[PATCH v2 2/8] btrfs: handle shrink_delalloc pages calculation differently

[PATCH v2 2/8] btrfs: handle shrink_delalloc pages calculation differently

[Josef Bacik]

We have been hitting some early ENOSPC issues in production with more
recent kernels, and I tracked it down to us simply not flushing delalloc
as aggressively as we should be.  With tracing I was seeing us failing
all tickets with all of the block rsvs at or around 0, with very little
pinned space, but still around 120MiB of outstanding bytes_may_used.
Upon further investigation I saw that we were flushing around 14 pages
per shrink call for delalloc, despite having around 2GiB of delalloc
outstanding.

Consider the example of a 8 way machine, all CPUs trying to create a
file in parallel, which at the time of this commit requires 5 items to
do.  Assuming a 16k leaf size, we have 10MiB of total metadata reclaim
size waiting on reservations.  Now assume we have 128MiB of delalloc
outstanding.  With our current math we would set items to 20, and then
set to_reclaim to 20 * 256k, or 5MiB.

Assuming that we went through this loop all 3 times, for both
FLUSH_DELALLOC and FLUSH_DELALLOC_WAIT, and then did the full loop
twice, we'd only flush 60MiB of the 128MiB delalloc space.  This could
leave a fair bit of delalloc reservations still hanging around by the
time we go to ENOSPC out all the remaining tickets.

Fix this two ways.  First, change the calculations to be a fraction of
the total delalloc bytes on the system.  Prior to this change we were
calculating based on dirty inodes so our math made more sense, now it's
just completely unrelated to what we're actually doing.

Second add a FLUSH_DELALLOC_FULL state, that we hold off until we've
gone through the flush states at least once.  This will empty the system
of all delalloc so we're sure to be truly out of space when we start
failing tickets.

I'm tagging stable 5.10 and forward, because this is where we started
using the page stuff heavily again.  This affects earlier kernel
versions as well, but would be a pain to backport to them as the
flushing mechanisms aren't the same.

CC: stable@vger.kernel.org # 5.10+
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/ctree.h             |  9 +++++----
 fs/btrfs/space-info.c        | 35 ++++++++++++++++++++++++++---------
 include/trace/events/btrfs.h |  1 +
 3 files changed, 32 insertions(+), 13 deletions(-)

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index d7ef4d7d2c1a..232ff1a49ca6 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -2783,10 +2783,11 @@ enum btrfs_flush_state {
 	FLUSH_DELAYED_REFS	=	4,
 	FLUSH_DELALLOC		=	5,
 	FLUSH_DELALLOC_WAIT	=	6,
-	ALLOC_CHUNK		=	7,
-	ALLOC_CHUNK_FORCE	=	8,
-	RUN_DELAYED_IPUTS	=	9,
-	COMMIT_TRANS		=	10,
+	FLUSH_DELALLOC_FULL	=	7,
+	ALLOC_CHUNK		=	8,
+	ALLOC_CHUNK_FORCE	=	9,
+	RUN_DELAYED_IPUTS	=	10,
+	COMMIT_TRANS		=	11,
 };
 
 int btrfs_subvolume_reserve_metadata(struct btrfs_root *root,
diff --git a/fs/btrfs/space-info.c b/fs/btrfs/space-info.c
index af161eb808a2..0c539a94c6d9 100644
--- a/fs/btrfs/space-info.c
+++ b/fs/btrfs/space-info.c
@@ -494,6 +494,9 @@ static void shrink_delalloc(struct btrfs_fs_info *fs_info,
 	long time_left;
 	int loops;
 
