Re: [PATCH 0/3] remove rw_page() from brd, pmem and btt

Minchan Kim
2017-08-03T00:13:18Z

Hi Ross,

On Wed, Aug 02, 2017 at 04:13:59PM -0600, Ross Zwisler wrote:
> On Fri, Jul 28, 2017 at 10:31:43AM -0700, Matthew Wilcox wrote:
> > On Fri, Jul 28, 2017 at 10:56:01AM -0600, Ross Zwisler wrote:
> > > Dan Williams and Christoph Hellwig have recently expressed doubt about
> > > whether the rw_page() interface made sense for synchronous memory drivers
> > > [1][2]. It's unclear whether this interface has any performance benefit
> > > for these drivers, but as we continue to fix bugs it is clear that it does
> > > have a maintenance burden. This series removes the rw_page()
> > > implementations in brd, pmem and btt to relieve this burden.
> >
> > Why don't you measure whether it has performance benefits? I don't
> > understand why zram would see performance benefits and not other drivers.
> > If it's going to be removed, then the whole interface should be removed,
> > not just have the implementations removed from some drivers.
>
> Okay, I've run a bunch of performance tests with the PMEM and with BTT entry
> points for rw_pages() in a swap workload, and in all cases I do see an
> improvement over the code when rw_pages() is removed. Here are the results
> from my random lab box:
>
> Average latency of swap_writepage()
> +------+------------+---------+-------------+
> | | no rw_page | rw_page | Improvement |
> +-------------------------------------------+
> | PMEM | 5.0 us | 4.7 us | 6% |
> +-------------------------------------------+
> | BTT | 6.8 us | 6.1 us | 10% |
> +------+------------+---------+-------------+
>
> Average latency of swap_readpage()
> +------+------------+---------+-------------+
> | | no rw_page | rw_page | Improvement |
> +-------------------------------------------+
> | PMEM | 3.3 us | 2.9 us | 12% |
> +-------------------------------------------+
> | BTT | 3.7 us | 3.4 us | 8% |
> +------+------------+---------+-------------+
>
> The workload was pmbench, a memory benchmark, run on a system where I had
> severely restricted the amount of memory in the system with the 'mem' kernel
> command line parameter. The benchmark was set up to test more memory than I
> allowed the OS to have so it spilled over into swap.
>
> The PMEM or BTT device was set up as my swap device, and during the test I got
> a few hundred thousand samples of each of swap_writepage() and
> swap_writepage(). The PMEM/BTT device was just memory reserved with the
> memmap kernel command line parameter.
>
> Thanks, Matthew, for asking for performance data. It looks like removing this
> code would have been a mistake.

By suggestion of Christoph Hellwig, I made a quick patch which does IO without
dynamic bio allocation for swap IO. Actually, it's not formal patch to be
worth to send mainline yet but I believe it's enough to test the improvement.

Could you test patchset on pmem and btt without rw_page?

For working the patch, block drivers need to declare it's synchronous IO
device via BDI_CAP_SYNC but if it's hard, you can just make every swap IO
comes from (sis->flags & SWP_SYNC_IO) with removing condition check

if (!(sis->flags & SWP_SYNC_IO)) in swap_[read|write]page.

Patchset is based on 4.13-rc3.

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 856d5dc02451..b1c5e9bf3ad5 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -125,9 +125,9 @@ static inline bool is_partial_io(struct bio_vec *bvec)
static void zram_revalidate_disk(struct zram *zram)
{
revalidate_disk(zram->disk);
- /* revalidate_disk reset the BDI_CAP_STABLE_WRITES so set again */
+ /* revalidate_disk reset the BDI capability so set again */
zram->disk->queue->backing_dev_info->capabilities |=
- BDI_CAP_STABLE_WRITES;
+ (BDI_CAP_STABLE_WRITES|BDI_CAP_SYNC);
}

