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Date: Wed, 9 May 2018 09:18:17 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Mike Galbraith <efault@....de>,
Paolo Valente <paolo.valente@...aro.org>
Cc: Christoph Hellwig <hch@....de>,
linux-block <linux-block@...r.kernel.org>,
Ulf Hansson <ulf.hansson@...aro.org>,
LKML <linux-kernel@...r.kernel.org>,
Linus Walleij <linus.walleij@...aro.org>,
Oleksandr Natalenko <oleksandr@...alenko.name>
Subject: Re: bug in tag handling in blk-mq?
On 5/8/18 10:11 PM, Mike Galbraith wrote:
> On Tue, 2018-05-08 at 19:09 -0600, Jens Axboe wrote:
>>
>> Alright, I managed to reproduce it. What I think is happening is that
>> BFQ is limiting the inflight case to something less than the wake
>> batch for sbitmap, which can lead to stalls. I don't have time to test
>> this tonight, but perhaps you can give it a go when you are back at it.
>> If not, I'll try tomorrow morning.
>>
>> If this is the issue, I can turn it into a real patch. This is just to
>> confirm that the issue goes away with the below.
>
> Confirmed. Impressive high speed bug stomping.
Well, that's good news. Can I get you to try this patch? Needs to be
split, but it'll be good to know if this fixes it too (since it's an
ACTUAL attempt at a fix, not just a masking).
diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index ebc264c87a09..b0dbfd297d20 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -533,19 +533,20 @@ static void bfq_update_depths(struct bfq_data *bfqd, struct sbitmap_queue *bt)
* Limit depths of async I/O and sync writes so as to counter both
* problems.
*/
-static void bfq_limit_depth(unsigned int op, struct blk_mq_alloc_data *data)
+static int bfq_limit_depth(unsigned int op, struct blk_mq_alloc_data *data)
{
struct blk_mq_tags *tags = blk_mq_tags_from_data(data);
struct bfq_data *bfqd = data->q->elevator->elevator_data;
struct sbitmap_queue *bt;
+ int old_depth;
if (op_is_sync(op) && !op_is_write(op))
- return;
+ return 0;
if (data->flags & BLK_MQ_REQ_RESERVED) {
if (unlikely(!tags->nr_reserved_tags)) {
WARN_ON_ONCE(1);
- return;
+ return 0;
}
bt = &tags->breserved_tags;
} else
@@ -554,12 +555,18 @@ static void bfq_limit_depth(unsigned int op, struct blk_mq_alloc_data *data)
if (unlikely(bfqd->sb_shift != bt->sb.shift))
bfq_update_depths(bfqd, bt);
+ old_depth = data->shallow_depth;
data->shallow_depth =
bfqd->word_depths[!!bfqd->wr_busy_queues][op_is_sync(op)];
bfq_log(bfqd, "[%s] wr_busy %d sync %d depth %u",
__func__, bfqd->wr_busy_queues, op_is_sync(op),
data->shallow_depth);
+
+ if (old_depth != data->shallow_depth)
+ return data->shallow_depth;
+
+ return 0;
}
static struct bfq_queue *
diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index 25c14c58385c..0c53a254671f 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -16,6 +16,32 @@
#include "blk-mq-tag.h"
#include "blk-wbt.h"
+void blk_mq_sched_limit_depth(struct elevator_queue *e,
+ struct blk_mq_alloc_data *data, unsigned int op)
+{
+ struct blk_mq_tags *tags = blk_mq_tags_from_data(data);
+ struct sbitmap_queue *bt;
+ int ret;
+
+ /*
+ * Flush requests are special and go directly to the
+ * dispatch list.
+ */
+ if (op_is_flush(op) || !e->type->ops.mq.limit_depth)
+ return;
+
+ ret = e->type->ops.mq.limit_depth(op, data);
+ if (!ret)
+ return;
+
+ if (data->flags & BLK_MQ_REQ_RESERVED)
+ bt = &tags->breserved_tags;
+ else
+ bt = &tags->bitmap_tags;
+
+ sbitmap_queue_shallow_depth(bt, ret);
+}
+
void blk_mq_sched_free_hctx_data(struct request_queue *q,
void (*exit)(struct blk_mq_hw_ctx *))
{
diff --git a/block/blk-mq-sched.h b/block/blk-mq-sched.h
index 1e9c9018ace1..6abebc1b9ae0 100644
--- a/block/blk-mq-sched.h
+++ b/block/blk-mq-sched.h
@@ -5,6 +5,9 @@
#include "blk-mq.h"
#include "blk-mq-tag.h"
+void blk_mq_sched_limit_depth(struct elevator_queue *e,
+ struct blk_mq_alloc_data *data, unsigned int op);
+
void blk_mq_sched_free_hctx_data(struct request_queue *q,
void (*exit)(struct blk_mq_hw_ctx *));
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 4e9d83594cca..1bb7aa40c192 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -357,13 +357,7 @@ static struct request *blk_mq_get_request(struct request_queue *q,
if (e) {
data->flags |= BLK_MQ_REQ_INTERNAL;
-
- /*
- * Flush requests are special and go directly to the
- * dispatch list.
