Re: [PATCH v5 2/6] block: wire-up support for passthrough plugging

Ming Lei
2022-05-12T05:25:50Z

Hello,

On Wed, May 11, 2022 at 11:17:46AM +0530, Kanchan Joshi wrote:
> From: Jens Axboe <axboe@kernel.dk>
>
> Add support for plugging in passthrough path. When plugging is enabled, the
> requests are added to a plug instead of getting dispatched to the driver.
> And when the plug is finished, the whole batch gets dispatched via
> ->queue_rqs which turns out to be more efficient. Otherwise dispatching
> used to happen via ->queue_rq, one request at a time.
>
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> ---
> block/blk-mq.c | 73 +++++++++++++++++++++++++++-----------------------
> 1 file changed, 39 insertions(+), 34 deletions(-)
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 84d749511f55..2cf011b57cf9 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2340,6 +2340,40 @@ void __blk_mq_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
> blk_mq_hctx_mark_pending(hctx, ctx);
> }
>
> +/*
> + * Allow 2x BLK_MAX_REQUEST_COUNT requests on plug queue for multiple
> + * queues. This is important for md arrays to benefit from merging
> + * requests.
> + */
> +static inline unsigned short blk_plug_max_rq_count(struct blk_plug *plug)
> +{
> + if (plug->multiple_queues)
> + return BLK_MAX_REQUEST_COUNT * 2;
> + return BLK_MAX_REQUEST_COUNT;
> +}
> +
> +static void blk_add_rq_to_plug(struct blk_plug *plug, struct request *rq)
> +{
> + struct request *last = rq_list_peek(&plug->mq_list);
> +
> + if (!plug->rq_count) {
> + trace_block_plug(rq->q);
> + } else if (plug->rq_count >= blk_plug_max_rq_count(plug) ||
> + (!blk_queue_nomerges(rq->q) &&
> + blk_rq_bytes(last) >= BLK_PLUG_FLUSH_SIZE)) {
> + blk_mq_flush_plug_list(plug, false);
> + trace_block_plug(rq->q);
> + }
> +
> + if (!plug->multiple_queues && last && last->q != rq->q)
> + plug->multiple_queues = true;
> + if (!plug->has_elevator && (rq->rq_flags & RQF_ELV))
> + plug->has_elevator = true;
> + rq->rq_next = NULL;
> + rq_list_add(&plug->mq_list, rq);
> + plug->rq_count++;
> +}
> +
> /**
> * blk_mq_request_bypass_insert - Insert a request at dispatch list.
> * @rq: Pointer to request to be inserted.
> @@ -2353,7 +2387,12 @@ void blk_mq_request_bypass_insert(struct request *rq, bool at_head,
> bool run_queue)
> {
> struct blk_mq_hw_ctx *hctx = rq->mq_hctx;
> + struct blk_plug *plug = current->plug;
>
> + if (plug) {
> + blk_add_rq_to_plug(plug, rq);
> + return;
> + }

This way may cause nested plugging, and breaks xfstests generic/131.
Also may cause io hang since request can't be polled before flushing
plug in blk_execute_rq().

I'd suggest to apply the plug in blk_execute_rq_nowait(), such as:

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 2cf011b57cf9..60c29c0229d5 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1169,6 +1169,62 @@ static void blk_end_sync_rq(struct request *rq, blk_status_t error)
complete(waiting);
}

