Re: [Qemu-devel] [Qemu-block] [RFC] block-backend: fix double inc/dec inflight requests number

[Kevin Wolf <kwolf@redhat.com>]

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Am 25.01.2018 um 12:37 hat Stefan Hajnoczi geschrieben:
> On Mon, Jan 22, 2018 at 05:45:49PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> > Is it a bug or a feature? Why do we call inc/dec twice for read/write?
> > We don't do this for flush and discard..
> 
> It's non-obvious and I asked Paolo the same question previously.
> 
> > -    bdrv_inc_in_flight(bs);
> > -
> >      /* throttling disk I/O */
> >      if (blk->public.throttle_group_member.throttle_state) {
> >          throttle_group_co_io_limits_intercept(&blk->public.throttle_group_member,
> >                  bytes, false);
> >      }
> 
> ^^^ HINT HINT HINT ^^^
> 
> >  
> > -    ret = bdrv_co_preadv(blk->root, offset, bytes, qiov, flags);
> > -    bdrv_dec_in_flight(bs);
> > -    return ret;
> > +    return bdrv_co_preadv(blk->root, offset, bytes, qiov, flags);
> 
> The problem is what happens if the request is throttled?
> 
> Even throttled requests must be counted so that bdrv_drain() and friends
> work.
> 
> It may be possible to eliminate this now that throttling is a BDS node.
> It used to be implemented as a completely separate API outside the BDS
> node graph.  Now there is a throttling node in the graph, so maybe we
> can stop taking the extra reference but some refactoring may be
> necessary.

The patches to use a throttling node even when the legacy options are
used have not been merged yet.

Kevin

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