[PATCH 03/11] x86/intel_rdt: Cache Allocation documentation

[Vikas Shivappa]

Adds a description of Cache allocation technology, overview of kernel
framework implementation. The framework has APIs to manage class of
service, capacity bitmask(CBM), scheduling support and other
architecture specific implementation. The APIs are used to build the
cgroup interface in later patches.

Cache allocation is a sub-feature of Resource Director Technology (RDT)
or Platform Shared resource control which provides support to control
Platform shared resources like L3 cache.

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 tasks are
grouped into CLOS (class of service). OS uses MSR writes to indicate the
CLOSid of the thread when scheduling in and to indicate the cache
capacity associated with the CLOSid. Currently cache allocation is
supported for L3 cache.

More information can be found in the Intel SDM June 2015, Volume 3,
section 17.16.

Signed-off-by: Vikas Shivappa <vikas.shivappa@xxxxxxxxxxxxxxx>
---
 Documentation/x86/intel_rdt.txt | 109 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 109 insertions(+)
 create mode 100644 Documentation/x86/intel_rdt.txt

diff --git a/Documentation/x86/intel_rdt.txt b/Documentation/x86/intel_rdt.txt
new file mode 100644
index 0000000..05ec819
--- /dev/null
+++ b/Documentation/x86/intel_rdt.txt
@@ -0,0 +1,109 @@
+        Intel RDT
+        ---------
+
+Copyright (C) 2014 Intel Corporation
+Written by vikas.shivappa@xxxxxxxxxxxxxxx
+
+CONTENTS:
+=========
+
+1. Cache Allocation Technology
+  1.1 What is RDT and Cache allocation ?
+  1.2 Why is Cache allocation needed ?
+  1.3 Cache allocation implementation overview
+  1.4 Assignment of CBM and CLOS
+  1.5 Scheduling and Context Switch
+
+1. Cache Allocation Technology
+===================================
+
+1.1 What is RDT and Cache allocation
+------------------------------------
+
+Cache allocation is a sub-feature of Resource Director Technology (RDT)
+Allocation or Platform Shared resource control which provides support to
+control Platform shared resources like L3 cache. Currently L3 Cache is
+the only resource that is supported in RDT. More information can be
+found in the Intel SDM June 2015, Volume 3, section 17.16.
+
+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 Cache allocation needed
+----------------------------------
+
+In todays new processors the number of cores is continuously increasing
+especially in large scale usage models where VMs are used like
+webservers and datacenters. The number of cores increase the number of
+threads or workloads that can simultaneously be run. When
+multi-threaded-applications, VMs, workloads run concurrently they
+compete for shared resources including L3 cache.
+
+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.
+
+This technique may be useful in managing large computer server systems
+with large L3 cache, in the cloud and container context. 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 by a centralized root accessible
+interface.
+
+A specific use case may be to solve the noisy neighbour issue when a app
+which is constantly copying data like streaming app is using large
+amount of cache which could have otherwise been used by a high priority
+computing application. Using the cache allocation feature, the streaming
+application can be confined to use a smaller cache and the high priority
+application be awarded a larger amount of cache space.
+
+1.3 Cache allocation implementation Overview
+--------------------------------------------
+
+Kernel has a new field in the task_struct called 'closid' which
+represents the Class of service ID of the task.
+
+There is a 1:1 CLOSid <-> CBM (capacity bit mask) mapping. A CLOS (Class
+of service) is represented by a CLOSid. Each closid would have one CBM
+and would just represent one cache 'subset'.  The tasks would get to
+fill the L3 cache represented by the capacity bit mask or CBM.
+
+The APIs to manage the closid and CBM can be used to develop user
+interfaces.
+
+1.4 Assignment of CBM, CLOS
+---------------------------
+
+The framework provides APIs to manage the closid and CBM which can be
+used to develop user/kernel mode interfaces.
+
+1.5 Scheduling and Context Switch
+---------------------------------
+
+During context switch kernel implements this by writing the CLOSid of
+the task 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.
+
+The following considerations are done for the PQR MSR write so that it
+has minimal impact on scheduling hot path:
+ - This path doesn't exist on any non-intel platforms.
+ - On Intel platforms, this would not exist by default unless INTEL_RDT
+ is enabled.
+ - remains a no-op when INTEL_RDT is enabled and intel hardware does
+ not support the feature.
+ - When feature is available, does not do MSR write till the user
+ starts using the feature *and* assigns a new cache capacity mask.
+ - per cpu PQR values are cached and the MSR write is only done when
+ there is a task with different PQR is scheduled on the CPU. Typically
+ if the task groups are bound to be scheduled on a set of CPUs, the
+ number of MSR writes is greatly reduced.
-- 
1.9.1

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