- */
- if (!op_is_flush(op) && e->type->ops.mq.limit_depth)
- e->type->ops.mq.limit_depth(op, data);
+ blk_mq_sched_limit_depth(e, data, op);
}
tag = blk_mq_get_tag(data);
diff --git a/block/kyber-iosched.c b/block/kyber-iosched.c
index 564967fafe5f..d2622386c115 100644
--- a/block/kyber-iosched.c
+++ b/block/kyber-iosched.c
@@ -433,17 +433,23 @@ static void rq_clear_domain_token(struct kyber_queue_data *kqd,
}
}
-static void kyber_limit_depth(unsigned int op, struct blk_mq_alloc_data *data)
+static int kyber_limit_depth(unsigned int op, struct blk_mq_alloc_data *data)
{
+ struct kyber_queue_data *kqd = data->q->elevator->elevator_data;
+
+ if (op_is_sync(op))
+ return 0;
+
/*
* We use the scheduler tags as per-hardware queue queueing tokens.
* Async requests can be limited at this stage.
*/
- if (!op_is_sync(op)) {
- struct kyber_queue_data *kqd = data->q->elevator->elevator_data;
-
+ if (data->shallow_depth != kqd->async_depth) {
data->shallow_depth = kqd->async_depth;
+ return data->shallow_depth;
}
+
+ return 0;
}
static void kyber_prepare_request(struct request *rq, struct bio *bio)
diff --git a/include/linux/elevator.h b/include/linux/elevator.h
index 6d9e230dffd2..b2712f4ca9f1 100644
--- a/include/linux/elevator.h
+++ b/include/linux/elevator.h
@@ -105,7 +105,7 @@ struct elevator_mq_ops {
int (*request_merge)(struct request_queue *q, struct request **, struct bio *);
void (*request_merged)(struct request_queue *, struct request *, enum elv_merge);
void (*requests_merged)(struct request_queue *, struct request *, struct request *);
- void (*limit_depth)(unsigned int, struct blk_mq_alloc_data *);
+ int (*limit_depth)(unsigned int, struct blk_mq_alloc_data *);
void (*prepare_request)(struct request *, struct bio *bio);
void (*finish_request)(struct request *);
void (*insert_requests)(struct blk_mq_hw_ctx *, struct list_head *, bool);
diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
index 841585f6e5f2..99059789f45f 100644
--- a/include/linux/sbitmap.h
+++ b/include/linux/sbitmap.h
@@ -164,6 +164,17 @@ static inline void sbitmap_free(struct sbitmap *sb)
void sbitmap_resize(struct sbitmap *sb, unsigned int depth);
/**
+ * sbitmap_queue_shallow_depth() - Inform sbitmap about shallow depth changes
+ * @sbq: Bitmap queue in question
+ * @depth: Shallow depth limit
+ *
+ * Due to how sbitmap does batched wakes, if a user of sbitmap updates the
+ * shallow depth, then we might need to update our batched wake counts.
+ *
+ */
+void sbitmap_queue_shallow_depth(struct sbitmap_queue *sbq, unsigned int depth);
+
+/**
* sbitmap_get() - Try to allocate a free bit from a &struct sbitmap.
* @sb: Bitmap to allocate from.
* @alloc_hint: Hint for where to start searching for a free bit.
diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index e6a9c06ec70c..563ae9d75fb8 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -327,7 +327,8 @@ int sbitmap_queue_init_node(struct sbitmap_queue *sbq, unsigned int depth,
}
EXPORT_SYMBOL_GPL(sbitmap_queue_init_node);
-void sbitmap_queue_resize(struct sbitmap_queue *sbq, unsigned int depth)
+static void sbitmap_queue_update_batch_wake(struct sbitmap_queue *sbq,
+ unsigned int depth)
{
unsigned int wake_batch = sbq_calc_wake_batch(depth);
int i;
@@ -342,6 +343,11 @@ void sbitmap_queue_resize(struct sbitmap_queue *sbq, unsigned int depth)
for (i = 0; i < SBQ_WAIT_QUEUES; i++)
atomic_set(&sbq->ws[i].wait_cnt, 1);
}
+}
+
+void sbitmap_queue_resize(struct sbitmap_queue *sbq, unsigned int depth)
+{
+ sbitmap_queue_update_batch_wake(sbq, depth);
sbitmap_resize(&sbq->sb, depth);
}
EXPORT_SYMBOL_GPL(sbitmap_queue_resize);
@@ -403,6 +409,15 @@ int __sbitmap_queue_get_shallow(struct sbitmap_queue *sbq,
}
EXPORT_SYMBOL_GPL(__sbitmap_queue_get_shallow);
+/*
+ * User has limited the shallow depth to 'depth', update batch wake counts
+ */
+void sbitmap_queue_shallow_depth(struct sbitmap_queue *sbq, unsigned int depth)
+{
+ sbitmap_queue_update_batch_wake(sbq, depth);
+}
+EXPORT_SYMBOL_GPL(sbitmap_queue_shallow_depth);
+
static struct sbq_wait_state *sbq_wake_ptr(struct sbitmap_queue *sbq)
{
int i, wake_index;
--
Jens Axboe
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