Re: [PATCH] block: forbid to re-enable I/O stat accounting

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

On Sat, Mar 21 2009, Andrew Morton wrote:
> 
> Please fix your email headers - what I received was
> 
> From: Jerome Marchand <jmarchan@redhat.com>
> To: unlisted-recipients:;;@imap1.linux-foundation.org (no To-header on input)
> Cc: Jens Axboe <axboe@kernel.dk>
> 
> 
> On Thu, 19 Mar 2009 11:36:50 +0100 Jerome Marchand <jmarchan@redhat.com> wrote:
> 
> > When we stop I/O stat accounting we stop to update the in-flight
> > requests counter and we need this counter to be reliable for
> > accounting I/O stats. Unfortunately updating in_flight field may
> > affect performance. So, until we have a better solution, just forbid
> > to re-enable I/O stat accounting after it has been disabled.
> 
> hm.  Is it really so hard to just quiesce the device until all in-flight
> requests have drained?  freeze_bdev() might be a suitable starting point?

freeze_bdev() is too far up, I think. It would be better to just reuse
the quiscing code we use for switching io schedulers on-the-fly.
Basically something ala:

        spin_lock_irq(q->queue_lock);
        queue_flag_set(QUEUE_FLAG_ELVSWITCH, q);
        elv_drain_elevator(q);
        while (q->rq.elvpriv) {
                blk_start_queueing(q);
                spin_unlock_irq(q->queue_lock);
                msleep(10);
                spin_lock_irq(q->queue_lock);
                elv_drain_elevator(q);
        }

        re-enable IO accounting;
        spin_unlock_irq(q->queue_lock);

and then don't account requests that don't have REQ_ELVPRIV set. I think
that should be acceptable. I was pretty sure I had a patch for something
else that yanked this logic into start/stop helpers... Ah, found it:

http://git.kernel.dk/?p=linux-2.6-block.git;a=commitdiff;h=8cb9f793b82024cd43ed5d4583a5287d002abdf8

-- 
Jens Axboe