Re: [PATCH v4 2/3] locking: Implement an algorithm choice for Wound-Wait mutexes

[Peter Zijlstra]

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@xxxxxxxxxx>
> Cc: Jonathan Corbet <corbet@xxxxxxx>
> Cc: Gustavo Padovan <gustavo@xxxxxxxxxxx>
> Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
> Cc: Sean Paul <seanpaul@xxxxxxxxxxxx>
> Cc: David Airlie <airlied@xxxxxxxx>
> Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Kate Stewart <kstewart@xxxxxxxxxxxxxxxxxxx>
> Cc: Philippe Ombredanne <pombredanne@xxxxxxxx>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: linux-doc@xxxxxxxxxxxxxxx
> Cc: linux-media@xxxxxxxxxxxxxxx
> Cc: linaro-mm-sig@xxxxxxxxxxxxxxxx
> Co-authored-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Thomas Hellstrom <thellstrom@xxxxxxxxxx>
> 
> ---
>  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@xxxxxxxxxxxxx>