+	delalloc_bytes = percpu_counter_sum_positive(&fs_info->delalloc_bytes);
+	ordered_bytes = percpu_counter_sum_positive(&fs_info->ordered_bytes);
+
 	/* Calc the number of the pages we need flush for space reservation */
 	if (to_reclaim == U64_MAX) {
 		items = U64_MAX;
@@ -501,19 +504,21 @@ static void shrink_delalloc(struct btrfs_fs_info *fs_info,
 		/*
 		 * to_reclaim is set to however much metadata we need to
 		 * reclaim, but reclaiming that much data doesn't really track
-		 * exactly, so increase the amount to reclaim by 2x in order to
-		 * make sure we're flushing enough delalloc to hopefully reclaim
-		 * some metadata reservations.
+		 * exactly.  What we really want to do is reclaim full inode's
+		 * worth of reservations, however that's not available to us
+		 * here.  We will take a fraction of the delalloc bytes for our
+		 * flushing loops and hope for the best.  Delalloc will expand
+		 * the amount we write to cover an entire dirty extent, which
+		 * will reclaim the metadata reservation for that range.  If
+		 * it's not enough subsequent flush stages will be more
+		 * aggressive.
 		 */
+		to_reclaim = max(to_reclaim, delalloc_bytes >> 3);
 		items = calc_reclaim_items_nr(fs_info, to_reclaim) * 2;
-		to_reclaim = items * EXTENT_SIZE_PER_ITEM;
 	}
 
 	trans = (struct btrfs_trans_handle *)current->journal_info;
 
-	delalloc_bytes = percpu_counter_sum_positive(
-						&fs_info->delalloc_bytes);
-	ordered_bytes = percpu_counter_sum_positive(&fs_info->ordered_bytes);
 	if (delalloc_bytes == 0 && ordered_bytes == 0)
 		return;
 
@@ -596,8 +601,11 @@ static void flush_space(struct btrfs_fs_info *fs_info,
 		break;
 	case FLUSH_DELALLOC:
 	case FLUSH_DELALLOC_WAIT:
+	case FLUSH_DELALLOC_FULL:
+		if (state == FLUSH_DELALLOC_FULL)
+			num_bytes = U64_MAX;
 		shrink_delalloc(fs_info, space_info, num_bytes,
-				state == FLUSH_DELALLOC_WAIT, for_preempt);
+				state != FLUSH_DELALLOC, for_preempt);
 		break;
 	case FLUSH_DELAYED_REFS_NR:
 	case FLUSH_DELAYED_REFS:
@@ -907,6 +915,14 @@ static void btrfs_async_reclaim_metadata_space(struct work_struct *work)
 				commit_cycles--;
 		}
 
+		/*
+		 * We do not want to empty the system of delalloc unless we're
+		 * under heavy pressure, so allow one trip through the flushing
+		 * logic before we start doing a FLUSH_DELALLOC_FULL.
+		 */
+		if (flush_state == FLUSH_DELALLOC_FULL && !commit_cycles)
+			flush_state++;
+
 		/*
 		 * We don't want to force a chunk allocation until we've tried
 		 * pretty hard to reclaim space.  Think of the case where we
@@ -1070,7 +1086,7 @@ static void btrfs_preempt_reclaim_metadata_space(struct work_struct *work)
  *   so if we now have space to allocate do the force chunk allocation.
  */
 static const enum btrfs_flush_state data_flush_states[] = {
-	FLUSH_DELALLOC_WAIT,
+	FLUSH_DELALLOC_FULL,
 	RUN_DELAYED_IPUTS,
 	COMMIT_TRANS,
 	ALLOC_CHUNK_FORCE,
@@ -1159,6 +1175,7 @@ static const enum btrfs_flush_state evict_flush_states[] = {
 	FLUSH_DELAYED_REFS,
 	FLUSH_DELALLOC,
 	FLUSH_DELALLOC_WAIT,
+	FLUSH_DELALLOC_FULL,
 	ALLOC_CHUNK,
 	COMMIT_TRANS,
 };
diff --git a/include/trace/events/btrfs.h b/include/trace/events/btrfs.h
index 3d81ba8c37b9..ddf5c250726c 100644
--- a/include/trace/events/btrfs.h
+++ b/include/trace/events/btrfs.h
@@ -94,6 +94,7 @@ struct btrfs_space_info;
 	EM( FLUSH_DELAYED_ITEMS,	"FLUSH_DELAYED_ITEMS")		\
 	EM( FLUSH_DELALLOC,		"FLUSH_DELALLOC")		\
 	EM( FLUSH_DELALLOC_WAIT,	"FLUSH_DELALLOC_WAIT")		\
+	EM( FLUSH_DELALLOC_FULL,	"FLUSH_DELALLOC_FULL")		\
 	EM( FLUSH_DELAYED_REFS_NR,	"FLUSH_DELAYED_REFS_NR")	\
 	EM( FLUSH_DELAYED_REFS,		"FLUSH_ELAYED_REFS")		\
 	EM( ALLOC_CHUNK,		"ALLOC_CHUNK")			\
-- 
2.26.3