/*
@@ -1096,7 +1096,7 @@ static int zram_open(struct block_device *bdev, fmode_t mode)
static const struct block_device_operations zram_devops = {
.open = zram_open,
.swap_slot_free_notify = zram_slot_free_notify,
- .rw_page = zram_rw_page,
+ // .rw_page = zram_rw_page,
.owner = THIS_MODULE
};

diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index 854e1bdd0b2a..05eee145d964 100644
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -130,6 +130,7 @@ int bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned int max_ratio);
#define BDI_CAP_STABLE_WRITES 0x00000008
#define BDI_CAP_STRICTLIMIT 0x00000010
#define BDI_CAP_CGROUP_WRITEBACK 0x00000020
+#define BDI_CAP_SYNC 0x00000040

#define BDI_CAP_NO_ACCT_AND_WRITEBACK \
(BDI_CAP_NO_WRITEBACK | BDI_CAP_NO_ACCT_DIRTY | BDI_CAP_NO_ACCT_WB)
@@ -177,6 +178,11 @@ long wait_iff_congested(struct pglist_data *pgdat, int sync, long timeout);
int pdflush_proc_obsolete(struct ctl_table *table, int write,
void __user *buffer, size_t *lenp, loff_t *ppos);

+static inline bool bdi_cap_sync_io_required(struct backing_dev_info *bdi)
+{
+ return bdi->capabilities & BDI_CAP_SYNC;
+}
+
static inline bool bdi_cap_stable_pages_required(struct backing_dev_info *bdi)
{
return bdi->capabilities & BDI_CAP_STABLE_WRITES;
diff --git a/include/linux/swap.h b/include/linux/swap.h
index d83d28e53e62..86457dbfd300 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -152,8 +152,9 @@ enum {
SWP_AREA_DISCARD = (1 << 8), /* single-time swap area discards */
SWP_PAGE_DISCARD = (1 << 9), /* freed swap page-cluster discards */
SWP_STABLE_WRITES = (1 << 10), /* no overwrite PG_writeback pages */
+ SWP_SYNC_IO = (1 << 11),
/* add others here before... */
- SWP_SCANNING = (1 << 11), /* refcount in scan_swap_map */
+ SWP_SCANNING = (1 << 12), /* refcount in scan_swap_map */
};

#define SWAP_CLUSTER_MAX 32UL
diff --git a/mm/page_io.c b/mm/page_io.c
index b6c4ac388209..2c85e5182364 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -263,7 +263,6 @@ static sector_t swap_page_sector(struct page *page)
int __swap_writepage(struct page *page, struct writeback_control *wbc,
bio_end_io_t end_write_func)
{
- struct bio *bio;
int ret;
struct swap_info_struct *sis = page_swap_info(page);

@@ -316,25 +315,44 @@ int __swap_writepage(struct page *page, struct writeback_control *wbc,
}

ret = 0;
- bio = get_swap_bio(GFP_NOIO, page, end_write_func);
- if (bio == NULL) {
- set_page_dirty(page);
+ count_vm_event(PSWPOUT);
+
+ if (!(sis->flags & SWP_SYNC_IO)) {
+ struct bio *bio;
+
+ bio = get_swap_bio(GFP_NOIO, page, end_write_func);
+ if (bio == NULL) {
+ set_page_dirty(page);
+ unlock_page(page);
+ ret = -ENOMEM;
+ goto out;
+ }
+ bio->bi_opf = REQ_OP_WRITE | wbc_to_write_flags(wbc);
+ set_page_writeback(page);
unlock_page(page);
- ret = -ENOMEM;
- goto out;
+ submit_bio(bio);
+ } else {
+ struct bio bio;
+ struct bio_vec bvec;
+
+ bio_init(&bio, &bvec, 1);
+
+ bio.bi_iter.bi_sector = map_swap_page(page, &bio.bi_bdev);
+ bio.bi_iter.bi_sector <<= PAGE_SHIFT - 9;
+ bio.bi_end_io = end_write_func;
+ bio_add_page(&bio, page, PAGE_SIZE, 0);
+ bio.bi_opf = REQ_OP_WRITE | wbc_to_write_flags(wbc);
+ bio_get(&bio);
+ set_page_writeback(page);
+ unlock_page(page);
+ submit_bio(&bio);
}
- bio->bi_opf = REQ_OP_WRITE | wbc_to_write_flags(wbc);
- count_vm_event(PSWPOUT);
- set_page_writeback(page);
- unlock_page(page);
- submit_bio(bio);
out:
return ret;
}

