm' needs to be a  subset of the parent node's 'cbm'.
+Any contiguous subset of these bits(with a minimum of 2 bits) maybe 
+set to indicate the cache mapping desired.  The 'cbm' between 2 
+directories can overlap. The 'cbm' would represent the cache 'subset' 
+of the CAT cgroup. 
+For ex: on a system with 16 bits of max cbm bits,
+if the directory has the least significant 4 bits set in its 'cbm' 
+file(meaning the 'cbm' is just 0xf), it
+would be allocated the right quarter of the Last level cache which
+means the tasks belonging to this CAT cgroup can use the right quarter
+of the cache to fill. If it has the most significant 8 bits set ,it
+would be allocated the left half of the cache(8 bits  out of 16
+represents 50%).
+
+The cache portion defined in the CBM file is available to all tasks 
+within the cgroup to fill and these task are not allowed to allocate 
+space in other parts of the cache. 
+
+1.5 Scheduling and Context Switch
+---------------------------------
+
+During context switch kernel implements this by writing the
+CLOSid (internally maintained by kernel) of the cgroup to which the task 
+belongs to the CPU's IA32_PQR_ASSOC MSR. The MSR is only written when 
+there is a change in the CLOSid for the CPU in order to minimize the 
+latency incurred during context switch.
+
+2. Usage examples and syntax
+============================
+
+To check if CAT was enabled on your system 
+
+dmesg | grep -i intel_rdt
+should output : intel_rdt: cbmlength:xx, Closs:xx
+the length of cbm and CLOS should depend on the system you use.
+
+
+Following would mount the cache allocation cgroup subsystem and create
+2 directories. Please refer to Documentation/cgroups/cgroups.txt on
+details about how to use cgroups.
+
+  cd /sys/fs/cgroup 
+  mkdir rdt
+  mount -t cgroup -ordt rdt /sys/fs/cgroup/rdt 
+  cd rdt
+
+Create 2 rdt cgroups 
+
+  mkdir group1 
+  mkdir group2
+
+Following are some of the Files in the directory
+
+  ls 
+  rdt.cbm 
+  tasks 
+
+Say if the cache is 2MB and cbm supports 16 bits, then setting the
+below allocates the 'right 1/4th(512KB)' of the cache to group2 
+
+Edit the CBM for group2 to set the least significant 4 bits.  This
+allocates 'right quarter' of the cache. 
+
+  cd group2 
+  /bin/echo 0xf > cat.cbm 
+
+
+Edit the CBM for group2 to set the least significant 8 bits.This
+allocates the right half of the cache to 'group2'.
+
+  cd group2 
+  /bin/echo 0xff > rdt.cbm 
+
+Assign tasks to the group2
+  
+  /bin/echo PID1 > tasks 
+  /bin/echo PID2 > tasks 
+
+  Meaning now threads
+  PID1 and PID2 get to fill the 'right half' of
+  the cache as the belong to cgroup group2. 
+
+Create a group under group2
+
+  cd group2 
+  mkdir group21
+  cat rdt.cbm 
+   0xff - inherits parents mask.
+
+  /bin/echo 0xfff > rdt.cbm - throws error as mask has to parent's mask's subset
+
-- 
1.9.1

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