Date:   Tue, 21 Jul 2020 01:42:54 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     Theodore Ts'o <tytso@....edu>
Cc:     Ext4 Developers List <linux-ext4@...r.kernel.org>,
        Alex Zhuravlev <bzzz@...mcloud.com>,
        Shuichi Ihara <sihara@....com>
Subject: Re: [PATCH 1/4] ext4: add prefetching for block allocation bitmaps

On Jul 17, 2020, at 9:53 AM, Theodore Ts'o <tytso@....edu> wrote:
> 
> From: Alex Zhuravlev <bzzz@...mcloud.com>
> 
> This should significantly improve bitmap loading, especially for flex
> groups as it tries to load all bitmaps within a flex.group instead of
> one by one synchronously.
> 
> Prefetching is done in 8 * flex_bg groups, so it should be 8 read-ahead
> reads for a single allocating thread. At the end of allocation the
> thread waits for read-ahead completion and initializes buddy information
> so that read-aheads are not lost in case of memory pressure.
> 
> At cr=0 the number of prefetching IOs is limited per allocation context
> to prevent a situation when mballoc loads thousands of bitmaps looking
> for a perfect group and ignoring groups with good chunks.
> 
> Together with the patch "ext4: limit scanning of uninitialized groups"
> the mount time (which includes few tiny allocations) of a 1PB filesystem
> is reduced significantly:
> 
>               0% full    50%-full unpatched    patched
>  mount time       33s                9279s       563s
> 
> [ Restructured by tytso; removed the state flags in the allocation
>  context, so it can be used to lazily prefetch the allocation bitmaps
>  immediately after the file system is mounted.  Skip prefetching
>  block groups which are unitialized.  Finally pass in the REQ_RAHEAD
>  flag to the block layer while prefetching. ]
> 
> Signed-off-by: Alex Zhuravlev <bzzz@...mcloud.com>
> Reviewed-by: Andreas Dilger <adilger@...mcloud.com>

I re-reviewed the patch with the changes, and it looks OK.  I see that
you reduced the prefetch limit from 32 to 4 group blocks, presumably to
keep the latency low?  It would be useful to see what impact that has
on the mount time and IO performance of a large filesystem.

Shuichi, do you have a properly populated large OST that you could test
this?  Since it is a tunable (/sys/fs/ext4/<dev>/mb_prefetch_limit), it
should be possible to see the effect on allocation performance at least
without recompiling the module, though this tunable only appears after
mount, so you will have a chance to change it right after mount to see
the effect. Given the long mount time with a bad parameter, this should
not be hard to observe.

