On Fri, May 15, 2020 at 05:38:47PM +0200, Christoph Hellwig wrote:
> On Fri, May 15, 2020 at 09:41:52AM +0800, Ming Lei wrote:
> > Disable preempt for a little while during allocating request tag, so
> > request's tag(internal tag) is always allocated on the cpu of data->ctx,
> > prepare for improving to handle cpu hotplug during allocating request.
> >
> > In the following patch, hctx->state will be checked to see if it becomes
> > inactive which is always set on the last CPU of hctx->cpumask.
>
> I like the idea, but I hate the interface. I really think we need
> to moving assigning the ctx and hctx entirely into blk_mq_get_tag,
> and then just unconditionally disable preemption in there.
Maybe something vaguely like this (on top of the previous patch):
diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index c8e34c1f547fe..f75be6c431ac6 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -111,7 +111,7 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
if (data->flags & BLK_MQ_REQ_RESERVED) {
if (unlikely(!tags->nr_reserved_tags)) {
WARN_ON_ONCE(1);
- return BLK_MQ_TAG_FAIL;
+ goto fail;
}
bt = &tags->breserved_tags;
tag_offset = 0;
@@ -120,12 +120,22 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
tag_offset = tags->nr_reserved_tags;
}
+ if (!data->driver_tag) {
+ preempt_disable();
+ data->ctx = blk_mq_get_ctx(data->q);
+ data->hctx = blk_mq_map_queue(data->q, data->cmd_flags,
+ data->ctx);
+ }
+
+ if (!(data->flags & BLK_MQ_REQ_INTERNAL))
+ data->shared = blk_mq_tag_busy(data->hctx);
+
tag = __blk_mq_get_tag(data, bt);
if (tag != -1)
goto found_tag;
if (data->flags & BLK_MQ_REQ_NOWAIT)
- return BLK_MQ_TAG_FAIL;
+ goto fail;
ws = bt_wait_ptr(bt, data->hctx);
do {
@@ -152,14 +162,14 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
if (tag != -1)
break;
- if (data->preempt_disabled)
+ if (!data->driver_tag)
preempt_enable();
bt_prev = bt;
io_schedule();
sbitmap_finish_wait(bt, ws, &wait);
- if (data->preempt_disabled)
+ if (!data->driver_tag)
preempt_disable();
data->ctx = blk_mq_get_ctx(data->q);
data->hctx = blk_mq_map_queue(data->q, data->cmd_flags,
@@ -184,17 +194,20 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
sbitmap_finish_wait(bt, ws, &wait);
found_tag:
- /*
- * we are in request allocation, check if the current hctx is inactive.
- * If yes, give up this allocation. We will retry on another new online
- * CPU.
- */
- if (data->preempt_disabled && unlikely(test_bit(BLK_MQ_S_INACTIVE,
- &data->hctx->state))) {
- blk_mq_put_tag(tags, data->ctx, tag + tag_offset);
- return BLK_MQ_TAG_FAIL;
+ if (!data->driver_tag) {
+ /* fail allocations for inactive contexts. */
+ if (unlikely(!test_bit(BLK_MQ_S_INACTIVE, &data->hctx->state)))
+ goto fail_free_tag;
+ preempt_enable();
}
return tag + tag_offset;
+
+fail_free_tag:
+ blk_mq_put_tag(tags, data->ctx, tag + tag_offset);
+fail:
+ data->ctx = NULL;
+ data->hctx = NULL;
+ return BLK_MQ_TAG_FAIL;
}
void blk_mq_put_tag(struct blk_mq_tags *tags, struct blk_mq_ctx *ctx,
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 52b8917ea7cb6..298557d00e081 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -347,10 +347,6 @@ static void __blk_mq_get_request(struct blk_mq_alloc_data *data)
WARN_ON_ONCE(data->ctx || data->hctx);
- preempt_disable();
- data->preempt_disabled = true;
- data->ctx = blk_mq_get_ctx(q);
- data->hctx = blk_mq_map_queue(q, data->cmd_flags, data->ctx);
if (data->cmd_flags & REQ_NOWAIT)
data->flags |= BLK_MQ_REQ_NOWAIT;
@@ -366,15 +362,10 @@ static void __blk_mq_get_request(struct blk_mq_alloc_data *data)
e->type->ops.limit_depth &&
!(data->flags & BLK_MQ_REQ_RESERVED))
e->type->ops.limit_depth(data->cmd_flags, data);
- } else {
- blk_mq_tag_busy(data->hctx);
}
tag = blk_mq_get_tag(data);
- preempt_enable();
if (tag == BLK_MQ_TAG_FAIL) {
- data->ctx = NULL;
- data->hctx = NULL;
blk_queue_exit(q);
return;
}
@@ -1042,8 +1033,8 @@ bool blk_mq_get_driver_tag(struct request *rq)
.hctx = rq->mq_hctx,
.flags = BLK_MQ_REQ_NOWAIT,
.cmd_flags = rq->cmd_flags,
+ .driver_tag = true,
};
- bool shared;
if (rq->tag != -1)
return true;
@@ -1051,10 +1042,9 @@ bool blk_mq_get_driver_tag(struct request *rq)
if (blk_mq_tag_is_reserved(data.hctx->sched_tags, rq->internal_tag))
data.flags |= BLK_MQ_REQ_RESERVED;
- shared = blk_mq_tag_busy(data.hctx);
rq->tag = blk_mq_get_tag(&data);
if (rq->tag >= 0) {
- if (shared) {
+ if (data.shared) {
rq->rq_flags |= RQF_MQ_INFLIGHT;
atomic_inc(&data.hctx->nr_active);
}
diff --git a/block/blk-mq.h b/block/blk-mq.h
index e07be009831e9..563d096ea0cf4 100644
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -153,14 +153,15 @@ struct blk_mq_alloc_data {
blk_mq_req_flags_t flags;
unsigned int shallow_depth;
unsigned int cmd_flags;
- bool preempt_disabled;
+ bool driver_tag;
/* input & output parameter */
struct blk_mq_ctx *ctx;
struct blk_mq_hw_ctx *hctx;
- /* output parameter for __blk_mq_get_request */
+ /* output parameters */
struct request *rq;
+ bool shared;
};
static inline struct blk_mq_tags *blk_mq_tags_from_data(struct blk_mq_alloc_data *data)
