[dpdk-dev] [PATCH 0/3] lib/rcu: add RCU library supporting QSBR mechanism

Honnappa Nagarahalli honnappa.nagarahalli at arm.com
Tue Mar 19 05:52:25 CET 2019

Lock-less data structures provide scalability and determinism.
They enable use cases where locking may not be allowed
(for ex: real-time applications).

In the following paras, the term 'memory' refers to memory allocated
by typical APIs like malloc or anything that is representative of
memory, for ex: an index of a free element array.

Since these data structures are lock less, the writers and readers
are accessing the data structures concurrently. Hence, while removing
an element from a data structure, the writers cannot return the memory
to the allocator, without knowing that the readers are not
referencing that element/memory anymore. Hence, it is required to
separate the operation of removing an element into 2 steps:

Delete: in this step, the writer removes the reference to the element from
the data structure but does not return the associated memory to the
allocator. This will ensure that new readers will not get a reference to
the removed element. Removing the reference is an atomic operation.

Free(Reclaim): in this step, the writer returns the memory to the
memory allocator, only after knowing that all the readers have stopped
referencing the deleted element.

This library helps the writer determine when it is safe to free the

This library makes use of thread Quiescent State (QS). QS can be
defined as 'any point in the thread execution where the thread does
not hold a reference to shared memory'. It is upto the application to
determine its quiescent state. Let us consider the following diagram:

    Time -------------------------------------------------->

                                |     |
  RT1   $++++****D1****+++***D2*|**+++|+++**D3*****++++$
                                |     |
  RT2          $++++****D1****++|+**D2|***++++++**D3*****++++$
                                |     |
  RT3      $++++****D1****+++***|D2***|++++++**D2*****++++$
                                |     |
                               Del | Free
                              Cannot free memory
                              during this period
                              (Grace Period)

RTx - Reader thread
< and > - Start and end of while(1) loop
***Dx*** - Reader thread is accessing the shared data structure Dx.
           i.e. critical section.
+++ - Reader thread is not accessing any shared data structure.
      i.e. non critical section or quiescent state.
Del - Point in time when the reference to the entry is removed using
      atomic operation.
Free - Point in time when the writer can free the entry.
Grace Period - Time duration between Del and Free, during which memory cannot
               be freed.

As shown, thread RT1 acesses data structures D1, D2 and D3. When it is
accessing D2, if the writer has to remove an element from D2, the
writer cannot free the memory associated with that element immediately.
The writer can return the memory to the allocator only after the reader
stops referencng D2. In other words, reader thread RT1 has to enter
a quiescent state.

Similarly, since thread RT3 is also accessing D2, writer has to wait till
RT3 enters quiescent state as well.

However, the writer does not need to wait for RT2 to enter quiescent state.
Thread RT2 was not accessing D2 when the delete operation happened.
So, RT2 will not get a reference to the deleted entry.

It can be noted that, the critical sections for D2 and D3 are quiescent states
for D1. i.e. for a given data structure Dx, any point in the thread execution
that does not reference Dx is a quiescent state.

Since memory is not freed immediately, there might be a need for
provisioning of additional memory, depending on the application requirements.

It is important to make sure that this library keeps the over head of
identifying the end of grace period and subsequent freeing of memory,
to a minimum. The following paras explain how grace period and critical
section affect this overhead.

The writer has to poll the readers to identify the end of grace period.
Polling introduces memory accesses and wastes CPU cycles. The memory
is not available for reuse during grace period. Longer grace periods
exasperate these conditions.

The length of the critical section and the number of reader threads
is proportional to the duration of the grace period. Keeping the critical
sections smaller will keep the grace period smaller. However, keeping the
critical sections smaller requires additional CPU cycles(due to additional
reporting) in the readers.

