Re: BLK_TA_ISSUE

[Jens Axboe <jens.axboe@oracle.com>]

On Thu, Apr 12 2007, Ming Zhang wrote:
> On Thu, 2007-04-12 at 13:29 +0200, Jens Axboe wrote:
> > On Wed, Apr 11 2007, Ming Zhang wrote:
> > > Hi All
> > > 
> > > For this ISSUE event, currently it is in elv_next_request(), any idea
> > > why it is not in elv_dequeue_request() which is where the request marked
> > > as on-the-fly and send to lower level?
> > 
> > elv_next_request() is the driver hand-off point, so should be pretty
> > close to the issue time unless the request gets requeued due to some
> > busy condition (which will also be logged). elv_dequeue_request() may
> > happen much later, some drivers do it right before calling the io
> > completion handler - IDE does this - since it leaves the request on the
> > queue list for the duration of the operation. So moving the ISSUE event
> > to elv_dequeue_request() would not be correct.
> > 
> 
> ic. i assumed all requests will be removed from queue before llDD handle
> it.
> 
> in 2.6.20 ele_dequeue_request
> 
> 771 
> 772         /*
> 773          * the time frame between a request being removed from the lists
> 774          * and to it is freed is accounted as io that is in progress at
> 775          * the driver side.
> 776          */
> 777         if (blk_account_rq(rq))
> 778                 q->in_flight++;
> 
> then this in_flight counter is more likely to be how many outstanding
> requests that not in the queue and before it is free. and it might be
> less than how many undergoing IOs?

It's good enough for what ->in_flight is used for. Your assumption on
that all low level drivers dequeue before handling a request is wrong.
Usually only drivers that do queueing do this.

-- 
Jens Axboe