linux-kernel - Re: [PATCH v3 2/3] virtio-balloon: improve update_balloon_size

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Date:   Mon, 14 Jan 2019 20:00:27 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Wei Wang <wei.w.wang@...el.com>
Cc:     virtio-dev@...ts.oasis-open.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        cohuck@...hat.com, borntraeger@...ibm.com, pbonzini@...hat.com,
        dgilbert@...hat.com, pasic@...ux.ibm.com
Subject: Re: [PATCH v3 2/3] virtio-balloon: improve update_balloon_size_func

On Mon, Jan 07, 2019 at 03:01:05PM +0800, Wei Wang wrote:
> There is no need to update the balloon actual register when there is no
> ballooning request. This patch avoids update_balloon_size when diff is 0.
> 
> Signed-off-by: Wei Wang <wei.w.wang@...el.com>
> Reviewed-by: Cornelia Huck <cohuck@...hat.com>
> Reviewed-by: Halil Pasic <pasic@...ux.ibm.com>

It sounds reasonable but it does have a side effect of
not writing into actual anymore.

I am not sure actual never can get out of sync as a result.

So better deferred, pls send this in the next merge window.




> ---
>  drivers/virtio/virtio_balloon.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index fb12fe2..e33dc8e 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -457,9 +457,12 @@ static void update_balloon_size_func(struct work_struct *work)
>  			  update_balloon_size_work);
>  	diff = towards_target(vb);
>  
> +	if (!diff)
> +		return;
> +
>  	if (diff > 0)
>  		diff -= fill_balloon(vb, diff);
> -	else if (diff < 0)
> +	else
>  		diff += leak_balloon(vb, -diff);
>  	update_balloon_size(vb);
>  
> -- 
> 2.7.4