Cheers, Andreas

> Signed-off-by: Theodore Ts'o <tytso@....edu>
> ---
> fs/ext4/balloc.c  |  14 +++--
> fs/ext4/ext4.h    |   8 ++-
> fs/ext4/mballoc.c | 132 +++++++++++++++++++++++++++++++++++++++++++++-
> fs/ext4/sysfs.c   |   4 ++
> 4 files changed, 152 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/ext4/balloc.c b/fs/ext4/balloc.c
> index 1ba46d87cdf1..aaa9ec5212c8 100644
> --- a/fs/ext4/balloc.c
> +++ b/fs/ext4/balloc.c
> @@ -413,7 +413,8 @@ static int ext4_validate_block_bitmap(struct super_block *sb,
>  * Return buffer_head on success or an ERR_PTR in case of failure.
>  */
> struct buffer_head *
> -ext4_read_block_bitmap_nowait(struct super_block *sb, ext4_group_t block_group)
> +ext4_read_block_bitmap_nowait(struct super_block *sb, ext4_group_t block_group,
> +			      bool ignore_locked)
> {
> 	struct ext4_group_desc *desc;
> 	struct ext4_sb_info *sbi = EXT4_SB(sb);
> @@ -441,6 +442,12 @@ ext4_read_block_bitmap_nowait(struct super_block *sb, ext4_group_t block_group)
> 		return ERR_PTR(-ENOMEM);
> 	}
> 
> +	if (ignore_locked && buffer_locked(bh)) {
> +		/* buffer under IO already, return if called for prefetching */
> +		put_bh(bh);
> +		return NULL;
> +	}
> +
> 	if (bitmap_uptodate(bh))
> 		goto verify;
> 
> @@ -490,7 +497,8 @@ ext4_read_block_bitmap_nowait(struct super_block *sb, ext4_group_t block_group)
> 	trace_ext4_read_block_bitmap_load(sb, block_group);
> 	bh->b_end_io = ext4_end_bitmap_read;
> 	get_bh(bh);
> -	submit_bh(REQ_OP_READ, REQ_META | REQ_PRIO, bh);
> +	submit_bh(REQ_OP_READ, REQ_META | REQ_PRIO |
> +		  ignore_locked ? REQ_RAHEAD : 0, bh);
> 	return bh;
> verify:
> 	err = ext4_validate_block_bitmap(sb, desc, block_group, bh);
> @@ -534,7 +542,7 @@ ext4_read_block_bitmap(struct super_block *sb, ext4_group_t block_group)
> 	struct buffer_head *bh;
> 	int err;
> 
> -	bh = ext4_read_block_bitmap_nowait(sb, block_group);
> +	bh = ext4_read_block_bitmap_nowait(sb, block_group, false);
> 	if (IS_ERR(bh))
> 		return bh;
> 	err = ext4_wait_block_bitmap(sb, block_group, bh);
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 42f5060f3cdf..7451662e092a 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -1505,6 +1505,8 @@ struct ext4_sb_info {
> 	/* where last allocation was done - for stream allocation */
> 	unsigned long s_mb_last_group;
> 	unsigned long s_mb_last_start;
> +	unsigned int s_mb_prefetch;
> +	unsigned int s_mb_prefetch_limit;
> 
> 	/* stats for buddy allocator */
> 	atomic_t s_bal_reqs;	/* number of reqs with len > 1 */
> @@ -2446,7 +2448,8 @@ extern struct ext4_group_desc * ext4_get_group_desc(struct super_block * sb,
> extern int ext4_should_retry_alloc(struct super_block *sb, int *retries);
> 
> extern struct buffer_head *ext4_read_block_bitmap_nowait(struct super_block *sb,
> -						ext4_group_t block_group);
> +						ext4_group_t block_group,
> +						bool ignore_locked);
> extern int ext4_wait_block_bitmap(struct super_block *sb,
> 				  ext4_group_t block_group,
> 				  struct buffer_head *bh);
> @@ -3145,6 +3148,7 @@ struct ext4_group_info {
> 	(1 << EXT4_GROUP_INFO_BBITMAP_CORRUPT_BIT)
> #define EXT4_GROUP_INFO_IBITMAP_CORRUPT		\
> 	(1 << EXT4_GROUP_INFO_IBITMAP_CORRUPT_BIT)
> +#define EXT4_GROUP_INFO_BBITMAP_READ_BIT	4
> 
> #define EXT4_MB_GRP_NEED_INIT(grp)	\
> 	(test_bit(EXT4_GROUP_INFO_NEED_INIT_BIT, &((grp)->bb_state)))
> @@ -3159,6 +3163,8 @@ struct ext4_group_info {
> 	(set_bit(EXT4_GROUP_INFO_WAS_TRIMMED_BIT, &((grp)->bb_state)))
> #define EXT4_MB_GRP_CLEAR_TRIMMED(grp)	\
> 	(clear_bit(EXT4_GROUP_INFO_WAS_TRIMMED_BIT, &((grp)->bb_state)))
> +#define EXT4_MB_GRP_TEST_AND_SET_READ(grp)	\
> +	(test_and_set_bit(EXT4_GROUP_INFO_BBITMAP_READ_BIT, &((grp)->bb_state)))