Hence, we need the characteristics of small grace period and large critical
section. This library addresses this by allowing the writer to do
other work without having to block till the readers report their quiescent

For DPDK applications, the start and end of while(1) loop (where no
references to shared data structures are kept) act as perfect quiescent
states. This will combine all the shared data structure accesses into a
single, large critical section which helps keep the over head on the
reader side to a minimum.

DPDK supports pipeline model of packet processing and service cores.
In these use cases, a given data structure may not be used by all the
workers in the application. The writer does not have to wait for all
the workers to report their quiescent state. To provide the required
flexibility, this library has a concept of QS variable. The application
can create one QS variable per data structure to help it track the
end of grace period for each data structure. This helps keep the grace
period to a minimum.

The application has to allocate memory and initialize a QS variable.

Application can call rte_rcu_qsbr_get_memsize to calculate the size
of memory to allocate. This API takes maximum number of reader threads,
using this variable, as a parameter. Currently, a maximum of 1024 threads
are supported.

Further, the application can initialize a QS variable using the API

Each reader thread is assumed to have a unique thread ID. Currently, the
management of the thread ID (for ex: allocation/free) is left to the
application. The thread ID should be in the range of 0 to
maximum number of threads provided while creating the QS variable.
The application could also use lcore_id as the thread ID where applicable.

rte_rcu_qsbr_thread_register API will register a reader thread
to report its quiescent state. This can be called from a reader thread.
A control plane thread can also call this on behalf of a reader thread.
The reader thread must call rte_rcu_qsbr_thread_online API to start reporting
its quiescent state.

Some of the use cases might require the reader threads to make
blocking API calls (for ex: while using eventdev APIs). The writer thread
should not wait for such reader threads to enter quiescent state.
The reader thread must call rte_rcu_qsbr_thread_offline API, before calling
blocking APIs. It can call rte_rcu_qsbr_thread_online API once the blocking
API call returns.

The writer thread can trigger the reader threads to report their quiescent
state by calling the API rte_rcu_qsbr_start. It is possible for multiple
writer threads to query the quiescent state status simultaneously. Hence,
rte_rcu_qsbr_start returns a token to each caller.

The writer thread has to call rte_rcu_qsbr_check API with the token to get the
current quiescent state status. Option to block till all the reader threads
enter the quiescent state is provided. If this API indicates that all the
reader threads have entered the quiescent state, the application can free the
deleted entry.

The APIs rte_rcu_qsbr_start and rte_rcu_qsbr_check are lock free. Hence, they
can be called concurrently from multiple writers even while running
as worker threads.

The separation of triggering the reporting from querying the status provides
the writer threads flexibility to do useful work instead of blocking for the
reader threads to enter the quiescent state or go offline. This reduces the
memory accesses due to continuous polling for the status.

rte_rcu_qsbr_synchronize API combines the functionality of rte_rcu_qsbr_start
and blocking rte_rcu_qsbr_check into a single API. This API triggers the reader
threads to report their quiescent state and polls till all the readers enter
the quiescent state or go offline. This API does not allow the writer to
do useful work while waiting and also introduces additional memory accesses
due to continuous polling.

The reader thread must call rte_rcu_qsbr_thread_offline and
rte_rcu_qsbr_thread_unregister APIs to remove itself from reporting its
quiescent state. The rte_rcu_qsbr_check API will not wait for this reader
thread to report the quiescent state status anymore.

The reader threads should call rte_rcu_qsbr_update API to indicate that they
entered a quiescent state. This API checks if a writer has triggered a
quiescent state query and update the state accordingly.

1) Library changes
   a) Changed the maximum number of reader threads to 1024
   b) Renamed rte_rcu_qsbr_register/unregister_thread to
   c) Added rte_rcu_qsbr_thread_online/offline API. These are optimized
      version of rte_rcu_qsbr_thread_register/unregister API. These
      also provide the flexibility for performance when the requested
      maximum number of threads is higher than the current number of
   d) Corrected memory orderings in rte_rcu_qsbr_update
   e) Changed the signature of rte_rcu_qsbr_start API to return the token
   f) Changed the signature of rte_rcu_qsbr_start API to not take the
      expected number of QS states to wait.
   g) Added debug logs
   h) Added API and programmer guide documentation.

