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SubjectRe: [PATCH 9/9] block: make blk-throttle preserve the issuing task on delayed bios
On Thu, Feb 16, 2012 at 02:37:58PM -0800, Tejun Heo wrote:
> Make blk-throttle call bio_associate_current() on bios being delayed
> such that they get issued to block layer with the original io_context.
> This allows stacking blk-throttle and cfq-iosched propio policies.
> bios will always be issued with the correct ioc and blkcg whether it
> gets delayed by blk-throttle or not.
>
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Cc: Vivek Goyal <vgoyal@redhat.com>
> ---
> block/blk-throttle.c | 4 ++++
> 1 files changed, 4 insertions(+), 0 deletions(-)
>
> diff --git a/block/blk-throttle.c b/block/blk-throttle.c
> index a7c8e0b..ad8e16b 100644
> --- a/block/blk-throttle.c
> +++ b/block/blk-throttle.c
> @@ -894,6 +894,9 @@ bool blk_throtl_bio(struct request_queue *q, struct bio *bio)
> goto out;
> }
>
> + /* bio_associate_current() needs ioc, try creating */
> + create_io_context(GFP_ATOMIC, q->node);
> +
> /*
> * A throtl_grp pointer retrieved under rcu can be used to access
> * basic fields like stats and io rates. If a group has no rules,
> @@ -958,6 +961,7 @@ queue_bio:
> tg->io_disp[rw], tg->iops[rw],
> tg->nr_queued[READ], tg->nr_queued[WRITE]);
>
> + bio_associate_current(bio);

blk-throttle is just one of places where we lose submitter's context info.
There are others too.

As Kent said, doing this at more generic level will probably make sense.
I think dm layer also might benefit from it. We had issues with losing
this process context info while bio is being submitted with a helper thread.

https://lkml.org/lkml/2010/10/24/59

But I guess this will require some changes at dm layer too so that it
takes extra bio->ioc and bio->blkcg references at cloning time.

Thanks
Vivek


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