Hi Suwan,
On Mon, Aug 29, 2022 at 11:49 AM Suwan Kim <suwan.kim027@xxxxxxxxx> wrote:
>
> On Fri, Aug 26, 2022 at 10:41:39AM +0900, Alexandre Courbot wrote:
> > On Thu, Aug 25, 2022 at 11:50 PM Suwan Kim <suwan.kim027@xxxxxxxxx> wrote:
> > >
> > > On Thu, Aug 25, 2022 at 2:32 AM Stefan Hajnoczi <stefanha@xxxxxxxxxx> wrote:
> > > >
> > > > On Wed, Aug 24, 2022 at 10:16:10PM +0900, Kim Suwan wrote:
> > > > > Hi Stefan,
> > > > >
> > > > > On Wed, Aug 24, 2022 at 5:56 AM Stefan Hajnoczi <stefanha@xxxxxxxxxx> wrote:
> > > > > >
> > > > > > On Tue, Aug 23, 2022 at 11:50:05PM +0900, Suwan Kim wrote:
> > > > > > > @@ -409,6 +409,8 @@ static bool virtblk_add_req_batch(struct virtio_blk_vq *vq,
> > > > > > > virtblk_unmap_data(req, vbr);
> > > > > > > virtblk_cleanup_cmd(req);
> > > > > > > rq_list_add(requeue_list, req);
> > > > > > > + } else {
> > > > > > > + blk_mq_start_request(req);
> > > > > > > }
> > > > > >
> > > > > > The device may see new requests as soon as virtblk_add_req() is called
> > > > > > above. Therefore the device may complete the request before
> > > > > > blk_mq_start_request() is called.
> > > > > >
> > > > > > rq->io_start_time_ns = ktime_get_ns() will be after the request was
> > > > > > actually submitted.
> > > > > >
> > > > > > I think blk_mq_start_request() needs to be called before
> > > > > > virtblk_add_req().
> > > > > >
> > > > >
> > > > > But if blk_mq_start_request() is called before virtblk_add_req()
> > > > > and virtblk_add_req() fails, it can trigger WARN_ON_ONCE() at
> > > > > virtio_queue_rq().
> > > > >
> > > > > With regard to the race condition between virtblk_add_req() and
> > > > > completion, I think that the race condition can not happen because
> > > > > virtblk_add_req() holds vq lock with irq saving and completion side
> > > > > (virtblk_done, virtblk_poll) need to acquire the vq lock also.
> > > > > Moreover, virtblk_done() is irq context so I think it can not be
> > > > > executed until virtblk_add_req() releases the lock.
> > > >
> > > > I agree there is no race condition regarding the ordering of
> > > > blk_mq_start_request() and request completion. The spinlock prevents
> > > > that and I wasn't concerned about that part.
> > > >
> > > > The issue is that the timestamp will be garbage. If we capture the
> > > > timestamp during/after the request is executing, then the collected
> > > > statistics will be wrong.
> > > >
> > > > Can you look for another solution that doesn't break the timestamp?
> > > >
> > > > FWIW I see that the rq->state state machine allows returning to the idle
> > > > state once the request has been started: __blk_mq_requeue_request().
> > >
> > > I considered blk_mq_requeue_request() to handle error cases but
> > > I didn't use it because I think it can make the error path request
> > > processing slower than requeuing an error request to plug list again.
> > >
> > > But there doesn't seem to be any other option that doesn't break
> > > the timestamp.
> > >
> > > As you said, I will use __blk_mq_requeue_request() and send
> > > new patch soon.
> > >
> > > To Alexandre,
> > >
> > > I will share new diff soon. Could you please test one more time?
> >
> > Absolutely! Thanks for looking into this.
>
> Hi Alexandre,
>
> Could you test this path?
> If it works, I will send v2 patch.
This version is working fine for me!
Cheers,
Alex.
