linux-kernel - Re: [PATCH 2/3] vdpa_sim_blk: limit the number of request handled per batch

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Date:   Thu, 23 Jun 2022 10:58:30 +0200
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     virtualization <virtualization@...ts.linux-foundation.org>,
        Eugenio Pérez <eperezma@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] vdpa_sim_blk: limit the number of request handled
 per batch

On Thu, Jun 23, 2022 at 11:50:22AM +0800, Jason Wang wrote:
>On Wed, Jun 22, 2022 at 12:09 AM Stefano Garzarella <sgarzare@...hat.com> wrote:
>>
>> Limit the number of requests (4 per queue as for vdpa_sim_net) handled
>> in a batch to prevent the worker from using the CPU for too long.
>>
>> Suggested-by: Eugenio Pérez <eperezma@...hat.com>
>> Signed-off-by: Stefano Garzarella <sgarzare@...hat.com>
>> ---
>>  drivers/vdpa/vdpa_sim/vdpa_sim_blk.c | 15 ++++++++++++++-
>>  1 file changed, 14 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/vdpa/vdpa_sim/vdpa_sim_blk.c b/drivers/vdpa/vdpa_sim/vdpa_sim_blk.c
>> index a83a5c76f620..ac86478845b6 100644
>> --- a/drivers/vdpa/vdpa_sim/vdpa_sim_blk.c
>> +++ b/drivers/vdpa/vdpa_sim/vdpa_sim_blk.c
>> @@ -197,6 +197,7 @@ static bool vdpasim_blk_handle_req(struct vdpasim *vdpasim,
>>  static void vdpasim_blk_work(struct work_struct *work)
>>  {
>>         struct vdpasim *vdpasim = container_of(work, struct vdpasim, work);
>> +       bool reschedule = false;
>>         int i;
>>
>>         spin_lock(&vdpasim->lock);
>> @@ -206,11 +207,15 @@ static void vdpasim_blk_work(struct work_struct *work)
>>
>>         for (i = 0; i < VDPASIM_BLK_VQ_NUM; i++) {
>>                 struct vdpasim_virtqueue *vq = &vdpasim->vqs[i];
>> +               bool vq_work = true;
>> +               int reqs = 0;
>>
>>                 if (!vq->ready)
>>                         continue;
>>
>> -               while (vdpasim_blk_handle_req(vdpasim, vq)) {
>> +               while (vq_work) {
>> +                       vq_work = vdpasim_blk_handle_req(vdpasim, vq);
>> +
>
>Is it better to check and exit the loop early here?

Maybe, but I'm not sure.

In vdpa_sim_net we call vringh_complete_iotlb() and send notification 
also in the error path, so I thought was better to send notification 
also when vdpasim_blk_handle_req() return false, since we will update 
the used.idx.

However, I don't think it's a common path, so if you think it's better 
to exit the loop early, I can do it.

Thanks,
Stefano