Re: [PATCH v2 2/8] btrfs: handle shrink_delalloc pages calculation differently

[Nikolay Borisov]

On 29.06.21 г. 16:59, Josef Bacik wrote:
> We have been hitting some early ENOSPC issues in production with more
> recent kernels, and I tracked it down to us simply not flushing delalloc
> as aggressively as we should be.  With tracing I was seeing us failing
> all tickets with all of the block rsvs at or around 0, with very little
> pinned space, but still around 120MiB of outstanding bytes_may_used.
> Upon further investigation I saw that we were flushing around 14 pages
> per shrink call for delalloc, despite having around 2GiB of delalloc
> outstanding.
> 
> Consider the example of a 8 way machine, all CPUs trying to create a
> file in parallel, which at the time of this commit requires 5 items to
> do.  Assuming a 16k leaf size, we have 10MiB of total metadata reclaim
> size waiting on reservations.  Now assume we have 128MiB of delalloc
> outstanding.  With our current math we would set items to 20, and then
> set to_reclaim to 20 * 256k, or 5MiB.
> 
> Assuming that we went through this loop all 3 times, for both
> FLUSH_DELALLOC and FLUSH_DELALLOC_WAIT, and then did the full loop
> twice, we'd only flush 60MiB of the 128MiB delalloc space.  This could
> leave a fair bit of delalloc reservations still hanging around by the
> time we go to ENOSPC out all the remaining tickets.
> 
> Fix this two ways.  First, change the calculations to be a fraction of
> the total delalloc bytes on the system.  Prior to this change we were
> calculating based on dirty inodes so our math made more sense, now it's
> just completely unrelated to what we're actually doing.
> 
> Second add a FLUSH_DELALLOC_FULL state, that we hold off until we've
> gone through the flush states at least once.  This will empty the system
> of all delalloc so we're sure to be truly out of space when we start
> failing tickets.
> 
> I'm tagging stable 5.10 and forward, because this is where we started
> using the page stuff heavily again.  This affects earlier kernel
> versions as well, but would be a pain to backport to them as the
> flushing mechanisms aren't the same.
> 
> CC: stable@vger.kernel.org # 5.10+
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
>  fs/btrfs/ctree.h             |  9 +++++----
>  fs/btrfs/space-info.c        | 35 ++++++++++++++++++++++++++---------
>  include/trace/events/btrfs.h |  1 +
>  3 files changed, 32 insertions(+), 13 deletions(-)
> 
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index d7ef4d7d2c1a..232ff1a49ca6 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -2783,10 +2783,11 @@ enum btrfs_flush_state {
>  	FLUSH_DELAYED_REFS	=	4,
>  	FLUSH_DELALLOC		=	5,
>  	FLUSH_DELALLOC_WAIT	=	6,
> -	ALLOC_CHUNK		=	7,
> -	ALLOC_CHUNK_FORCE	=	8,
> -	RUN_DELAYED_IPUTS	=	9,
> -	COMMIT_TRANS		=	10,
> +	FLUSH_DELALLOC_FULL	=	7,
> +	ALLOC_CHUNK		=	8,
> +	ALLOC_CHUNK_FORCE	=	9,
> +	RUN_DELAYED_IPUTS	=	10,
> +	COMMIT_TRANS		=	11,
>  };
>  
>  int btrfs_subvolume_reserve_metadata(struct btrfs_root *root,
> diff --git a/fs/btrfs/space-info.c b/fs/btrfs/space-info.c
> index af161eb808a2..0c539a94c6d9 100644
> --- a/fs/btrfs/space-info.c
> +++ b/fs/btrfs/space-info.c
> @@ -494,6 +494,9 @@ static void shrink_delalloc(struct btrfs_fs_info *fs_info,
>  	long time_left;
>  	int loops;
>  
> +	delalloc_bytes = percpu_counter_sum_positive(&fs_info->delalloc_bytes);
> +	ordered_bytes = percpu_counter_sum_positive(&fs_info->ordered_bytes);
> +
>  	/* Calc the number of the pages we need flush for space reservation */
>  	if (to_reclaim == U64_MAX) {
>  		items = U64_MAX;
> @@ -501,19 +504,21 @@ static void shrink_delalloc(struct btrfs_fs_info *fs_info,
>  		/*
>  		 * to_reclaim is set to however much metadata we need to
>  		 * reclaim, but reclaiming that much data doesn't really track
> -		 * exactly, so increase the amount to reclaim by 2x in order to
> -		 * make sure we're flushing enough delalloc to hopefully reclaim
> -		 * some metadata reservations.
> +		 * exactly.  What we really want to do is reclaim full inode's
> +		 * worth of reservations, however that's not available to us
> +		 * here.  We will take a fraction of the delalloc bytes for our
> +		 * flushing loops and hope for the best.  Delalloc will expand
> +		 * the amount we write to cover an entire dirty extent, which
> +		 * will reclaim the metadata reservation for that range.  If
> +		 * it's not enough subsequent flush stages will be more
> +		 * aggressive.
>  		 */
> +		to_reclaim = max(to_reclaim, delalloc_bytes >> 3);
>  		items = calc_reclaim_items_nr(fs_info, to_reclaim) * 2;
> -		to_reclaim = items * EXTENT_SIZE_PER_ITEM;
>  	}
>  
>  	trans = (struct btrfs_trans_handle *)current->journal_info;
>  
> -	delalloc_bytes = percpu_counter_sum_positive(
> -						&fs_info->delalloc_bytes);
> -	ordered_bytes = percpu_counter_sum_positive(&fs_info->ordered_bytes);
>  	if (delalloc_bytes == 0 && ordered_bytes == 0)
>  		return;

