• Waiman Long's avatar
    locking/pvqspinlock: Queue node adaptive spinning · cd0272fa
    Waiman Long authored
    
    
    In an overcommitted guest where some vCPUs have to be halted to make
    forward progress in other areas, it is highly likely that a vCPU later
    in the spinlock queue will be spinning while the ones earlier in the
    queue would have been halted. The spinning in the later vCPUs is then
    just a waste of precious CPU cycles because they are not going to
    get the lock soon as the earlier ones have to be woken up and take
    their turn to get the lock.
    
    This patch implements an adaptive spinning mechanism where the vCPU
    will call pv_wait() if the previous vCPU is not running.
    
    Linux kernel builds were run in KVM guest on an 8-socket, 4
    cores/socket Westmere-EX system and a 4-socket, 8 cores/socket
    Haswell-EX system. Both systems are configured to have 32 physical
    CPUs. The kernel build times before and after the patch were:
    
    		    Westmere			Haswell
      Patch		32 vCPUs    48 vCPUs	32 vCPUs    48 vCPUs
      -----		--------    --------    --------    --------
      Before patch   3m02.3s     5m00.2s     1m43.7s     3m03.5s
      After patch    3m03.0s     4m37.5s	 1m43.0s     2m47.2s
    
    For 32 vCPUs, this patch doesn't cause any noticeable change in
    performance. For 48 vCPUs (over-committed), there is about 8%
    performance improvement.
    Signed-off-by: default avatarWaiman Long <Waiman.Long@hpe.com>
    Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Davidlohr Bueso <dave@stgolabs.net>
    Cc: Douglas Hatch <doug.hatch@hpe.com>
    Cc: H. Peter Anvin <hpa@zytor.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Scott J Norton <scott.norton@hpe.com>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Link: http://lkml.kernel.org/r/1447114167-47185-8-git-send-email-Waiman.Long@hpe.com
    
    Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
    cd0272fa
qspinlock.c 13.8 KB