+/*
+ * Allow 2x BLK_MAX_REQUEST_COUNT requests on plug queue for multiple
+ * queues. This is important for md arrays to benefit from merging
+ * requests.
+ */
+static inline unsigned short blk_plug_max_rq_count(struct blk_plug *plug)
+{
+ if (plug->multiple_queues)
+ return BLK_MAX_REQUEST_COUNT * 2;
+ return BLK_MAX_REQUEST_COUNT;
+}
+
+static void blk_add_rq_to_plug(struct blk_plug *plug, struct request *rq)
+{
+ struct request *last = rq_list_peek(&plug->mq_list);
+
+ if (!plug->rq_count) {
+ trace_block_plug(rq->q);
+ } else if (plug->rq_count >= blk_plug_max_rq_count(plug) ||
+ (!blk_queue_nomerges(rq->q) &&
+ blk_rq_bytes(last) >= BLK_PLUG_FLUSH_SIZE)) {
+ blk_mq_flush_plug_list(plug, false);
+ trace_block_plug(rq->q);
+ }
+
+ if (!plug->multiple_queues && last && last->q != rq->q)
+ plug->multiple_queues = true;
+ if (!plug->has_elevator && (rq->rq_flags & RQF_ELV))
+ plug->has_elevator = true;
+ rq->rq_next = NULL;
+ rq_list_add(&plug->mq_list, rq);
+ plug->rq_count++;
+}
+
+static void __blk_execute_rq_nowait(struct request *rq, bool at_head,
+ rq_end_io_fn *done, bool use_plug)
+{
+ WARN_ON(irqs_disabled());
+ WARN_ON(!blk_rq_is_passthrough(rq));
+
+ rq->end_io = done;
+
+ blk_account_io_start(rq);
+
+ if (use_plug && current->plug) {
+ blk_add_rq_to_plug(current->plug, rq);
+ return;
+ }
+ /*
+ * don't check dying flag for MQ because the request won't
+ * be reused after dying flag is set
+ */
+ blk_mq_sched_insert_request(rq, at_head, true, false);
+}
+
+
/**
* blk_execute_rq_nowait - insert a request to I/O scheduler for execution
* @rq: request to insert
@@ -1184,18 +1240,8 @@ static void blk_end_sync_rq(struct request *rq, blk_status_t error)
*/
void blk_execute_rq_nowait(struct request *rq, bool at_head, rq_end_io_fn *done)
{
- WARN_ON(irqs_disabled());
- WARN_ON(!blk_rq_is_passthrough(rq));
-
- rq->end_io = done;
-
- blk_account_io_start(rq);
+ __blk_execute_rq_nowait(rq, at_head, done, true);

- /*
- * don't check dying flag for MQ because the request won't
- * be reused after dying flag is set
- */
- blk_mq_sched_insert_request(rq, at_head, true, false);
}
EXPORT_SYMBOL_GPL(blk_execute_rq_nowait);

@@ -1234,7 +1280,7 @@ blk_status_t blk_execute_rq(struct request *rq, bool at_head)
unsigned long hang_check;

rq->end_io_data = &wait;
- blk_execute_rq_nowait(rq, at_head, blk_end_sync_rq);
+ __blk_execute_rq_nowait(rq, at_head, blk_end_sync_rq, false);

/* Prevent hang_check timer from firing at us during very long I/O */
hang_check = sysctl_hung_task_timeout_secs;
@@ -2340,40 +2386,6 @@ void __blk_mq_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
blk_mq_hctx_mark_pending(hctx, ctx);
}

-/*
- * Allow 2x BLK_MAX_REQUEST_COUNT requests on plug queue for multiple
- * queues. This is important for md arrays to benefit from merging
- * requests.
- */
-static inline unsigned short blk_plug_max_rq_count(struct blk_plug *plug)
-{
- if (plug->multiple_queues)
- return BLK_MAX_REQUEST_COUNT * 2;
- return BLK_MAX_REQUEST_COUNT;
-}
-
-static void blk_add_rq_to_plug(struct blk_plug *plug, struct request *rq)
-{
- struct request *last = rq_list_peek(&plug->mq_list);
-
- if (!plug->rq_count) {
- trace_block_plug(rq->q);
- } else if (plug->rq_count >= blk_plug_max_rq_count(plug) ||
- (!blk_queue_nomerges(rq->q) &&
- blk_rq_bytes(last) >= BLK_PLUG_FLUSH_SIZE)) {
- blk_mq_flush_plug_list(plug, false);
- trace_block_plug(rq->q);
- }
-
- if (!plug->multiple_queues && last && last->q != rq->q)
- plug->multiple_queues = true;
- if (!plug->has_elevator && (rq->rq_flags & RQF_ELV))
- plug->has_elevator = true;
- rq->rq_next = NULL;
- rq_list_add(&plug->mq_list, rq);
- plug->rq_count++;
-}
-
/**
* blk_mq_request_bypass_insert - Insert a request at dispatch list.
* @rq: Pointer to request to be inserted.
@@ -2387,12 +2399,7 @@ void blk_mq_request_bypass_insert(struct request *rq, bool at_head,
bool run_queue)
{
struct blk_mq_hw_ctx *hctx = rq->mq_hctx;
- struct blk_plug *plug = current->plug;

- if (plug) {
- blk_add_rq_to_plug(plug, rq);
- return;
- }
spin_lock(&hctx->lock);
if (at_head)
list_add(&rq->queuelist, &hctx->dispatch);

Thanks,
Ming