RFC v3:
1) Library changes
   a) Rebased to latest master
   b) Added new API rte_rcu_qsbr_get_memsize
   c) Add support for memory allocation for QSBR variable (Konstantin)
   d) Fixed a bug in rte_rcu_qsbr_check (Konstantin)
2) Testcase changes
   a) Separated stress tests into a performance test case file
   b) Added performance statistics

RFC v2:
1) Cover letter changes
   a) Explian the parameters that affect the overhead of using RCU
      and their effect
   b) Explain how this library addresses these effects to keep
      the overhead to minimum
2) Library changes
   a) Rename the library to avoid confusion (Matias, Bruce, Konstantin)
   b) Simplify the code/remove APIs to keep this library inline with
      other synchronisation mechanisms like locks (Konstantin)
   c) Change the design to support more than 64 threads (Konstantin)
   d) Fixed version map to remove static inline functions
3) Testcase changes
   a) Add boundary and additional functional test cases
   b) Add stress test cases (Paul E. McKenney)

Next Steps:
1) Update the cover letter to indicate the addition of rte_rcu_get_memsize
2) rte_rcu_qsbr_register_thread/rte_rcu_qsbr_unregister_thread can be
   optimized to avoid accessing the common bitmap array. This is required
   as these are data plane APIs. Plan is to introduce
   rte_rcu_qsbr_thread_online/rte_rcu_qsbr_thread_offline which will not
   touch the common bitmap array.
3) Add debug logs to enable debugging
4) Documentation
5) Convert to patch

Dharmik Thakkar (1):
  test/rcu_qsbr: add API and functional tests

Honnappa Nagarahalli (2):
  rcu: add RCU library supporting QSBR mechanism
  doc/rcu: add lib_rcu documentation

 MAINTAINERS                                   |   5 +
 app/test/Makefile                             |   2 +
 app/test/autotest_data.py                     |  12 +
 app/test/meson.build                          |   7 +-
 app/test/test_rcu_qsbr.c                      | 927 ++++++++++++++++++
 app/test/test_rcu_qsbr_perf.c                 | 615 ++++++++++++
 config/common_base                            |   6 +
 doc/api/doxy-api-index.md                     |   3 +-
 doc/api/doxy-api.conf.in                      |   1 +
 .../prog_guide/img/rcu_general_info.svg       | 494 ++++++++++
 doc/guides/prog_guide/index.rst               |   1 +
 doc/guides/prog_guide/rcu_lib.rst             | 179 ++++
 lib/Makefile                                  |   2 +
 lib/librte_rcu/Makefile                       |  23 +
 lib/librte_rcu/meson.build                    |   5 +
 lib/librte_rcu/rte_rcu_qsbr.c                 |  99 ++
 lib/librte_rcu/rte_rcu_qsbr.h                 | 511 ++++++++++
 lib/librte_rcu/rte_rcu_version.map            |   9 +
 lib/meson.build                               |   2 +-
 mk/rte.app.mk                                 |   1 +
 20 files changed, 2901 insertions(+), 3 deletions(-)
 create mode 100644 app/test/test_rcu_qsbr.c
 create mode 100644 app/test/test_rcu_qsbr_perf.c
 create mode 100644 doc/guides/prog_guide/img/rcu_general_info.svg
 create mode 100644 doc/guides/prog_guide/rcu_lib.rst
 create mode 100644 lib/librte_rcu/Makefile
 create mode 100644 lib/librte_rcu/meson.build
 create mode 100644 lib/librte_rcu/rte_rcu_qsbr.c
 create mode 100644 lib/librte_rcu/rte_rcu_qsbr.h
 create mode 100644 lib/librte_rcu/rte_rcu_version.map


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