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SubjectRe: [PATCH 05/10] block: remove per-queue plugging
On 2011-04-11 06:50, NeilBrown wrote:
> On Tue, 5 Apr 2011 13:05:41 +1000 NeilBrown <neilb@suse.de> wrote:
>
>> On Wed, 9 Mar 2011 19:58:10 -0500 Mike Snitzer <snitzer@redhat.com> wrote:
>>
>>> Also, in your MD changes, you removed all calls to md_unplug() but
>>> didn't remove md_unplug(). Seems it should be removed along with the
>>> 'plug' member of 'struct mddev_t'? Neil?
>>
>> I've been distracted by other things and only just managed to have a look at
>> this.
>>
>> The new plugging code seems to completely ignore the needs of stacked devices
>> - or at least my needs in md.
>>
>> For RAID1 with a write-intent-bitmap, I queue all write requests and then on
>> an unplug I update the write-intent-bitmap to mark all the relevant blocks
>> and then release the writes.
>>
>> With the new code there is no way for an unplug event to wake up the raid1d
>> thread to start the writeout - I haven't tested it but I suspect it will just
>> hang.
>>
>> Similarly for RAID5 I gather write bios (long before they become 'struct
>> request' which is what the plugging code understands) and on an unplug event
>> I release the writes - hopefully with enough bios per stripe so that we don't
>> need to pre-read.
>>
>> Possibly the simplest fix would be to have a second list_head in 'struct
>> blk_plug' which contained callbacks (a function pointer a list_head in a
>> struct which is passed as an arg to the function!).
>> blk_finish_plug could then walk the list and call the call-backs.
>> It would be quite easy to hook into that.
>
> I've implemented this and it seems to work.
> Jens: could you please review and hopefully ack the patch below, and let
> me know if you will submit it or should I?
>
> My testing of this combined with some other patches which cause various md
> personalities to use it shows up a bug somewhere.
>
> The symptoms are crashes in various places in blk-core and sometimes
> elevator.c
> list_sort occurs fairly often included in the stack but not always.
>
> This patch
>
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 273d60b..903ce8d 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -2674,19 +2674,23 @@ static void flush_plug_list(struct blk_plug *plug)
> struct request_queue *q;
> unsigned long flags;
> struct request *rq;
> + struct list_head head;
>
> BUG_ON(plug->magic != PLUG_MAGIC);
>
> if (list_empty(&plug->list))
> return;
> + list_add(&head, &plug->list);
> + list_del_init(&plug->list);
>
> if (plug->should_sort)
> - list_sort(NULL, &plug->list, plug_rq_cmp);
> + list_sort(NULL, &head, plug_rq_cmp);
> + plug->should_sort = 0;
>
> q = NULL;
> local_irq_save(flags);
> - while (!list_empty(&plug->list)) {
> - rq = list_entry_rq(plug->list.next);
> + while (!list_empty(&head)) {
> + rq = list_entry_rq(head.next);
> list_del_init(&rq->queuelist);
> BUG_ON(!(rq->cmd_flags & REQ_ON_PLUG));
> BUG_ON(!rq->q);
>
>
> makes the symptom go away. It simply moves the plug list onto a separate
> list head before sorting and processing it.
> My test was simply writing to a RAID1 with dd:
> while true; do dd if=/dev/zero of=/dev/md0 size=4k; done
>
> Obviously all writes go to two devices so the plug list will always need
> sorting.
>
> The only explanation I can come up with is that very occasionally schedule on
> 2 separate cpus calls blk_flush_plug for the same task. I don't understand
> the scheduler nearly well enough to know if or how that can happen.
> However with this patch in place I can write to a RAID1 constantly for half
> an hour, and without it, the write rarely lasts for 3 minutes.

Or perhaps if the request_fn blocks, that would be problematic. So the
patch is likely a good idea even for that case.

I'll merge it, changing it to list_splice_init() as I think that would
be more clear.

> From 687b189c02276887dd7d5b87a817da9f67ed3c2c Mon Sep 17 00:00:00 2001
> From: NeilBrown <neilb@suse.de>
> Date: Thu, 7 Apr 2011 13:16:59 +1000
> Subject: [PATCH] Enhance new plugging support to support general callbacks.
>
> md/raid requires an unplug callback, but as it does not uses
> requests the current code cannot provide one.
>
> So allow arbitrary callbacks to be attached to the blk_plug.
>
> Cc: Jens Axboe <jaxboe@fusionio.com>
> Signed-off-by: NeilBrown <neilb@suse.de>
> ---
> block/blk-core.c | 13 +++++++++++++
> include/linux/blkdev.h | 7 ++++++-
> 2 files changed, 19 insertions(+), 1 deletions(-)
>
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 725091d..273d60b 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -2644,6 +2644,7 @@ void blk_start_plug(struct blk_plug *plug)
>
> plug->magic = PLUG_MAGIC;
> INIT_LIST_HEAD(&plug->list);
> + INIT_LIST_HEAD(&plug->cb_list);
> plug->should_sort = 0;
>
> /*
> @@ -2717,9 +2718,21 @@ static void flush_plug_list(struct blk_plug *plug)
> local_irq_restore(flags);
> }
>
> +static void flush_plug_callbacks(struct blk_plug *plug)
> +{
> + while (!list_empty(&plug->cb_list)) {
> + struct blk_plug_cb *cb = list_first_entry(&plug->cb_list,
> + struct blk_plug_cb,
> + list);
> + list_del(&cb->list);
> + cb->callback(cb);
> + }
> +}
> +
> static void __blk_finish_plug(struct task_struct *tsk, struct blk_plug *plug)
> {
> flush_plug_list(plug);
> + flush_plug_callbacks(plug);
>
> if (plug == tsk->plug)
> tsk->plug = NULL;
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 32176cc..3e5e604 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -857,8 +857,13 @@ extern void blk_put_queue(struct request_queue *);
> struct blk_plug {
> unsigned long magic;
> struct list_head list;
> + struct list_head cb_list;
> unsigned int should_sort;
> };
> +struct blk_plug_cb {
> + struct list_head list;
> + void (*callback)(struct blk_plug_cb *);
> +};
>
> extern void blk_start_plug(struct blk_plug *);
> extern void blk_finish_plug(struct blk_plug *);
> @@ -876,7 +881,7 @@ static inline bool blk_needs_flush_plug(struct task_struct *tsk)
> {
> struct blk_plug *plug = tsk->plug;
>
> - return plug && !list_empty(&plug->list);
> + return plug && (!list_empty(&plug->list) || !list_empty(&plug->cb_list));
> }
>
> /*

Maybe I'm missing something, but why do you need those callbacks? If
it's to use plugging yourself, perhaps we can just ensure that those
don't get assigned in the task - so it would be have to used with care.

It's not that I disagree to these callbacks, I just want to ensure I
understand why you need them.

--
Jens Axboe



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