On Thu, Jul 07, 2016 at 10:16:16AM +0200, Lars Ellenberg wrote:
> > > Instead, I suggest to distinguish between recursive calls to
> > > generic_make_request(), and pushing back the remainder part in
> > > blk_queue_split(), by pointing current->bio_lists to a
> > > struct recursion_to_iteration_bio_lists {
> > > struct bio_list recursion;
> > > struct bio_list remainder;
> > > }
> > >
> > > To have all bios targeted to drivers lower in the stack processed before
> > > processing the next piece of a bio targeted at the higher levels,
> > > as long as queued bios resulting from recursion are available,
> > > they will continue to be processed in FIFO order.
> > > Pushed back bio-parts resulting from blk_queue_split() will be processed
> > > in LIFO order, one-by-one, whenever the recursion list becomes empty.
> >
> > I really like this change. It seems to precisely address the problem.
> > The "problem" being that requests for "this" device are potentially
> > mixed up with requests from underlying devices.
> > However I'm not sure it is quite general enough.
> >
> > The "remainder" list is a stack of requests aimed at "this" level or
> > higher, and I think it will always exactly fit that description.
> > The "recursion" list needs to be a queue of requests aimed at the next
> > level down, and that doesn't quiet work, because once you start acting
> > on the first entry in that list, all the rest become "this" level.
>
> Uhm, well,
> that's how it has been since you introduced this back in 2007, d89d879.
> And it worked.
>
> > I think you can address this by always calling ->make_request_fn with an
> > empty "recursion", then after the call completes, splice the "recursion"
> > list that resulted (if any) on top of the "remainder" stack.
> >
> > This way, the "remainder" stack is always "requests for lower-level
> > devices before request for upper level devices" and the "recursion"
> > queue is always "requests for devices below the current level".
>
> Yes, I guess that would work as well,
> but may need "empirical proof" to check for performance regressions.
>
> > I also really *don't* like the idea of punting to a separate thread - it
> > seems to be just delaying the problem.
> >
> > Can you try move the bio_list_init(->recursion) call to just before
> > the ->make_request_fn() call, and adding
> > bio_list_merge_head(->remainder, ->recursion)
> > just after?
> > (or something like that) and confirm it makes sense, and works?
>
> Sure, will do.
Attached,
on top of the patch of my initial post.
Also fixes the issue for me.
> I'd suggest this would be a patch on its own though, on top of this one.
> Because it would change the order in which stacked bios are processed
> wrt the way it used to be since 2007 (my suggestion as is does not).
>
> Which may change performance metrics.
> It may even improve some of them,
> or maybe it does nothing, but we don't know.
>
> Thanks,
>
> Lars
>
>From 73254eae63786aca0af10e42e5b41465c90d8da8 Mon Sep 17 00:00:00 2001
From: Lars Ellenberg <lars.ellenberg@xxxxxxxxxx>
Date: Thu, 7 Jul 2016 11:03:30 +0200
Subject: [PATCH] block: generic_make_request() recursive bios: process deepest
levels first
By providing each q->make_request_fn() with an empty "recursion"
bio_list, then merging any recursively submitted bios to the
head of the "remainder" list, we can make the recursion-to-iteration
logic in generic_make_request() process deepest level bios first.
---
As suggested by Neil Brown while discussing
[RFC] block: fix blk_queue_split() resource exhaustion
https://lkml.org/lkml/2016/7/7/27
Stack: qA -> qB -> qC -> qD
=== Without this patch:
generic_make_request(bio_orig to qA)
recursion: empty, remainder: empty
qA->make_request_fn(bio_orig)
potential call to bio_queue_split()
result: bio_S, bio_R
recursion: empty, remainder: bio_R
bio_S
generic_make_request(bio_S to qB)
recursion: bio_S, remainder: bio_R
<- return
pop: recursion: empty, remainder: bio_R
qB->make_request_fn(bio_S)
remap, maybe many clones because of striping
generic_make_request(clones to qC)
recursion: bio_C1, bio_C2, bio_C3
remainder: bio_R
<- return
pop: recursion: bio_C2, bio_C3,
remainder: bio_R
qC->make_request_fn(bio_C1)
remap, ...
generic_make_request(clones to qD)
recursion: bio_C2, bio_C3, bio_D1_1, bio_D1_2
remainder: bio_R
<- return
pop: recursion: bio_C3, bio_D1_1, bio_D1_2
remainder: bio_R
qC->make_request_fn(bio_C2)
recursion: bio_C3, bio_D1_1, bio_D1_2, bio_D2_1, bio_D2_2
remainder: bio_R
<- return
pop: recursion: bio_D1_1, bio_D1_2, bio_D2_1, bio_D2_2
remainder: bio_R
qC->make_request_fn(bio_C3)
...
