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

Dan Williams
2017-08-03T00:32:23Z

[ adding Tim and Ying who have also been looking at swap optimization
and rw_page interactions ]

On Wed, Aug 2, 2017 at 5:13 PM, Minchan Kim <minchan@kernel.org> wrote:
> 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;