• Christian Borntraeger's avatar
    KVM: halt_polling: provide a way to qualify wakeups during poll · 3491caf2
    Christian Borntraeger authored
    Some wakeups should not be considered a sucessful poll. For example on
    s390 I/O interrupts are usually floating, which means that _ALL_ CPUs
    would be considered runnable - letting all vCPUs poll all the time for
    transactional like workload, even if one vCPU would be enough.
    This can result in huge CPU usage for large guests.
    This patch lets architectures provide a way to qualify wakeups if they
    should be considered a good/bad wakeups in regard to polls.
    For s390 the implementation will fence of halt polling for anything but
    known good, single vCPU events. The s390 implementation for floating
    interrupts does a wakeup for one vCPU, but the interrupt will be delivered
    by whatever CPU checks first for a pending interrupt. We prefer the
    woken up CPU by marking the poll of this CPU as "good" poll.
    This code will also mark several other wakeup reasons like IPI or
    expired timers as "good". This will of course also mark some events as
    not sucessful. As  KVM on z runs always as a 2nd level hypervisor,
    we prefer to not poll, unless we are really sure, though.
    This patch successfully limits the CPU usage for cases like uperf 1byte
    transactional ping pong workload or wakeup heavy workload like OLTP
    while still providing a proper speedup.
    This also introduced a new vcpu stat "halt_poll_no_tuning" that marks
    wakeups that are considered not good for polling.
    Signed-off-by: default avatarChristian Borntraeger <borntraeger@de.ibm.com>
    Acked-by: Radim Krčmář <rkrcmar@redhat.com> (for an earlier version)
    Cc: David Matlack <dmatlack@google.com>
    Cc: Wanpeng Li <kernellwp@gmail.com>
    [Rename config symbol. - Paolo]
    Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
kvm.h 8.76 KB