int swap_readpage(struct page *page, bool do_poll)
{
- struct bio *bio;
int ret = 0;
struct swap_info_struct *sis = page_swap_info(page);
blk_qc_t qc;
@@ -371,29 +389,49 @@ int swap_readpage(struct page *page, bool do_poll)
}

ret = 0;
- bio = get_swap_bio(GFP_KERNEL, page, end_swap_bio_read);
- if (bio == NULL) {
- unlock_page(page);
- ret = -ENOMEM;
- goto out;
- }
- bdev = bio->bi_bdev;
- bio->bi_private = current;
- bio_set_op_attrs(bio, REQ_OP_READ, 0);
- count_vm_event(PSWPIN);
- bio_get(bio);
- qc = submit_bio(bio);
- while (do_poll) {
- set_current_state(TASK_UNINTERRUPTIBLE);
- if (!READ_ONCE(bio->bi_private))
- break;
-
- if (!blk_mq_poll(bdev_get_queue(bdev), qc))
- break;
+ if (!(sis->flags & SWP_SYNC_IO)) {
+ struct bio *bio;
+
+ bio = get_swap_bio(GFP_KERNEL, page, end_swap_bio_read);
+ if (bio == NULL) {
+ unlock_page(page);
+ ret = -ENOMEM;
+ goto out;
+ }
+ bdev = bio->bi_bdev;
+ bio->bi_private = current;
+ bio_set_op_attrs(bio, REQ_OP_READ, 0);
+ bio_get(bio);
+ qc = submit_bio(bio);
+ while (do_poll) {
+ set_current_state(TASK_UNINTERRUPTIBLE);
+ if (!READ_ONCE(bio->bi_private))
+ break;
+
+ if (!blk_mq_poll(bdev_get_queue(bdev), qc))
+ break;
+ }
+ __set_current_state(TASK_RUNNING);
+ bio_put(bio);
+ } else {
+ struct bio bio;
+ struct bio_vec bvec;
+
+ bio_init(&bio, &bvec, 1);
+
+ bio.bi_iter.bi_sector = map_swap_page(page, &bio.bi_bdev);
+ bio.bi_iter.bi_sector <<= PAGE_SHIFT - 9;
+ bio.bi_end_io = end_swap_bio_read;
+ bio_add_page(&bio, page, PAGE_SIZE, 0);
+ bio.bi_private = current;
+ BUG_ON(bio.bi_iter.bi_size != PAGE_SIZE);
+ bio_set_op_attrs(&bio, REQ_OP_READ, 0);
+ /* end_swap_bio_read calls bio_put unconditionally */
+ bio_get(&bio);
+ submit_bio(&bio);
}
- __set_current_state(TASK_RUNNING);
- bio_put(bio);

+ count_vm_event(PSWPIN);
out:
return ret;
}
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 6ba4aab2db0b..855d50eeeaf9 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -2931,6 +2931,9 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
if (bdi_cap_stable_pages_required(inode_to_bdi(inode)))
p->flags |= SWP_STABLE_WRITES;

+ if (bdi_cap_sync_io_required(inode_to_bdi(inode)))
+ p->flags |= SWP_SYNC_IO;
+
if (p->bdev && blk_queue_nonrot(bdev_get_queue(p->bdev))) {
int cpu;
unsigned long ci, nr_cluster;