Re: [Xen-devel] [PATCH v8 00/10] qspinlock: a 4-byte queue spinlock with PV support

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, Apr 02, 2014 at 09:27:29AM -0400, Waiman Long wrote:
> N.B. Sorry for the duplicate. This patch series were resent as the
>      original one was rejected by the vger.kernel.org list server
>      due to long header. There is no change in content.
> 
> v7->v8:
>   - Remove one unneeded atomic operation from the slowpath, thus
>     improving performance.
>   - Simplify some of the codes and add more comments.
>   - Test for X86_FEATURE_HYPERVISOR CPU feature bit to enable/disable
>     unfair lock.
>   - Reduce unfair lock slowpath lock stealing frequency depending
>     on its distance from the queue head.
>   - Add performance data for IvyBridge-EX CPU.

FYI, your v7 patch with 32 VCPUs (on a 32 cpu socket machine) on an
HVM guest under Xen after a while stops working. The workload
is doing 'make -j32' on the Linux kernel.

Completely unresponsive. Thoughts?

(CC ing Marcos who had run the test)
> 
> v6->v7:
>   - Remove an atomic operation from the 2-task contending code
>   - Shorten the names of some macros
>   - Make the queue waiter to attempt to steal lock when unfair lock is
>     enabled.
>   - Remove lock holder kick from the PV code and fix a race condition
>   - Run the unfair lock & PV code on overcommitted KVM guests to collect
>     performance data.
> 
> v5->v6:
>  - Change the optimized 2-task contending code to make it fairer at the
>    expense of a bit of performance.
>  - Add a patch to support unfair queue spinlock for Xen.
>  - Modify the PV qspinlock code to follow what was done in the PV
>    ticketlock.
>  - Add performance data for the unfair lock as well as the PV
>    support code.
> 
> v4->v5:
>  - Move the optimized 2-task contending code to the generic file to
>    enable more architectures to use it without code duplication.
>  - Address some of the style-related comments by PeterZ.
>  - Allow the use of unfair queue spinlock in a real para-virtualized
>    execution environment.
>  - Add para-virtualization support to the qspinlock code by ensuring
>    that the lock holder and queue head stay alive as much as possible.
> 
> v3->v4:
>  - Remove debugging code and fix a configuration error
>  - Simplify the qspinlock structure and streamline the code to make it
>    perform a bit better
>  - Add an x86 version of asm/qspinlock.h for holding x86 specific
>    optimization.
>  - Add an optimized x86 code path for 2 contending tasks to improve
>    low contention performance.
> 
> v2->v3:
>  - Simplify the code by using numerous mode only without an unfair option.
>  - Use the latest smp_load_acquire()/smp_store_release() barriers.
>  - Move the queue spinlock code to kernel/locking.
>  - Make the use of queue spinlock the default for x86-64 without user
>    configuration.
>  - Additional performance tuning.
> 
> v1->v2:
>  - Add some more comments to document what the code does.
>  - Add a numerous CPU mode to support >= 16K CPUs
>  - Add a configuration option to allow lock stealing which can further
>    improve performance in many cases.
>  - Enable wakeup of queue head CPU at unlock time for non-numerous
>    CPU mode.
> 
> This patch set has 3 different sections:
>  1) Patches 1-4: Introduces a queue-based spinlock implementation that
>     can replace the default ticket spinlock without increasing the
>     size of the spinlock data structure. As a result, critical kernel
>     data structures that embed spinlock won't increase in size and
>     break data alignments.
>  2) Patches 5-6: Enables the use of unfair queue spinlock in a
>     para-virtualized execution environment. This can resolve some
>     of the locking related performance issues due to the fact that
>     the next CPU to get the lock may have been scheduled out for a
>     period of time.
>  3) Patches 7-10: Enable qspinlock para-virtualization support
>     by halting the waiting CPUs after spinning for a certain amount of
>     time. The unlock code will detect the a sleeping waiter and wake it
>     up. This is essentially the same logic as the PV ticketlock code.
> 
> The queue spinlock has slightly better performance than the ticket
> spinlock in uncontended case. Its performance can be much better
> with moderate to heavy contention.  This patch has the potential of
> improving the performance of all the workloads that have moderate to
> heavy spinlock contention.
> 
> The queue spinlock is especially suitable for NUMA machines with at
> least 2 sockets, though noticeable performance benefit probably won't
> show up in machines with less than 4 sockets.
> 
> The purpose of this patch set is not to solve any particular spinlock
> contention problems. Those need to be solved by refactoring the code
> to make more efficient use of the lock or finer granularity ones. The
> main purpose is to make the lock contention problems more tolerable
> until someone can spend the time and effort to fix them.
> 
> To illustrate the performance benefit of the queue spinlock, the
> ebizzy benchmark was run with the -m option in two different computers:
> 
>   Test machine                ticket-lock             queue-lock
>   ------------                -----------             ----------
>   4-socket 40-core    2316 rec/s              2899 rec/s
>   Westmere-EX (HT off)
>   2-socket 12-core    2130 rec/s              2176 rec/s
>   Westmere-EP (HT on)
> 
> Waiman Long (10):
>   qspinlock: A generic 4-byte queue spinlock implementation
>   qspinlock, x86: Enable x86-64 to use queue spinlock
>   qspinlock: More optimized code for smaller NR_CPUS
>   qspinlock: Optimized code path for 2 contending tasks
>   pvqspinlock, x86: Allow unfair spinlock in a PV guest
>   pvqspinlock: Enable lock stealing in queue lock waiters
>   pvqspinlock, x86: Rename paravirt_ticketlocks_enabled
>   pvqspinlock, x86: Add qspinlock para-virtualization support
>   pvqspinlock, x86: Enable qspinlock PV support for KVM
>   pvqspinlock, x86: Enable qspinlock PV support for XEN
> 
>  arch/x86/Kconfig                      |   12 +
>  arch/x86/include/asm/paravirt.h       |   17 +-
>  arch/x86/include/asm/paravirt_types.h |   16 +
>  arch/x86/include/asm/pvqspinlock.h    |  260 +++++++++
>  arch/x86/include/asm/qspinlock.h      |  191 +++++++
>  arch/x86/include/asm/spinlock.h       |    9 +-
>  arch/x86/include/asm/spinlock_types.h |    4 +
>  arch/x86/kernel/Makefile              |    1 +
>  arch/x86/kernel/kvm.c                 |  113 ++++-
>  arch/x86/kernel/paravirt-spinlocks.c  |   36 ++-
>  arch/x86/xen/spinlock.c               |  121 ++++-
>  include/asm-generic/qspinlock.h       |  126 ++++
>  include/asm-generic/qspinlock_types.h |   63 ++
>  kernel/Kconfig.locks                  |    7 +
>  kernel/locking/Makefile               |    1 +
>  kernel/locking/qspinlock.c            | 1010 
> +++++++++++++++++++++++++++++++++
>  16 files changed, 1975 insertions(+), 12 deletions(-)
>  create mode 100644 arch/x86/include/asm/pvqspinlock.h
>  create mode 100644 arch/x86/include/asm/qspinlock.h
>  create mode 100644 include/asm-generic/qspinlock.h
>  create mode 100644 include/asm-generic/qspinlock_types.h
>  create mode 100644 kernel/locking/qspinlock.c
> 

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