> 
> #define EXT4_MAX_CONTENTION		8
> #define EXT4_CONTENTION_THRESHOLD	2
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index c0a331e2feb0..8a1e6e03c088 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -922,7 +922,7 @@ static int ext4_mb_init_cache(struct page *page, char *incore, gfp_t gfp)
> 			bh[i] = NULL;
> 			continue;
> 		}
> -		bh[i] = ext4_read_block_bitmap_nowait(sb, group);
> +		bh[i] = ext4_read_block_bitmap_nowait(sb, group, false);
> 		if (IS_ERR(bh[i])) {
> 			err = PTR_ERR(bh[i]);
> 			bh[i] = NULL;
> @@ -2209,12 +2209,93 @@ static int ext4_mb_good_group_nolock(struct ext4_allocation_context *ac,
> 	return ret;
> }
> 
> +/*
> + * Start prefetching @nr block bitmaps starting at @group.
> + * Return the next group which needs to be prefetched.
> + */
> +static ext4_group_t
> +ext4_mb_prefetch(struct super_block *sb, ext4_group_t group,
> +		 unsigned int nr, int *cnt)
> +{
> +	ext4_group_t ngroups = ext4_get_groups_count(sb);
> +	struct buffer_head *bh;
> +	struct blk_plug plug;
> +
> +	blk_start_plug(&plug);
> +	while (nr-- > 0) {
> +		struct ext4_group_desc *gdp = ext4_get_group_desc(sb, group,
> +								  NULL);
> +		struct ext4_group_info *grp = ext4_get_group_info(sb, group);
> +
> +		/*
> +		 * Prefetch block groups with free blocks; but don't
> +		 * bother if it is marked uninitialized on disk, since
> +		 * it won't require I/O to read.  Also only try to
> +		 * prefetch once, so we avoid getblk() call, which can
> +		 * be expensive.
> +		 */
> +		if (!EXT4_MB_GRP_TEST_AND_SET_READ(grp) &&
> +		    EXT4_MB_GRP_NEED_INIT(grp) &&
> +		    ext4_free_group_clusters(sb, gdp) > 0 &&
> +		    !(ext4_has_group_desc_csum(sb) &&
> +		      (gdp->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT)))) {
> +			bh = ext4_read_block_bitmap_nowait(sb, group, true);
> +			if (bh && !IS_ERR(bh)) {
> +				if (!buffer_uptodate(bh) && cnt)
> +					(*cnt)++;
> +				brelse(bh);
> +			}
> +		}
> +		if (++group >= ngroups)
> +			group = 0;
> +	}
> +	blk_finish_plug(&plug);
> +	return group;
> +}
> +
> +/*
> + * Prefetching reads the block bitmap into the buffer cache; but we
> + * need to make sure that the buddy bitmap in the page cache has been
> + * initialized.  Note that ext4_mb_init_group() will block if the I/O
> + * is not yet completed, or indeed if it was not initiated by
> + * ext4_mb_prefetch did not start the I/O.
> + *
> + * TODO: We should actually kick off the buddy bitmap setup in a work
> + * queue when the buffer I/O is completed, so that we don't block
> + * waiting for the block allocation bitmap read to finish when
> + * ext4_mb_prefetch_fini is called from ext4_mb_regular_allocator().
> + */
> +static void
> +ext4_mb_prefetch_fini(struct super_block *sb, ext4_group_t group,
> +		      unsigned int nr)
> +{
> +	while (nr-- > 0) {
> +		struct ext4_group_desc *gdp = ext4_get_group_desc(sb, group,
> +								  NULL);
> +		struct ext4_group_info *grp = ext4_get_group_info(sb, group);
> +
> +		if (!group)
> +			group = ext4_get_groups_count(sb);
> +		group--;
> +		grp = ext4_get_group_info(sb, group);
> +
> +		if (EXT4_MB_GRP_NEED_INIT(grp) &&
> +		    ext4_free_group_clusters(sb, gdp) > 0 &&
> +		    !(ext4_has_group_desc_csum(sb) &&
> +		      (gdp->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT)))) {
> +			if (ext4_mb_init_group(sb, group, GFP_NOFS))
> +				break;
> +		}
> +	}
> +}
> +
> static noinline_for_stack int
> ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
> {
> -	ext4_group_t ngroups, group, i;
> +	ext4_group_t prefetch_grp = 0, ngroups, group, i;
> 	int cr = -1;
> 	int err = 0, first_err = 0;
> +	unsigned int nr = 0, prefetch_ios = 0;
> 	struct ext4_sb_info *sbi;
