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SubjectRe: [PATCH v4 2/3] locking: Implement an algorithm choice for Wound-Wait mutexes
On Tue, Jun 19, 2018 at 10:24:44AM +0200, Thomas Hellstrom wrote:
> The current Wound-Wait mutex algorithm is actually not Wound-Wait but
> Wait-Die. Implement also Wound-Wait as a per-ww-class choice. Wound-Wait
> is, contrary to Wait-Die a preemptive algorithm and is known to generate
> fewer backoffs. Testing reveals that this is true if the
> number of simultaneous contending transactions is small.
> As the number of simultaneous contending threads increases, Wait-Wound
> becomes inferior to Wait-Die in terms of elapsed time.
> Possibly due to the larger number of held locks of sleeping transactions.
>
> Update documentation and callers.
>
> Timings using git://people.freedesktop.org/~thomash/ww_mutex_test
> tag patch-18-06-15
>
> Each thread runs 100000 batches of lock / unlock 800 ww mutexes randomly
> chosen out of 100000. Four core Intel x86_64:
>
> Algorithm #threads Rollbacks time
> Wound-Wait 4 ~100 ~17s.
> Wait-Die 4 ~150000 ~19s.
> Wound-Wait 16 ~360000 ~109s.
> Wait-Die 16 ~450000 ~82s.
>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: Gustavo Padovan <gustavo@padovan.org>
> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> Cc: Sean Paul <seanpaul@chromium.org>
> Cc: David Airlie <airlied@linux.ie>
> Cc: Davidlohr Bueso <dave@stgolabs.net>
> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> Cc: Josh Triplett <josh@joshtriplett.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Kate Stewart <kstewart@linuxfoundation.org>
> Cc: Philippe Ombredanne <pombredanne@nexb.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: linux-doc@vger.kernel.org
> Cc: linux-media@vger.kernel.org
> Cc: linaro-mm-sig@lists.linaro.org
> Co-authored-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
>
> ---
> Documentation/locking/ww-mutex-design.txt | 57 +++++++++--
> drivers/dma-buf/reservation.c | 2 +-
> drivers/gpu/drm/drm_modeset_lock.c | 2 +-
> include/linux/ww_mutex.h | 17 ++-
> kernel/locking/locktorture.c | 2 +-
> kernel/locking/mutex.c | 165 +++++++++++++++++++++++++++---
> kernel/locking/test-ww_mutex.c | 2 +-
> lib/locking-selftest.c | 2 +-
> 8 files changed, 213 insertions(+), 36 deletions(-)

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

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