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

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Date:   Tue, 19 Jun 2018 11:45:00 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Hellstrom <thellstrom@...are.com>
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        linux-graphics-maintainer@...are.com, pv-drivers@...are.com,
        Ingo Molnar <mingo@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Gustavo Padovan <gustavo@...ovan.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Sean Paul <seanpaul@...omium.org>,
        David Airlie <airlied@...ux.ie>,
        Davidlohr Bueso <dave@...olabs.net>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Josh Triplett <josh@...htriplett.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-doc@...r.kernel.org, linux-media@...r.kernel.org,
        linaro-mm-sig@...ts.linaro.org
Subject: Re: [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@...hat.com>
> Cc: Jonathan Corbet <corbet@....net>
> Cc: Gustavo Padovan <gustavo@...ovan.org>
> Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>
> Cc: Sean Paul <seanpaul@...omium.org>
> Cc: David Airlie <airlied@...ux.ie>
> Cc: Davidlohr Bueso <dave@...olabs.net>
> Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> Cc: Josh Triplett <josh@...htriplett.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Kate Stewart <kstewart@...uxfoundation.org>
> Cc: Philippe Ombredanne <pombredanne@...b.com>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: linux-doc@...r.kernel.org
> Cc: linux-media@...r.kernel.org
> Cc: linaro-mm-sig@...ts.linaro.org
> Co-authored-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Thomas Hellstrom <thellstrom@...are.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@...radead.org>