> 	struct super_block *sb;
> 	struct ext4_buddy e4b;
> @@ -2282,6 +2363,7 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
> 		 * from the goal value specified
> 		 */
> 		group = ac->ac_g_ex.fe_group;
> +		prefetch_grp = group;
> 
> 		for (i = 0; i < ngroups; group++, i++) {
> 			int ret = 0;
> @@ -2293,6 +2375,29 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
> 			if (group >= ngroups)
> 				group = 0;
> 
> +			/*
> +			 * Batch reads of the block allocation bitmaps
> +			 * to get multiple READs in flight; limit
> +			 * prefetching at cr=0/1, otherwise mballoc can
> +			 * spend a lot of time loading imperfect groups
> +			 */
> +			if ((prefetch_grp == group) &&
> +			    (cr > 1 ||
> +			     prefetch_ios < sbi->s_mb_prefetch_limit)) {
> +				unsigned int curr_ios = prefetch_ios;
> +
> +				nr = sbi->s_mb_prefetch;
> +				if (ext4_has_feature_flex_bg(sb)) {
> +					nr = (group / sbi->s_mb_prefetch) *
> +						sbi->s_mb_prefetch;
> +					nr = nr + sbi->s_mb_prefetch - group;
> +				}
> +				prefetch_grp = ext4_mb_prefetch(sb, group,
> +							nr, &prefetch_ios);
> +				if (prefetch_ios == curr_ios)
> +					nr = 0;
> +			}
> +
> 			/* This now checks without needing the buddy page */
> 			ret = ext4_mb_good_group_nolock(ac, group, cr);
> 			if (ret <= 0) {
> @@ -2367,6 +2472,10 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
> 	mb_debug(sb, "Best len %d, origin len %d, ac_status %u, ac_flags 0x%x, cr %d ret %d\n",
> 		 ac->ac_b_ex.fe_len, ac->ac_o_ex.fe_len, ac->ac_status,
> 		 ac->ac_flags, cr, err);
> +
> +	if (nr)
> +		ext4_mb_prefetch_fini(sb, prefetch_grp, nr);
> +
> 	return err;
> }
> 
> @@ -2613,6 +2722,25 @@ static int ext4_mb_init_backend(struct super_block *sb)
> 			goto err_freebuddy;
> 	}
> 
> +	if (ext4_has_feature_flex_bg(sb)) {
> +		/* a single flex group is supposed to be read by a single IO */
> +		sbi->s_mb_prefetch = 1 << sbi->s_es->s_log_groups_per_flex;
> +		sbi->s_mb_prefetch *= 8; /* 8 prefetch IOs in flight at most */
> +	} else {
> +		sbi->s_mb_prefetch = 32;
> +	}
> +	if (sbi->s_mb_prefetch > ext4_get_groups_count(sb))
> +		sbi->s_mb_prefetch = ext4_get_groups_count(sb);
> +	/* now many real IOs to prefetch within a single allocation at cr=0
> +	 * given cr=0 is an CPU-related optimization we shouldn't try to
> +	 * load too many groups, at some point we should start to use what
> +	 * we've got in memory.
> +	 * with an average random access time 5ms, it'd take a second to get
> +	 * 200 groups (* N with flex_bg), so let's make this limit 4 */
> +	sbi->s_mb_prefetch_limit = sbi->s_mb_prefetch * 4;
> +	if (sbi->s_mb_prefetch_limit > ext4_get_groups_count(sb))
> +		sbi->s_mb_prefetch_limit = ext4_get_groups_count(sb);
> +
> 	return 0;
> 
> err_freebuddy:
> diff --git a/fs/ext4/sysfs.c b/fs/ext4/sysfs.c
> index 6c9fc9e21c13..31e0db726d21 100644
> --- a/fs/ext4/sysfs.c
> +++ b/fs/ext4/sysfs.c
> @@ -240,6 +240,8 @@ EXT4_RO_ATTR_ES_STRING(last_error_func, s_last_error_func, 32);
> EXT4_ATTR(first_error_time, 0444, first_error_time);
> EXT4_ATTR(last_error_time, 0444, last_error_time);
> EXT4_ATTR(journal_task, 0444, journal_task);
> +EXT4_RW_ATTR_SBI_UI(mb_prefetch, s_mb_prefetch);
> +EXT4_RW_ATTR_SBI_UI(mb_prefetch_limit, s_mb_prefetch_limit);
> 
> static unsigned int old_bump_val = 128;
> EXT4_ATTR_PTR(max_writeback_mb_bump, 0444, pointer_ui, &old_bump_val);
> @@ -283,6 +285,8 @@ static struct attribute *ext4_attrs[] = {
> #ifdef CONFIG_EXT4_DEBUG
> 	ATTR_LIST(simulate_fail),
> #endif
> +	ATTR_LIST(mb_prefetch),
> +	ATTR_LIST(mb_prefetch_limit),
> 	NULL,
> };
> ATTRIBUTE_GROUPS(ext4);
> --
> 2.24.1
> 


Cheers, Andreas






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