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

[Vivek Goyal <vgoyal@redhat.com>]

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.

Thanks
Vivek