Hi Andreas,

> 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.

Sure, I will test this on large OST and check if there are any good/bad performance impacts with it.

Thanks
Ihara

> On Jul 21, 2020, at 16:42, Andreas Dilger <[email protected]> wrote:
>
> On Jul 17, 2020, at 9:53 AM, Theodore Ts'o <[email protected]> wrote:
>>
>> From: Alex Zhuravlev <[email protected]>
>>
>> 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 <[email protected]>
>> Reviewed-by: Andreas Dilger <[email protected]>
>
> 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 <[email protected]>
>> ---
>> 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
>
>
>
>
>

2020-07-23 15:02:05

by Theodore Ts'o

[permalink] [raw]

Subject: Re: [PATCH 1/4] ext4: add prefetching for block allocation bitmaps

On Tue, Jul 21, 2020 at 01:42:54AM -0600, Andreas Dilger wrote:
>
> 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.

No, I didn't change it. As before, it's 4 times the size of the
flex_bg size (assuming the file system has flex bg's enabled);
otherwise it's 128 (4 times 32).

I'm actually worried that this is too aggressive on storage devies
where LBA's which are adjacent to each other aren't necessarily
adjacent to each other on the physical storage device. For example,
on dm-thin, some qemu-img formats, and potentially some cloud virtual
block devices....

Unfortunately, there isn't a good way to query a block device to
determine whether adjacent LBA's are really adjacent in the real world.

- Ted

2020-07-24 11:29:40

by Artem Blagodarenko

[permalink] [raw]

Subject: Re: [PATCH 2/4] ext4: skip non-loaded groups at cr=0/1 when scanning for good groups

Looks good.

Reviewed-by: Artem Blagodarenko <[email protected]>

> On 17 Jul 2020, at 18:53, Theodore Ts'o <[email protected]> wrote:
>
> From: Alex Zhuravlev <[email protected]>
>
> cr=0 is supposed to be an optimization to save CPU cycles, but if
> buddy data (in memory) is not initialized then all this makes no sense
> as we have to do sync IO taking a lot of cycles. also, at cr=0
> mballoc doesn't store any avaibale chunk. cr=1 also skips groups using
> heuristic based on avg. fragment size. it's more useful to skip such
> groups and switch to cr=2 where groups will be scanned for available
> chunks.
>
> using sparse image and dm-slow virtual device of 120TB was
> simulated. then the image was formatted and filled using debugfs to
> mark ~85% of available space as busy. mount process w/o the patch
> couldn't complete in half an hour (according to vmstat it would take
> ~10-11 hours). With the patch applied mount took ~20 seconds.
>
> Lustre-bug-id: https://jira.whamcloud.com/browse/LU-12988
> Signed-off-by: Alex Zhuravlev <[email protected]>
> Reviewed-by: Andreas Dilger <[email protected]>
> ---
> fs/ext4/mballoc.c | 13 ++++++++++++-
> 1 file changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 8a1e6e03c088..172994349bf6 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -2195,7 +2195,18 @@ static int ext4_mb_good_group_nolock(struct ext4_allocation_context *ac,
>
> /* We only do this if the grp has never been initialized */
> if (unlikely(EXT4_MB_GRP_NEED_INIT(grp))) {
> - ret = ext4_mb_init_group(ac->ac_sb, group, GFP_NOFS);
> + struct ext4_group_desc *gdp = ext4_get_group_desc(sb, group,
> + NULL);
> + int ret;
> +
> + /* cr=0/1 is a very optimistic search to find large
> + * good chunks almost for free. if buddy data is
> + * not ready, then this optimization makes no sense */
> + if (cr < 2 &&
> + !(ext4_has_group_desc_csum(sb) &&
> + (gdp->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT))))
> + return 0;
> + ret = ext4_mb_init_group(sb, group, GFP_NOFS);
> if (ret)
> return ret;
> }
> --
> 2.24.1
>

2020-07-24 12:07:38

by Artem Blagodarenko

[permalink] [raw]

Subject: Re: [PATCH 3/4] ext4: indicate via a block bitmap read is prefetched via a tracepoint

I have used this tracepoint for verifying other patches in the series. Useful. The patch looks good.

