Re: [PATCH 4/5] kvm: Replace vcpu->swait with rcuwait

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On 2020-04-22 05:07, Davidlohr Bueso wrote:
The use of any sort of waitqueue (simple or regular) for
wait/waking vcpus has always been an overkill and semantically
wrong. Because this is per-vcpu (which is blocked) there is
only ever a single waiting vcpu, thus no need for any sort of
queue.

As such, make use of the rcuwait primitive, with the following
considerations:

  - rcuwait already provides the proper barriers that serialize
  concurrent waiter and waker.

  - Task wakeup is done in rcu read critical region, with a
  stable task pointer.

  - Because there is no concurrency among waiters, we need
  not worry about rcuwait_wait_event() calls corrupting
  the wait->task. As a consequence, this saves the locking
  done in swait when modifying the queue. This also applies
  to per-vcore wait for powerpc kvm-hv.

The x86 tscdeadline_latency test mentioned in 8577370fb0cb
("KVM: Use simple waitqueue for vcpu->wq") shows that, on avg,
latency is reduced by around 15-20% with this change.

Cc: Paul Mackerras <paulus@xxxxxxxxxx>
Cc: kvmarm@xxxxxxxxxxxxxxxxxxxxx
Cc: linux-mips@xxxxxxxxxxxxxxx
Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx>

Reviewed-by: Marc Zyngier <maz@xxxxxxxxxx>

Thanks,

        M.
--
Who you jivin' with that Cosmik Debris?



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