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

[Marc Zyngier]

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.
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