Re: [PATCH 2/2] cfq-iosched: get rid of the need for __GFP_FAIL in cfq_find_alloc_queue()

Jens Axboe
2009-06-29T17:34:27Z

On Mon, Jun 29 2009, Jeff Moyer wrote:
> Jens Axboe <jens.axboe@oracle.com> writes:
>
> > On Fri, Jun 26 2009, Jeff Moyer wrote:
> >> Jens Axboe <jens.axboe@oracle.com> writes:
> >>
> >> > Setup an emergency fallback cfqq that we allocate at IO scheduler init
> >> > time. If the slab allocation fails in cfq_find_alloc_queue(), we'll just
> >> > punt IO to that cfqq instead. This ensures that cfq_find_alloc_queue()
> >> > never fails without having to ensure free memory.
> >> >
> >> > Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
> >> > ---
> >> > block/cfq-iosched.c | 124 +++++++++++++++++++++++++++-----------------------
> >> > 1 files changed, 67 insertions(+), 57 deletions(-)
> >> >
> >> > diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> >> > index c760ae7..91e7e0b 100644
> >> > --- a/block/cfq-iosched.c
> >> > +++ b/block/cfq-iosched.c
> >> > + /*
> >> > + * Fallback dummy cfqq for extreme OOM conditions
> >> > + */
> >> > + struct cfq_queue oom_cfqq;
> >>
> >> OK, so you're embedding a cfqq into the cfqd. That's 136 bytes, so I
> >> guess that's not too bad.
> >>
> >> > + /*
> >> > + * Our fallback cfqq if cfq_find_alloc_queue() runs into OOM issues.
> >> > + * Grab a permanent reference to it, so that the normal code flow
> >> > + * will not attempt to free it.
> >> > + */
> >> > + cfq_init_cfqq(cfqd, &cfqd->oom_cfqq, 1, 0);
> >> > + atomic_inc(&cfqd->oom_cfqq.ref);
> >> > +
> >>
> >> I guess this is so we never try to free it, good. ;)
> >>
> >> One issue I have with this patch is that, if a task happens to run into
> >> this condition, there is no way out. It will always have the oom_cfqq
> >> as it's cfqq. Can't we fix that if we recover from the OOM condition?
> >
> > Yeah, I fixed that about an hour after posting the patches. See:
> >
> > http://git.kernel.dk/?p=linux-2.6-block.git;a=commit;h=0370bc158cb1d5faa4b8a38c0de3211f0fd5bd64
> >
> > I didn't post the 3/2 patch though.
>
> OK, that looks better. Are you reposting the series, then?

Yeah, I'll fold the last two patches together and repost.

--
Jens Axboe