nit: That check should also be moved alongside delalloc/ordered _bytes
read out.

<snip>

[PATCH v2 4/8] btrfs: wake up async_delalloc_pages waiters after submit

[Josef Bacik]

We use the async_delalloc_pages mechanism to make sure that we've
completed our async work before trying to continue our delalloc
flushing.  The reason for this is we need to see any ordered extents
that were created by our delalloc flushing.  However we're waking up
before we do the submit work, which is before we create the ordered
extents.  This is a pretty wide race window where we could potentially
think there are no ordered extents and thus exit shrink_delalloc
prematurely.  Fix this by waking us up after we've done the work to
create ordered extents.

cc: stable@vger.kernel.org
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/inode.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index b1f02e3fea5d..e388153c4ae4 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -1290,11 +1290,6 @@ static noinline void async_cow_submit(struct btrfs_work *work)
 	nr_pages = (async_chunk->end - async_chunk->start + PAGE_SIZE) >>
 		PAGE_SHIFT;
 
-	/* atomic_sub_return implies a barrier */
-	if (atomic_sub_return(nr_pages, &fs_info->async_delalloc_pages) <
-	    5 * SZ_1M)
-		cond_wake_up_nomb(&fs_info->async_submit_wait);
-
 	/*
 	 * ->inode could be NULL if async_chunk_start has failed to compress,
 	 * in which case we don't have anything to submit, yet we need to
@@ -1303,6 +1298,11 @@ static noinline void async_cow_submit(struct btrfs_work *work)
 	 */
 	if (async_chunk->inode)
 		submit_compressed_extents(async_chunk);
+
+	/* atomic_sub_return implies a barrier */
+	if (atomic_sub_return(nr_pages, &fs_info->async_delalloc_pages) <
+	    5 * SZ_1M)
+		cond_wake_up_nomb(&fs_info->async_submit_wait);
 }
 
 static noinline void async_cow_free(struct btrfs_work *work)
-- 
2.26.3

Re: [PATCH v2 4/8] btrfs: wake up async_delalloc_pages waiters after submit

[Nikolay Borisov]

On 29.06.21 г. 16:59, Josef Bacik wrote:
> We use the async_delalloc_pages mechanism to make sure that we've
> completed our async work before trying to continue our delalloc
> flushing.  The reason for this is we need to see any ordered extents
> that were created by our delalloc flushing.  However we're waking up
> before we do the submit work, which is before we create the ordered
> extents.  This is a pretty wide race window where we could potentially
> think there are no ordered extents and thus exit shrink_delalloc
> prematurely.  Fix this by waking us up after we've done the work to
> create ordered extents.
> 
> cc: stable@vger.kernel.org
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>

Reviewed-by: Nikolay Borisov <nborisov@suse.com>

This is an independent change from the rest of the series and it can go
before the rest.