=== With this patch:
generic_make_request(bio_orig to qA)
recursion: empty, remainder: empty
qA->make_request_fn(bio_orig)
potential call to bio_queue_split()
result: bio_S, bio_R
recursion: empty, remainder: bio_R
bio_S
generic_make_request(bio_S to qB)
recursion: bio_S, remainder: bio_R
<- return
merge_head:
recursion: empty, remainder: bio_S, bio_R
pop: recursion: empty, remainder: bio_R
qB->make_request_fn(bio_S)
remap, maybe many clones because of striping
generic_make_request(clones to qC)
recursion: bio_C1, bio_C2, bio_C3
remainder: bio_R
<- return
merge_head:
recursion: empty
remainder: bio_C1, bio_C2, bio_C3, bio_R
pop: remainder: bio_C2, bio_C3, bio_R
qC->make_request_fn(bio_C1)
remap, ...
generic_make_request(clones to qD)
recursion: bio_D1_1, bio_D1_2
remainder: bio_C2, bio_C3, bio_R
<- return
merge_head:
recursion: empty
remainder: bio_D1_1, bio_D1_2, bio_C2, bio_C3, bio_R
pop
qC->make_request_fn(bio_D1_1)
remainder: bio_D1_2, bio_C2, bio_C3, bio_R
...
---
block/bio.c | 17 ++++++++++++++---
block/blk-core.c | 10 +++++-----
2 files changed, 19 insertions(+), 8 deletions(-)
diff --git a/block/bio.c b/block/bio.c
index 2ffcea0..92733ce 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -366,13 +366,17 @@ static void punt_bios_to_rescuer(struct bio_set *bs)
*/
bio_list_init(&punt);
- bio_list_init(&nopunt);
+ bio_list_init(&nopunt);
while ((bio = bio_list_pop(¤t->bio_lists->recursion)))
bio_list_add(bio->bi_pool == bs ? &punt : &nopunt, bio);
-
current->bio_lists->recursion = nopunt;
+ bio_list_init(&nopunt);
+ while ((bio = bio_list_pop(¤t->bio_lists->remainder)))
+ bio_list_add(bio->bi_pool == bs ? &punt : &nopunt, bio);
+ current->bio_lists->remainder = nopunt;
+
spin_lock(&bs->rescue_lock);
bio_list_merge(&bs->rescue_list, &punt);
spin_unlock(&bs->rescue_lock);
@@ -380,6 +384,13 @@ static void punt_bios_to_rescuer(struct bio_set *bs)
queue_work(bs->rescue_workqueue, &bs->rescue_work);
}
+static bool current_has_pending_bios(void)
+{
+ return current->bio_lists &&
+ (!bio_list_empty(¤t->bio_lists->recursion) ||
+ !bio_list_empty(¤t->bio_lists->remainder));
+}
+
/**
* bio_alloc_bioset - allocate a bio for I/O
* @gfp_mask: the GFP_ mask given to the slab allocator
@@ -459,7 +470,7 @@ struct bio *bio_alloc_bioset(gfp_t gfp_mask, int nr_iovecs, struct bio_set *bs)
* workqueue before we retry with the original gfp_flags.
*/
- if (current->bio_lists && !bio_list_empty(¤t->bio_lists->recursion))
+ if (current_has_pending_bios())
gfp_mask &= ~__GFP_DIRECT_RECLAIM;
p = mempool_alloc(bs->bio_pool, gfp_mask);
diff --git a/block/blk-core.c b/block/blk-core.c
index f03ff4c..675131b 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -2070,22 +2070,22 @@ blk_qc_t generic_make_request(struct bio *bio)
* bio_list, and call into ->make_request() again.
*/
BUG_ON(bio->bi_next);
- bio_list_init(&bio_lists_on_stack.recursion);
bio_list_init(&bio_lists_on_stack.remainder);
current->bio_lists = &bio_lists_on_stack;
do {
struct request_queue *q = bdev_get_queue(bio->bi_bdev);
if (likely(blk_queue_enter(q, false) == 0)) {
+ bio_list_init(&bio_lists_on_stack.recursion);
ret = q->make_request_fn(q, bio);
-
blk_queue_exit(q);
+ bio_list_merge_head(&bio_lists_on_stack.remainder,
+ &bio_lists_on_stack.recursion);
+ /* XXX bio_list_init(&bio_lists_on_stack.recursion); */
} else {
bio_io_error(bio);
}
- bio = bio_list_pop(¤t->bio_lists->recursion);
- if (!bio)
- bio = bio_list_pop(¤t->bio_lists->remainder);
+ bio = bio_list_pop(¤t->bio_lists->remainder);
} while (bio);
current->bio_lists = NULL; /* deactivate */
--
1.9.1
--
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