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

Jeff Moyer
2009-06-29T13:46:59Z

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?

Cheers,
Jeff