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Date:   Wed, 29 Mar 2023 12:26:10 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Albert Huang <huangjie.albert@bytedance.com>
Cc:     Jason Wang <jasowang@redhat.com>,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3] virtio_ring: interrupt disable flag updated to vq
 even with event_triggered is set
Message-ID: <20230329121354-mutt-send-email-mst@kernel.org>
References: <20230329012908-mutt-send-email-mst@kernel.org>
 <20230329072135.44757-1-huangjie.albert@bytedance.com>
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In-Reply-To: <20230329072135.44757-1-huangjie.albert@bytedance.com>
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Ican't parse the subject at all. I think the subject from v2
was fine - we are skipping event index updates on get buf.
Or maybe go higher level and describe the effect of the patch:

virtio_ring: reduce interrupt rate with event idx enabled


On Wed, Mar 29, 2023 at 03:21:35PM +0800, Albert Huang wrote:
> From: "huangjie.albert" <huangjie.albert@bytedance.com>
> 
> in virtio_net, if we disable the napi_tx, when we triger a tx interrupt,
> the vq->event_triggered will be set to true. It will no longer be set to
> false.

this last sentence is redundant

> Unless we explicitly call virtqueue_enable_cb_delayed or
> virtqueue_enable_cb_prepare.
> 
> If we disable the napi_tx, it will only be called when the tx ring
> buffer is relatively small.
> 
> Because event_triggered is true. Therefore, VRING_AVAIL_F_NO_INTERRUPT or
> VRING_PACKED_EVENT_FLAG_DISABLE will not be set. So we update
> vring_used_event(&vq->split.vring) or vq->packed.vring.driver->off_wrap
> every time we call virtqueue_get_buf_ctx.This bring more interruptions.

This will bring more interrupts. And pls add space after ".".

> 
> To summarize:
> 1) event_triggered was set to true in vring_interrupt()
> 2) after this nothing will happen for virtqueue_disable_cb() so
>    VRING_AVAIL_F_NO_INTERRUPT is not set in avail_flags_shadow
> 3) virtqueue_get_buf_ctx_split() will still think the cb is enabled
>    then it tries to publish new event
> 
> To fix:
> update VRING_AVAIL_F_NO_INTERRUPT or VRING_PACKED_EVENT_FLAG_DISABLE to vq
> when we call virtqueue_disable_cb even the event_triggered is set to true.

bad grammar here and way too much detail.  better:

	make disable_cb set VRING_AVAIL_F_NO_INTERRUPT or
	VRING_PACKED_EVENT_FLAG_DISABLE in flags shadow to make get_buf
	correctly detect that callbacks are disabled.

> 
> Tested with iperf:
> iperf3 tcp stream:
> vm1 -----------------> vm2
> vm2 just receives tcp data stream from vm1, and sends the ack to vm1,
> there are many tx interrupts in vm2.
> but without event_triggered there are just a few tx interrupts.
>

put changelog after --- please.
 
> v2->v3:
> -update the interrupt disable flag even with the event_triggered is set,
> -instead of checking whether event_triggered is set in
> -virtqueue_get_buf_ctx_{packed/split}, will cause the drivers  which have
> -not called virtqueue_{enable/disable}_cb to miss notifications.
> 
> Fixes: 8d622d21d248 ("virtio: fix up virtio_disable_cb")
> Signed-off-by: huangjie.albert <huangjie.albert@bytedance.com>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
>  drivers/virtio/virtio_ring.c | 24 +++++++++++++++++-------
>  1 file changed, 17 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 307e139cb11d..ad74463a48ee 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -812,6 +812,14 @@ static void virtqueue_disable_cb_split(struct virtqueue *_vq)
>  
>  	if (!(vq->split.avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)) {
>  		vq->split.avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
> +
> +		/*
> +		 * If device triggered an event already it won't trigger one again:
> +		 * no need to disable.
> +		 */
> +		if (vq->event_triggered)
> +			return;
> +
>  		if (vq->event)
>  			/* TODO: this is a hack. Figure out a cleaner value to write. */
>  			vring_used_event(&vq->split.vring) = 0x0;
> @@ -1544,8 +1552,16 @@ static void virtqueue_disable_cb_packed(struct virtqueue *_vq)
>  {
>  	struct vring_virtqueue *vq = to_vvq(_vq);
>  
> -	if (vq->packed.event_flags_shadow != VRING_PACKED_EVENT_FLAG_DISABLE) {
> +	if (!(vq->packed.event_flags_shadow & VRING_PACKED_EVENT_FLAG_DISABLE)) {


Why are you making this change? It's not great because it
only works because VRING_PACKED_EVENT_FLAG_DISABLE happens
to equal 0x1. does not the patch work with the original
if (vq->packed.event_flags_shadow != VRING_PACKED_EVENT_FLAG_DISABLE)
?

Besides, if you are making unrelated changes commit log should
describe them.

>  		vq->packed.event_flags_shadow = VRING_PACKED_EVENT_FLAG_DISABLE;
> +
> +		/*
> +		 * If device triggered an event already it won't trigger one again:
> +		 * no need to disable.
> +		 */
> +		if (vq->event_triggered)
> +			return;
> +
>  		vq->packed.vring.driver->flags =
>  			cpu_to_le16(vq->packed.event_flags_shadow);
>  	}
> @@ -2063,12 +2079,6 @@ void virtqueue_disable_cb(struct virtqueue *_vq)
>  {
>  	struct vring_virtqueue *vq = to_vvq(_vq);
>  
> -	/* If device triggered an event already it won't trigger one again:
> -	 * no need to disable.
> -	 */
> -	if (vq->event_triggered)
> -		return;
> -
>  	if (vq->packed_ring)
>  		virtqueue_disable_cb_packed(_vq);
>  	else
> -- 
> 2.31.1