Reviewed-by: Artem Blagodarenko <[email protected]>

> On 17 Jul 2020, at 18:53, Theodore Ts'o <[email protected]> wrote:
>
> Modify the ext4_read_block_bitmap_load tracepoint so that it tells us
> whether a block bitmap is being prefetched.
>
> Signed-off-by: Theodore Ts'o <[email protected]>
> ---
> fs/ext4/balloc.c | 2 +-
> include/trace/events/ext4.h | 24 ++++++++++++++++++++----
> 2 files changed, 21 insertions(+), 5 deletions(-)
>
> diff --git a/fs/ext4/balloc.c b/fs/ext4/balloc.c
> index aaa9ec5212c8..5a2f8837200c 100644
> --- a/fs/ext4/balloc.c
> +++ b/fs/ext4/balloc.c
> @@ -494,7 +494,7 @@ ext4_read_block_bitmap_nowait(struct super_block *sb, ext4_group_t block_group,
> * submit the buffer_head for reading
> */
> set_buffer_new(bh);
> - trace_ext4_read_block_bitmap_load(sb, block_group);
> + trace_ext4_read_block_bitmap_load(sb, block_group, ignore_locked);
> bh->b_end_io = ext4_end_bitmap_read;
> get_bh(bh);
> submit_bh(REQ_OP_READ, REQ_META | REQ_PRIO |
> diff --git a/include/trace/events/ext4.h b/include/trace/events/ext4.h
> index cc41d692ae8e..cbcd2e1a608d 100644
> --- a/include/trace/events/ext4.h
> +++ b/include/trace/events/ext4.h
> @@ -1312,18 +1312,34 @@ DEFINE_EVENT(ext4__bitmap_load, ext4_mb_buddy_bitmap_load,
> TP_ARGS(sb, group)
> );
>
> -DEFINE_EVENT(ext4__bitmap_load, ext4_read_block_bitmap_load,
> +DEFINE_EVENT(ext4__bitmap_load, ext4_load_inode_bitmap,
>
> TP_PROTO(struct super_block *sb, unsigned long group),
>
> TP_ARGS(sb, group)
> );
>
> -DEFINE_EVENT(ext4__bitmap_load, ext4_load_inode_bitmap,
> +TRACE_EVENT(ext4_read_block_bitmap_load,
> + TP_PROTO(struct super_block *sb, unsigned long group, bool prefetch),
>
> - TP_PROTO(struct super_block *sb, unsigned long group),
> + TP_ARGS(sb, group, prefetch),
>
> - TP_ARGS(sb, group)
> + TP_STRUCT__entry(
> + __field( dev_t, dev )
> + __field( __u32, group )
> + __field( bool, prefetch )
> +
> + ),
> +
> + TP_fast_assign(
> + __entry->dev = sb->s_dev;
> + __entry->group = group;
> + __entry->prefetch = prefetch;
> + ),
> +
> + TP_printk("dev %d,%d group %u prefetch %d",
> + MAJOR(__entry->dev), MINOR(__entry->dev),
> + __entry->group, __entry->prefetch)
> );
>
> TRACE_EVENT(ext4_direct_IO_enter,
> --
> 2.24.1
>

2020-07-24 13:59:14

by Artem Blagodarenko

[permalink] [raw]

Subject: Re: [PATCH 4/4] ext4: add prefetch_block_bitmaps mount options

Hello,

Thanks for patch. I believe it can be useful in some case. I have tried this patch. The option works fine.
One comment is placed bellow.

