Re: [RFC PATCH 0/3] block: Fix fsync slowness with CFQ cgroups

[Konstantin Khlebnikov]

Vivek Goyal wrote:
> On Tue, Jun 28, 2011 at 03:00:39PM +0400, Konstantin Khlebnikov wrote:
> > Vivek Goyal wrote:
> > > This patch series seems to be working for me. I did testing for ext4 only.
> > > This series is based on for-3.1/core branch of Jen's block tree.
> > > Konstantin, can you please give it a try and see if it fixes your
> > > issue.
> >
> > It works for me too, for ext3 and ext4, on top 3.0-rc5, after these trivial fixes:
> >
> > --- a/block/cfq-iosched.c
> > +++ b/block/cfq-iosched.c
> > @@ -1511,7 +1519,7 @@ static void cfq_add_rq_rb(struct request *rq)
> >           * if that happens, put the alias on the dispatch list
> >           */
> >          while ((__alias = elv_rb_add(&cfqq->sort_list, rq)) != NULL)
> > -               cfq_dispatch_insert(cfqd->queue, __alias);
> > +               cfq_dispatch_insert(cfqd->queue, __alias, false);
> >
> >          if (!cfq_cfqq_on_rr(cfqq))
> >                  cfq_add_cfqq_rr(cfqd, cfqq);
> > @@ -3797,12 +3797,11 @@ cfq_set_depends_on_task(struct request_queue *q, struct task_struct *tsk)
> >           */
> >          rcu_read_lock();
> >          if (task_blkio_cgroup(current) == task_blkio_cgroup(tsk))
> > -               return;
> > -       rcu_read_unlock();
> > +               goto out_unlock_rcu;
> >
> >          cic = cfq_cic_lookup(cfqd, current->io_context);
> >          if (!cic)
> > -               return;
> > +               goto out_unlock_rcu;
>
> You have done this change because you want to keep cfq_cic_lookup() also
> in rcu read side critical section? I am assuming that it works even
> without this. Though keeping it under rcu is probably more correct as
> cic objects are freed in rcu manner.

No, your just forgot to relese rcu in case task_blkio_cgroup(current) == task_blkio_cgroup(tsk)

> Thanks
> Vivek

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