• Marc Zyngier's avatar
    ARM: KVM: Yield CPU when vcpu executes a WFE · 58d5ec8f
    Marc Zyngier authored
    On an (even slightly) oversubscribed system, spinlocks are quickly
    becoming a bottleneck, as some vcpus are spinning, waiting for a
    lock to be released, while the vcpu holding the lock may not be
    running at all.
    
    This creates contention, and the observed slowdown is 40x for
    hackbench. No, this isn't a typo.
    
    The solution is to trap blocking WFEs and tell KVM that we're
    now spinning. This ensures that other vpus will get a scheduling
    boost, allowing the lock to be released more quickly. Also, using
    CONFIG_HAVE_KVM_CPU_RELAX_INTERCEPT slightly improves the performance
    when the VM is severely overcommited.
    
    Quick test to estimate the performance: hackbench 1 process 1000
    
    2xA15 host (baseline):	1.843s
    
    2xA15 guest w/o patch:	2.083s
    4xA15 guest w/o patch:	80.212s
    8xA15 guest w/o patch:	Could not be bothered to find out
    
    2xA15 guest w/ patch:	2.102s
    4xA15 guest w/ patch:	3.205s
    8xA15 guest w/ patch:	6.887s
    
    So we go from a 40x degradation to 1.5x in the 2x overcommit case,
    which is vaguely more acceptable.
    Signed-off-by: default avatarMarc Zyngier <marc.zyngier@arm.com>
    Signed-off-by: default avatarChristoffer Dall <christoffer.dall@linaro.org>
    58d5ec8f
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