Best regards,
Artem Blagodarenko

> On 17 Jul 2020, at 18:53, Theodore Ts'o <[email protected]> wrote:
>
> For file systems where we can afford to keep the buddy bitmaps cached,
> we can speed up initial writes to large file systems by starting to
> load the block allocation bitmaps as soon as the file system is
> mounted. This won't work well for _super_ large file systems, or
> memory constrained systems, so we only enable this when it is
> requested via a mount option.
>
> Signed-off-by: Theodore Ts'o <[email protected]>
> ---
> fs/ext4/ext4.h | 13 ++++++++++++
> fs/ext4/mballoc.c | 10 ++++------
> fs/ext4/super.c | 51 +++++++++++++++++++++++++++++++++++++----------
> 3 files changed, 57 insertions(+), 17 deletions(-)
>
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 7451662e092a..c04d4ef0b77a 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -1172,6 +1172,7 @@ struct ext4_inode_info {
> #define EXT4_MOUNT_JOURNAL_CHECKSUM 0x800000 /* Journal checksums */
> #define EXT4_MOUNT_JOURNAL_ASYNC_COMMIT 0x1000000 /* Journal Async Commit */
> #define EXT4_MOUNT_WARN_ON_ERROR 0x2000000 /* Trigger WARN_ON on error */
> +#define EXT4_MOUNT_PREFETCH_BLOCK_BITMAPS 0x4000000
> #define EXT4_MOUNT_DELALLOC 0x8000000 /* Delalloc support */
> #define EXT4_MOUNT_DATA_ERR_ABORT 0x10000000 /* Abort on file data write */
> #define EXT4_MOUNT_BLOCK_VALIDITY 0x20000000 /* Block validity checking */
> @@ -2315,9 +2316,15 @@ struct ext4_lazy_init {
> struct mutex li_list_mtx;
> };
>
> +typedef enum {
> + EXT4_LI_MODE_ITABLE,
> + EXT4_LI_MODE_PREFETCH_BBITMAP
> +} ext4_li_mode;
> +
> struct ext4_li_request {
> struct super_block *lr_super;
> struct ext4_sb_info *lr_sbi;
> + ext4_li_mode lr_mode;
> ext4_group_t lr_next_group;
> struct list_head lr_request;
> unsigned long lr_next_sched;
> @@ -2657,6 +2664,12 @@ extern int ext4_mb_reserve_blocks(struct super_block *, int);
> extern void ext4_discard_preallocations(struct inode *);
> extern int __init ext4_init_mballoc(void);
> extern void ext4_exit_mballoc(void);
> +extern ext4_group_t ext4_mb_prefetch(struct super_block *sb,
> + ext4_group_t group,
> + unsigned int nr, int *cnt);
> +extern void ext4_mb_prefetch_fini(struct super_block *sb, ext4_group_t group,
> + unsigned int nr);
> +
> extern void ext4_free_blocks(handle_t *handle, struct inode *inode,
> struct buffer_head *bh, ext4_fsblk_t block,
> unsigned long count, int flags);
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 172994349bf6..c072d06d678d 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -2224,9 +2224,8 @@ static int ext4_mb_good_group_nolock(struct ext4_allocation_context *ac,
> * 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 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;
> @@ -2276,9 +2275,8 @@ ext4_mb_prefetch(struct super_block *sb, ext4_group_t group,
> * 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)
> +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,
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 330957ed1f05..9e19d5830745 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -1521,6 +1521,7 @@ enum {
> Opt_dioread_nolock, Opt_dioread_lock,
> Opt_discard, Opt_nodiscard, Opt_init_itable, Opt_noinit_itable,
> Opt_max_dir_size_kb, Opt_nojournal_checksum, Opt_nombcache,
> + Opt_prefetch_block_bitmaps,
> };
>
> static const match_table_t tokens = {
> @@ -1612,6 +1613,7 @@ static const match_table_t tokens = {
> {Opt_test_dummy_encryption, "test_dummy_encryption"},
> {Opt_nombcache, "nombcache"},
> {Opt_nombcache, "no_mbcache"}, /* for backward compatibility */
> + {Opt_prefetch_block_bitmaps, "prefetch_block_bitmaps"},
> {Opt_removed, "check=none"}, /* mount option from ext2/3 */
> {Opt_removed, "nocheck"}, /* mount option from ext2/3 */
> {Opt_removed, "reservation"}, /* mount option from ext2/3 */
> @@ -1829,6 +1831,8 @@ static const struct mount_opts {
> {Opt_max_dir_size_kb, 0, MOPT_GTE0},
> {Opt_test_dummy_encryption, 0, MOPT_STRING},
