@@ -1700,6 +1700,7 @@ EXPORT_SYMBOL_GPL(blk_rq_check_limits);
int blk_insert_cloned_request(struct request_queue *q, struct request *rq)
{
unsigned long flags;
+ int where = ELEVATOR_INSERT_BACK;
if (blk_rq_check_limits(q, rq))
return -EIO;
@@ -1716,7 +1717,10 @@ int blk_insert_cloned_request(struct request_queue *q, struct request *rq)
*/
BUG_ON(blk_queued_rq(rq));
- add_acct_request(q, rq, ELEVATOR_INSERT_BACK);
+ if (rq->cmd_flags & (REQ_FLUSH|REQ_FUA))
+ where = ELEVATOR_INSERT_FLUSH;
+
+ add_acct_request(q, rq, where);
spin_unlock_irqrestore(q->queue_lock, flags);
return 0;
@@ -2273,7 +2277,7 @@ static bool blk_end_bidi_request(struct request *rq, int error,
* %false - we are done with this request
* %true - still buffers pending for this request
**/
-static bool __blk_end_bidi_request(struct request *rq, int error,
+bool __blk_end_bidi_request(struct request *rq, int error,
unsigned int nr_bytes, unsigned int bidi_bytes)
{
if (blk_update_bidi_request(rq, error, nr_bytes, bidi_bytes))
@@ -123,7 +123,7 @@ static void blk_flush_restore_request(struct request *rq)
/* make @rq a normal request */
rq->cmd_flags &= ~REQ_FLUSH_SEQ;
- rq->end_io = NULL;
+ rq->end_io = rq->flush.saved_end_io;
}
/**
@@ -301,9 +301,6 @@ void blk_insert_flush(struct request *rq)
unsigned int fflags = q->flush_flags; /* may change, cache */
unsigned int policy = blk_flush_policy(fflags, rq);
- BUG_ON(rq->end_io);
- BUG_ON(!rq->bio || rq->bio != rq->biotail);
-
/*
* @policy now records what operations need to be done. Adjust
* REQ_FLUSH and FUA for the driver.
@@ -313,6 +310,19 @@ void blk_insert_flush(struct request *rq)
rq->cmd_flags &= ~REQ_FUA;
/*
+ * An empty flush handed down from a stacking driver may
+ * translate into nothing if the underlying device does not
+ * advertise a write-back cache. In this case, simply
+ * complete the request.
+ */
+ if (!policy) {
+ __blk_end_bidi_request(rq, 0, 0, 0);
+ return;
+ }
+
+ BUG_ON(!rq->bio || rq->bio != rq->biotail);
+
+ /*
* If there's data but flush is not necessary, the request can be
* processed directly without going through flush machinery. Queue
* for normal execution.
@@ -320,6 +330,7 @@ void blk_insert_flush(struct request *rq)
if ((policy & REQ_FSEQ_DATA) &&
!(policy & (REQ_FSEQ_PREFLUSH | REQ_FSEQ_POSTFLUSH))) {
list_add_tail(&rq->queuelist, &q->queue_head);
+ blk_run_queue_async(q);
return;
}
@@ -330,6 +341,7 @@ void blk_insert_flush(struct request *rq)
memset(&rq->flush, 0, sizeof(rq->flush));
INIT_LIST_HEAD(&rq->flush.list);
rq->cmd_flags |= REQ_FLUSH_SEQ;
+ rq->flush.saved_end_io = rq->end_io; /* Usually NULL */
rq->end_io = flush_data_end_io;
blk_flush_complete_seq(rq, REQ_FSEQ_ACTIONS & ~policy, 0);
@@ -17,6 +17,8 @@ int blk_rq_append_bio(struct request_queue *q, struct request *rq,
struct bio *bio);
void blk_dequeue_request(struct request *rq);
void __blk_queue_free_tags(struct request_queue *q);
+bool __blk_end_bidi_request(struct request *rq, int error,
+ unsigned int nr_bytes, unsigned int bidi_bytes);
void blk_rq_timed_out_timer(unsigned long data);
void blk_delete_timer(struct request *);
@@ -117,6 +117,7 @@ struct request {
struct {
unsigned int seq;
struct list_head list;
+ rq_end_io_fn *saved_end_io;
} flush;
};
Hi, Commit ae1b1539622fb46e51b4d13b3f9e5f4c713f86ae, block: reimplement FLUSH/FUA to support merge, introduced a performance regression when running any sort of fsyncing workload using dm-multipath and certain storage (in our case, an HP EVA). The test I ran was fs_mark, and it dropped from ~800 files/sec on ext4 to ~100 files/sec. It turns out that dm-multipath always advertised flush+fua support, and passed commands on down the stack, where those flags used to get stripped off. The above commit changed that behavior: static inline struct request *__elv_next_request(struct request_queue *q) { struct request *rq; while (1) { - while (!list_empty(&q->queue_head)) { + if (!list_empty(&q->queue_head)) { rq = list_entry_rq(q->queue_head.next); - if (!(rq->cmd_flags & (REQ_FLUSH | REQ_FUA)) || - (rq->cmd_flags & REQ_FLUSH_SEQ)) - return rq; - rq = blk_do_flush(q, rq); - if (rq) - return rq; + return rq; } Note that previously, a command would come in here, have REQ_FLUSH|REQ_FUA set, and then get handed off to blk_do_flush: struct request *blk_do_flush(struct request_queue *q, struct request *rq) { unsigned int fflags = q->flush_flags; /* may change, cache it */ bool has_flush = fflags & REQ_FLUSH, has_fua = fflags & REQ_FUA; bool do_preflush = has_flush && (rq->cmd_flags & REQ_FLUSH); bool do_postflush = has_flush && !has_fua && (rq->cmd_flags & REQ_FUA); unsigned skip = 0; ... if (blk_rq_sectors(rq) && !do_preflush && !do_postflush) { rq->cmd_flags &= ~REQ_FLUSH; if (!has_fua) rq->cmd_flags &= ~REQ_FUA; return rq; } So, the flush machinery was bypassed in such cases (q->flush_flags == 0 && rq->cmd_flags & (REQ_FLUSH|REQ_FUA)). Now, however, we don't get into the flush machinery at all. Instead, __elv_next_request just hands a request with flush and fua bits set to the scsi_request_fn, even if the underlying request_queue does not support flush or fua. The agreed upon approach is to fix the flush machinery to allow stacking. While this isn't used in practice (since there is only one request-based dm target, and that target will now reflect the flush flags of the underlying device), it does future-proof the solution, and make it function as designed. In order to make this work, I had to add a field to the struct request, inside the flush structure (to store the original req->end_io). Shaohua had suggested overloading the union with rb_node and completion_data, but the completion data is used by device mapper and can also be used by other drivers. So, I didn't see a way around the additional field. I tested this patch on an HP EVA with both ext4 and xfs, and it recovers the lost performance. Comments and other testers, as always, are appreciated. Cheers, Jeff Signed-off-by: Jeff Moyer <jmoyer@redhat.com> --- Changes from v2->v3: - Got rid of redundant blk_rq_bytes check in blk_insert_flush. Changes from v1->v2: - Moved the detection of empty flush requests into blk_insert_flush. - Got rid of REQ_FLUSH_SEQ in the CLONE_FLAGS. -- dm-devel mailing list dm-devel@redhat.com https://www.redhat.com/mailman/listinfo/dm-devel