[dm-devel] [PATCH v2] dm-kcopyd: avoid useless atomic operations

[Mikulas Patocka]

On Tue, 25 May 2021, Damien Le Moal wrote:

> On 2021/05/26 4:50, Mikulas Patocka wrote:
> > The functions set_bit and clear_bit are atomic. We don't need atomicity
> > when making flags for dm-kcopyd. So, change them to direct manipulation of
> > the flags.
> > 
> > Signed-off-by: Mikulas Patocka <mpatocka at redhat.com>
> > 
> > Index: linux-2.6/drivers/md/dm-kcopyd.c
> > ===================================================================
> > --- linux-2.6.orig/drivers/md/dm-kcopyd.c
> > +++ linux-2.6/drivers/md/dm-kcopyd.c
> > @@ -812,7 +812,7 @@ void dm_kcopyd_copy(struct dm_kcopyd_cli
> >  	if (!test_bit(DM_KCOPYD_WRITE_SEQ, &job->flags)) {
> >  		for (i = 0; i < job->num_dests; i++) {
> >  			if (bdev_zoned_model(dests[i].bdev) == BLK_ZONED_HM) {
> > -				set_bit(DM_KCOPYD_WRITE_SEQ, &job->flags);
> > +				job->flags |= 1UL << DM_KCOPYD_WRITE_SEQ;
> 
> How about using the BIT() macro ?
> 
> job->flags |= BIT(DM_KCOPYD_WRITE_SEQ);
> 
> But I know some do not like that macro :)

Yes - it is better to use it.
I also changed flags from unsigned long to unsigned, to make the structure 
smaller.


From: Mikulas Patocka <mpatocka at redhat.com>

dm-kcopyd: avoid useless atomic operations

The functions set_bit and clear_bit are atomic. We don't need atomicity
when making flags for dm-kcopyd. So, change them to direct manipulation of
the flags.

Signed-off-by: Mikulas Patocka <mpatocka at redhat.com>

Index: linux-2.6/drivers/md/dm-kcopyd.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-kcopyd.c
+++ linux-2.6/drivers/md/dm-kcopyd.c
@@ -341,7 +341,7 @@ static void client_free_pages(struct dm_
 struct kcopyd_job {
 	struct dm_kcopyd_client *kc;
 	struct list_head list;
-	unsigned long flags;
+	unsigned flags;
 
 	/*
 	 * Error state of the job.
@@ -418,7 +418,7 @@ static struct kcopyd_job *pop_io_job(str
 	 * constraint and sequential writes that are at the right position.
 	 */
 	list_for_each_entry(job, jobs, list) {
-		if (job->rw == READ || !test_bit(DM_KCOPYD_WRITE_SEQ, &job->flags)) {
+		if (job->rw == READ || !(job->flags & BIT(DM_KCOPYD_WRITE_SEQ))) {
 			list_del(&job->list);
 			return job;
 		}
@@ -529,7 +529,7 @@ static void complete_io(unsigned long er
 		else
 			job->read_err = 1;
 
-		if (!test_bit(DM_KCOPYD_IGNORE_ERROR, &job->flags)) {
+		if (!(job->flags & BIT(DM_KCOPYD_IGNORE_ERROR))) {
 			push(&kc->complete_jobs, job);
 			wake(kc);
 			return;
@@ -572,7 +572,7 @@ static int run_io_job(struct kcopyd_job
 	 * If we need to write sequentially and some reads or writes failed,
 	 * no point in continuing.
 	 */
-	if (test_bit(DM_KCOPYD_WRITE_SEQ, &job->flags) &&
+	if (job->flags & BIT(DM_KCOPYD_WRITE_SEQ) &&
 	    job->master_job->write_err) {
 		job->write_err = job->master_job->write_err;
 		return -EIO;
@@ -716,7 +716,7 @@ static void segment_complete(int read_er
 	 * Only dispatch more work if there hasn't been an error.
 	 */
 	if ((!job->read_err && !job->write_err) ||
-	    test_bit(DM_KCOPYD_IGNORE_ERROR, &job->flags)) {
+	    job->flags & BIT(DM_KCOPYD_IGNORE_ERROR)) {
 		/* get the next chunk of work */
 		progress = job->progress;
 		count = job->source.count - progress;
@@ -809,10 +809,10 @@ void dm_kcopyd_copy(struct dm_kcopyd_cli
 	 * we need to write sequentially. If one of the destination is a
 	 * host-aware device, then leave it to the caller to choose what to do.
 	 */
-	if (!test_bit(DM_KCOPYD_WRITE_SEQ, &job->flags)) {
+	if (!(job->flags & BIT(DM_KCOPYD_WRITE_SEQ))) {
 		for (i = 0; i < job->num_dests; i++) {
 			if (bdev_zoned_model(dests[i].bdev) == BLK_ZONED_HM) {
-				set_bit(DM_KCOPYD_WRITE_SEQ, &job->flags);
+				job->flags |= BIT(DM_KCOPYD_WRITE_SEQ);
 				break;
 			}
 		}
@@ -821,9 +821,9 @@ void dm_kcopyd_copy(struct dm_kcopyd_cli
 	/*
 	 * If we need to write sequentially, errors cannot be ignored.
 	 */
-	if (test_bit(DM_KCOPYD_WRITE_SEQ, &job->flags) &&
-	    test_bit(DM_KCOPYD_IGNORE_ERROR, &job->flags))
-		clear_bit(DM_KCOPYD_IGNORE_ERROR, &job->flags);
+	if (job->flags & BIT(DM_KCOPYD_WRITE_SEQ) &&
+	    job->flags & BIT(DM_KCOPYD_IGNORE_ERROR))
+		job->flags &= ~BIT(DM_KCOPYD_IGNORE_ERROR);
 
 	if (from) {
 		job->source = *from;
Index: linux-2.6/drivers/md/dm-raid1.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-raid1.c
+++ linux-2.6/drivers/md/dm-raid1.c
@@ -364,7 +364,7 @@ static void recover(struct mirror_set *m
 
 	/* hand to kcopyd */
 	if (!errors_handled(ms))
-		set_bit(DM_KCOPYD_IGNORE_ERROR, &flags);
+		flags |= BIT(DM_KCOPYD_IGNORE_ERROR);
 
 	dm_kcopyd_copy(ms->kcopyd_client, &from, ms->nr_mirrors - 1, to,
 		       flags, recovery_complete, reg);
Index: linux-2.6/drivers/md/dm-zoned-reclaim.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-zoned-reclaim.c
+++ linux-2.6/drivers/md/dm-zoned-reclaim.c
@@ -134,7 +134,7 @@ static int dmz_reclaim_copy(struct dmz_r
 	dst_zone_block = dmz_start_block(zmd, dst_zone);
 
 	if (dmz_is_seq(dst_zone))
-		set_bit(DM_KCOPYD_WRITE_SEQ, &flags);
+		flags |= BIT(DM_KCOPYD_WRITE_SEQ);
 
 	while (block < end_block) {
 		if (src_zone->dev->flags & DMZ_BDEV_DYING)