Re: Flush requests not going through IO scheduler

[Jens Axboe]

On 11/02/2015 05:20 AM, Jan Kara wrote:
> Hello,
>
> when looking into a performance issue, I've noticed one interesting thing
> in blktrace data:
>
>    8,0    2        0     1.745149746     0  m   N cfq320SN / dispatch_insert
>    8,0    2        0     1.745150258     0  m   N cfq320SN / dispatched a request
>    8,0    2        0     1.745150524     0  m   N cfq320SN / activate rq, drv=10
>    8,0    2     2893     1.745150644 30477  D  WS 495331192 + 192 [git]
>    8,0    1     3678     1.746851310     0  C  WS 495331192 + 192 [0]
>
> We wrote the data for transaction commit here.
>
>    8,0    1        0     1.746863220     0  m   N cfq320SN / complete rqnoidle 1
>    8,0    1        0     1.746863801     0  m   N cfq320SN / set_slice=27
>    8,0    1        0     1.746864439     0  m   N cfq320SN / arm_idle: 8 group_idle: 0
>
> Currently there is no IO queued from jbd2 thread so idle...
>
>    8,0    1     3679     1.746878424   320  A FWFS 495331384 + 8 <- (8,2) 478543928
>    8,0    1     3680     1.746879028   320  Q FWFS 495331384 + 8 [jbd2/sda2-8]
>    8,0    1     3681     1.746879673   320  G FWFS 495331384 + 8 [jbd2/sda2-8]
>    8,0    1     3682     1.746880227   320  I FWFS 495331384 + 8 [jbd2/sda2-8]
>
> Jbd2 thread now queues the commit block.
>
>    8,0    1        0     1.754263523     0  m   N cfq idle timer fired
>    8,0    1        0     1.754264733     0  m   N cfq320SN / slice expired t=0
>
> But it was not dispatched and we just idled until timer fired. Then we
> started dispatching for other queue and got to dispatching the commit block
> only much later.
>
> I've looked into the block layer code and the reason for this behavior
> (idling when there is in fact IO to dispatch) is the special handling of
> flush requests. When a flush request is submitted, we insert it with
> ELEVATOR_INSERT_FLUSH and blk_insert_flush() then handles it. That
> eventually just ends up doing something along the lines of:
>
> 	list_add_tail(&rq->queuelist, &q->queue_head);
>
> So we add request to the list of requests to dispatch but we don't notify
> IO scheduler in any way. Thus IO scheduler won't properly track the
> request, won't properly account IO time for it if I'm right etc...
>
> Ideally we should call q->elevator->type->ops.elevator_add_req_fn() to
> handle the request but I'm not sure it won't break some assumptions of the
> flush code. But at minimum shouldn't we at least try to dispatch the
> request?

Certainly, the current behavior is undoubtedly broken. The least 
intrusive fix would be to kick off scheduling when we add it to the 
request, but the elevator should handle it. Are you going to be up for 
hacking up a fix?

--
Jens Axboe

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