Re: [PATCH 15/20] blkio: Take care of preemptions across groups

[Vivek Goyal <vgoyal@redhat.com>]

On Fri, Nov 06, 2009 at 03:55:58PM +0800, Gui Jianfeng wrote:
> Vivek Goyal wrote:
> > o Additional preemption checks for groups where we travel up the hierarchy
> >   and see if one queue should preempt other or not.
> > 
> > o Also prevents preemption across groups in some cases to provide isolation
> >   between groups.
> > 
> > Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> > ---
> >  block/cfq-iosched.c |   33 +++++++++++++++++++++++++++++++++
> >  1 files changed, 33 insertions(+), 0 deletions(-)
> > 
> > diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> > index 87b1799..98dbead 100644
> > --- a/block/cfq-iosched.c
> > +++ b/block/cfq-iosched.c
> > @@ -2636,6 +2636,36 @@ cfq_update_idle_window(struct cfq_data *cfqd, struct cfq_queue *cfqq,
> >  	}
> >  }
> >  
> > +static bool cfq_should_preempt_group(struct cfq_data *cfqd,
> > +		struct cfq_queue *cfqq, struct cfq_queue *new_cfqq)
> > +{
> > +	struct cfq_entity *cfqe = &cfqq->entity;
> > +	struct cfq_entity *new_cfqe = &new_cfqq->entity;
> > +
> > +	if (cfqq_to_cfqg(cfqq) != &cfqd->root_group)
> > +		cfqe = parent_entity(&cfqq->entity);
> > +
> > +	if (cfqq_to_cfqg(new_cfqq) != &cfqd->root_group)
> > +		new_cfqe = parent_entity(&new_cfqq->entity);
> > +
> > +	/*
> > +	 * Allow an RT request to pre-empt an ongoing non-RT cfqq timeslice.
> > +	 */
> > +
> > +	if (new_cfqe->ioprio_class == IOPRIO_CLASS_RT
> > +	    && cfqe->ioprio_class != IOPRIO_CLASS_RT)
> > +		return true;
> > +	/*
> > +	 * Allow an BE request to pre-empt an ongoing IDLE clas timeslice.
> > +	 */
> > +
> > +	if (new_cfqe->ioprio_class == IOPRIO_CLASS_BE
> > +	    && cfqe->ioprio_class == IOPRIO_CLASS_IDLE)
> > +		return true;
> > +
> > +	return false;
> > +}
> > +
> >  /*
> >   * Check if new_cfqq should preempt the currently active queue. Return 0 for
> >   * no or if we aren't sure, a 1 will cause a preempt.
> > @@ -2666,6 +2696,9 @@ cfq_should_preempt(struct cfq_data *cfqd, struct cfq_queue *new_cfqq,
> >  	if (rq_is_sync(rq) && !cfq_cfqq_sync(cfqq))
> >  		return true;
> >  
> > +	if (cfqq_to_cfqg(new_cfqq) != cfqq_to_cfqg(cfqq))
> > +		return cfq_should_preempt_group(cfqd, cfqq, new_cfqq);
> > +
> 
> Vivek, why not put cfq_should_preempt_group() at the beginning of cfq_should_preempt()
> to prevent preemption across groups?

Hi Gui,

Currently the checks before the group check were not hurting much, that's
why.

The only contentious check will be if a sync IO in one group should
preempt the async IO in other group or not.

Thanks
Vivek