Re: [PATCH 1/1] KVM: halt_polling: provide a way to qualify wakeups during poll

["Radim Krčmář" <rkrcmar@redhat.com>]

2016-05-03 14:37+0200, Christian Borntraeger:
> 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: Christian Borntraeger <borntraeger@de.ibm.com>
> Cc: David Matlack <dmatlack@google.com>
> Cc: Wanpeng Li <kernellwp@gmail.com>
> ---

Thanks for all explanations,

Acked-by: Radim Krčmář <rkrcmar@redhat.com>