block: cleanup the memory stall accounting in submit_bio

Instead of a convoluted chain just check for REQ_OP_READ directly,
and keep all the memory stall code together in a single unlikely
branch.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-core.c

@@ -1148,10 +1148,6 @@ EXPORT_SYMBOL_GPL(direct_make_request);
  */
 blk_qc_t submit_bio(struct bio *bio)
 {
-	bool workingset_read = false;
-	unsigned long pflags;
-	blk_qc_t ret;
-
 	if (blkcg_punt_bio_submit(bio))
 		return BLK_QC_T_NONE;
 
@@ -1170,8 +1166,6 @@ blk_qc_t submit_bio(struct bio *bio)
 		if (op_is_write(bio_op(bio))) {
 			count_vm_events(PGPGOUT, count);
 		} else {
-			if (bio_flagged(bio, BIO_WORKINGSET))
-				workingset_read = true;
 			task_io_account_read(bio->bi_iter.bi_size);
 			count_vm_events(PGPGIN, count);
 		}
@@ -1187,20 +1181,24 @@ blk_qc_t submit_bio(struct bio *bio)
 	}
 
 	/*
-	 * If we're reading data that is part of the userspace
-	 * workingset, count submission time as memory stall. When the
-	 * device is congested, or the submitting cgroup IO-throttled,
-	 * submission can be a significant part of overall IO time.
+	 * If we're reading data that is part of the userspace workingset, count
+	 * submission time as memory stall.  When the device is congested, or
+	 * the submitting cgroup IO-throttled, submission can be a significant
+	 * part of overall IO time.
 	 */
-	if (workingset_read)
-		psi_memstall_enter(&pflags);
-
-	ret = generic_make_request(bio);
+	if (unlikely(bio_op(bio) == REQ_OP_READ &&
+	    bio_flagged(bio, BIO_WORKINGSET))) {
+		unsigned long pflags;
+		blk_qc_t ret;
 
-	if (workingset_read)
+		psi_memstall_enter(&pflags);
+		ret = generic_make_request(bio);
 		psi_memstall_leave(&pflags);
 
-	return ret;
+		return ret;
+	}
+
+	return generic_make_request(bio);
 }
 EXPORT_SYMBOL(submit_bio);