Messages in this thread | | | Date | Sat, 27 Jun 2009 20:26:17 +0200 | From | Jens Axboe <> | Subject | Re: [PATCH 2/2] cfq-iosched: get rid of the need for __GFP_FAIL in cfq_find_alloc_queue() |
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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.
-- Jens Axboe
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