> {Opt_nombcache, EXT4_MOUNT_NO_MBCACHE, MOPT_SET},
> + {Opt_prefetch_block_bitmaps, EXT4_MOUNT_PREFETCH_BLOCK_BITMAPS,
> + MOPT_SET},
> {Opt_err, 0, 0}
> };
>
> @@ -3197,19 +3201,33 @@ static void print_daily_error_info(struct timer_list *t)
> mod_timer(&sbi->s_err_report, jiffies + 24*60*60*HZ); /* Once a day */
> }
>
> +static int ext4_run_li_prefetch(struct ext4_li_request *elr,
> + struct super_block *sb, ext4_group_t group)
> +{
> + unsigned int prefetch_ios = 0;
> +
> + elr->lr_next_group = ext4_mb_prefetch(sb, group,
> + EXT4_SB(sb)->s_mb_prefetch,
> + &prefetch_ios);
> + if (prefetch_ios)
> + ext4_mb_prefetch_fini(sb, elr->lr_next_group, prefetch_ios);
> + return (group >= elr->lr_next_group);
> +}
> +
> /* Find next suitable group and run ext4_init_inode_table */
> static int ext4_run_li_request(struct ext4_li_request *elr)
> {
> struct ext4_group_desc *gdp = NULL;
> - ext4_group_t group, ngroups;
> - struct super_block *sb;
> + ext4_group_t group = elr->lr_next_group;
> + struct super_block *sb = elr->lr_super;
> + ext4_group_t ngroups = EXT4_SB(sb)->s_groups_count;
> unsigned long timeout = 0;
> int ret = 0;
>
> - sb = elr->lr_super;
> - ngroups = EXT4_SB(sb)->s_groups_count;
> + if (elr->lr_mode == EXT4_LI_MODE_PREFETCH_BBITMAP)
> + return ext4_run_li_prefetch(elr, sb, group);
>
> - for (group = elr->lr_next_group; group < ngroups; group++) {
> + for (; group < ngroups; group++) {
> gdp = ext4_get_group_desc(sb, group, NULL);
> if (!gdp) {
> ret = 1;
> @@ -3219,13 +3237,12 @@ static int ext4_run_li_request(struct ext4_li_request *elr)
> if (!(gdp->bg_flags & cpu_to_le16(EXT4_BG_INODE_ZEROED)))
> break;
> }
> -
> if (group >= ngroups)
> ret = 1;
>
> if (!ret) {
> timeout = jiffies;
> - ret = ext4_init_inode_table(sb, group,
> + ret = ext4_init_inode_table(elr->lr_super, group,
> elr->lr_timeout ? 0 : 1);
> if (elr->lr_timeout == 0) {
> timeout = (jiffies - timeout) *
> @@ -3234,6 +3251,10 @@ static int ext4_run_li_request(struct ext4_li_request *elr)
> }
> elr->lr_next_sched = jiffies + elr->lr_timeout;
> elr->lr_next_group = group + 1;
> + } else if (test_opt(sb, PREFETCH_BLOCK_BITMAPS)) {
> + elr->lr_mode = EXT4_LI_MODE_PREFETCH_BBITMAP;
> + elr->lr_next_group = 0;
> + ret = 0;
> }
> return ret;
> }
> @@ -3459,7 +3480,8 @@ static int ext4_li_info_new(void)
> }
>
> static struct ext4_li_request *ext4_li_request_new(struct super_block *sb,
> - ext4_group_t start)
> + ext4_group_t start,
> + ext4_li_mode mode)
> {
> struct ext4_sb_info *sbi = EXT4_SB(sb);
> struct ext4_li_request *elr;
> @@ -3468,6 +3490,7 @@ static struct ext4_li_request *ext4_li_request_new(struct super_block *sb,
> if (!elr)
> return NULL;
>
> + elr->lr_mode = mode;
> elr->lr_super = sb;
> elr->lr_sbi = sbi;
> elr->lr_next_group = start;
> @@ -3488,6 +3511,7 @@ int ext4_register_li_request(struct super_block *sb,
> struct ext4_sb_info *sbi = EXT4_SB(sb);
> struct ext4_li_request *elr = NULL;
> ext4_group_t ngroups = sbi->s_groups_count;
> + ext4_li_mode lr_mode = EXT4_LI_MODE_ITABLE;
> int ret = 0;
>
> mutex_lock(&ext4_li_mtx);
> @@ -3501,10 +3525,15 @@ int ext4_register_li_request(struct super_block *sb,
> }
>
> if (first_not_zeroed == ngroups || sb_rdonly(sb) ||
> - !test_opt(sb, INIT_INODE_TABLE))
> - goto out;
> + !test_opt(sb, INIT_INODE_TABLE)) {
> + if (test_opt(sb, PREFETCH_BLOCK_BITMAPS)) {
> + first_not_zeroed = 0;

ext4_register_li_request() can be called on EXT4_IOC_RESIZE_FS lctl as
err = ext4_register_li_request(sb, o_group);

In this case inode tables will be initialised started from new block range, but block bitmaps loading loop will start from group 0. Yes, this loaded groups will be skipped finally, but there are useless CPU time.

> + lr_mode = EXT4_LI_MODE_PREFETCH_BBITMAP;
> + } else
> + goto out;
> + }
>
> - elr = ext4_li_request_new(sb, first_not_zeroed);
> + elr = ext4_li_request_new(sb, first_not_zeroed, lr_mode);
> if (!elr) {
> ret = -ENOMEM;
> goto out;
> --
> 2.24.1
>