On Mon, 11 Apr 2011 11:19:58 +0200 Jens Axboe <jaxboe@xxxxxxxxxxxx> wrote: > On 2011-04-11 06:50, NeilBrown wrote: > > 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. OK - though I'm not 100% the patch fixes the problem - just that it hides the symptom for me. I might try instrumenting the code a bit more and see if I can find exactly where it is re-entering flush_plug_list - as that seems to be what is happening. And yeah - list_split_init is probably better. I just never remember exactly what list_split means and have to look it up every time, where as list_add/list_del are very clear to me. > > > From 687b189c02276887dd7d5b87a817da9f67ed3c2c Mon Sep 17 00:00:00 2001 > > From: NeilBrown <neilb@xxxxxxx> > > 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@xxxxxxxxxxxx> > > Signed-off-by: NeilBrown <neilb@xxxxxxx> > > --- > > 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. > I'm sure one of us is missing something (probably both) but I'm not sure what. The callback is central. It is simply to use plugging in md. Just like blk-core, md will notice that a blk_plug is active and will put requests aside. I then need something to call in to md when blk_finish_plug is called so that put-aside requests can be released. As md can be built as a module, that call must be a call-back of some sort. blk-core doesn't need to register blk_plug_flush because that is never in a module, so it can be called directly. But the md equivalent could be in a module, so I need to be able to register a call back. Does that help? Thanks, NeilBrown -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel