[PATCH] Btrfs: shrink_delalloc check bdi write congested

[PATCH] Btrfs: shrink_delalloc check bdi write congested

[Itaru Kitayama]

In srhink_delalloc(), writeback starts if idle, also check the bdi is
not write congested.
The patch is against the head of the btrfs-next.

Signed-off-by: Itaru Kitayama <kitayama@cl.bb4u.ne.jp>

 fs/btrfs/extent-tree.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index efb044e..caa74d3 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -3712,7 +3712,7 @@ static void shrink_delalloc(struct btrfs_root *root, u64 t
        while (delalloc_bytes && loops < 3) {
                max_reclaim = min(delalloc_bytes, to_reclaim);
                nr_pages = max_reclaim >> PAGE_CACHE_SHIFT;
-               writeback_inodes_sb_nr_if_idle(root->fs_info->sb, nr_pages,
+               if (!bdi_write_congested(root->fs_info->sb->s_bdi)) writeback_in
                                               WB_REASON_FS_FREE_SPACE);

                /*

Re: [PATCH] Btrfs: shrink_delalloc check bdi write congested

[David Sterba]

On Sat, Sep 29, 2012 at 10:20:09PM +0900, Itaru Kitayama wrote:
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -3712,7 +3712,7 @@ static void shrink_delalloc(struct btrfs_root *root, u64 t
>         while (delalloc_bytes && loops < 3) {
>                 max_reclaim = min(delalloc_bytes, to_reclaim);
>                 nr_pages = max_reclaim >> PAGE_CACHE_SHIFT;
> -               writeback_inodes_sb_nr_if_idle(root->fs_info->sb, nr_pages,
> +               if (!bdi_write_congested(root->fs_info->sb->s_bdi)) writeback_in
>                                                WB_REASON_FS_FREE_SPACE);

malformed patch

Re: [PATCH] Btrfs: shrink_delalloc check bdi write congested

[Itaru Kitayama]

Resubmit this after the checkpatch test.

In srhink_delalloc(), writeback starts if idle, also check the bdi is
not write congested.
The patch is against the head of the btrfs-next.

Signed-off-by: Itaru Kitayama <kitayama@cl.bb4.ne.jp>

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index efb044e..1aae046 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -3712,8 +3712,9 @@ static void shrink_delalloc(struct btrfs_root
*root, u64 to_reclaim, u64 orig,
        while (delalloc_bytes && loops < 3) {
                max_reclaim = min(delalloc_bytes, to_reclaim);
                nr_pages = max_reclaim >> PAGE_CACHE_SHIFT;
-               writeback_inodes_sb_nr_if_idle(root->fs_info->sb, nr_pages,
-                                              WB_REASON_FS_FREE_SPACE);
+               if (!bdi_write_congested(root->fs_info->sb->s_bdi))
+                       writeback_inodes_sb_nr_if_idle(root->fs_info, nr_page,
+                                                      WB_REASON_FS_FREE_SPACE);

                /*
                 * We need to wait for the async pages to actually start before


On Sun, Sep 30, 2012 at 1:21 AM, David Sterba <dave@jikos.cz> wrote:
> On Sat, Sep 29, 2012 at 10:20:09PM +0900, Itaru Kitayama wrote:
>> --- a/fs/btrfs/extent-tree.c
>> +++ b/fs/btrfs/extent-tree.c
>> @@ -3712,7 +3712,7 @@ static void shrink_delalloc(struct btrfs_root *root, u64 t
>>         while (delalloc_bytes && loops < 3) {
>>                 max_reclaim = min(delalloc_bytes, to_reclaim);
>>                 nr_pages = max_reclaim >> PAGE_CACHE_SHIFT;
>> -               writeback_inodes_sb_nr_if_idle(root->fs_info->sb, nr_pages,
>> +               if (!bdi_write_congested(root->fs_info->sb->s_bdi)) writeback_in
>>                                                WB_REASON_FS_FREE_SPACE);
>
> malformed patch

Re: [PATCH] Btrfs: shrink_delalloc check bdi write congested

[Itaru Kitayama]

This is the correct one.

Signed-off-by: Itaru Kitayama <kitayama@cl.bb4.ne.jp>

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index efb044e..c032dbe 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -3712,8 +3712,9 @@ static void shrink_delalloc(struct btrfs_root
*root, u64 to_reclaim, u64 orig,
        while (delalloc_bytes && loops < 3) {
                max_reclaim = min(delalloc_bytes, to_reclaim);
                nr_pages = max_reclaim >> PAGE_CACHE_SHIFT;
-               writeback_inodes_sb_nr_if_idle(root->fs_info->sb, nr_pages,
-                                              WB_REASON_FS_FREE_SPACE);
+               if (!bdi_write_congested(root->fs_info->sb->s_bdi))
+                       writeback_inodes_sb_nr_if_idle(root->fs_info, nr_pages,
+                                                      WB_REASON_FS_FREE_SPACE);

                /*
                 * We need to wait for the async pages to actually start before


On Sun, Sep 30, 2012 at 10:28 AM, Itaru Kitayama <kitayama@cl.bb4u.ne.jp> wrote:
> Resubmit this after the checkpatch test.
>
> In srhink_delalloc(), writeback starts if idle, also check the bdi is
> not write congested.
> The patch is against the head of the btrfs-next.
>
> Signed-off-by: Itaru Kitayama <kitayama@cl.bb4.ne.jp>
>
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index efb044e..1aae046 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -3712,8 +3712,9 @@ static void shrink_delalloc(struct btrfs_root
> *root, u64 to_reclaim, u64 orig,
>         while (delalloc_bytes && loops < 3) {
>                 max_reclaim = min(delalloc_bytes, to_reclaim);
>                 nr_pages = max_reclaim >> PAGE_CACHE_SHIFT;
> -               writeback_inodes_sb_nr_if_idle(root->fs_info->sb, nr_pages,
> -                                              WB_REASON_FS_FREE_SPACE);
> +               if (!bdi_write_congested(root->fs_info->sb->s_bdi))
> +                       writeback_inodes_sb_nr_if_idle(root->fs_info, nr_page,
> +                                                      WB_REASON_FS_FREE_SPACE);
>
>                 /*
>                  * We need to wait for the async pages to actually start before
>
>
> On Sun, Sep 30, 2012 at 1:21 AM, David Sterba <dave@jikos.cz> wrote:
>> On Sat, Sep 29, 2012 at 10:20:09PM +0900, Itaru Kitayama wrote:
>>> --- a/fs/btrfs/extent-tree.c
>>> +++ b/fs/btrfs/extent-tree.c
>>> @@ -3712,7 +3712,7 @@ static void shrink_delalloc(struct btrfs_root *root, u64 t
>>>         while (delalloc_bytes && loops < 3) {
>>>                 max_reclaim = min(delalloc_bytes, to_reclaim);
>>>                 nr_pages = max_reclaim >> PAGE_CACHE_SHIFT;
>>> -               writeback_inodes_sb_nr_if_idle(root->fs_info->sb, nr_pages,
>>> +               if (!bdi_write_congested(root->fs_info->sb->s_bdi)) writeback_in
>>>                                                WB_REASON_FS_FREE_SPACE);
>>
>> malformed patch

Re: [PATCH] Btrfs: shrink_delalloc check bdi write congested

[David Sterba]

Hi again,

On Sun, Sep 30, 2012 at 11:11:10AM +0900, Itaru Kitayama wrote:
> This is the correct one.
> 
> Signed-off-by: Itaru Kitayama <kitayama@cl.bb4.ne.jp>
> 
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index efb044e..c032dbe 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -3712,8 +3712,9 @@ static void shrink_delalloc(struct btrfs_root
> *root, u64 to_reclaim, u64 orig,
>         while (delalloc_bytes && loops < 3) {
>                 max_reclaim = min(delalloc_bytes, to_reclaim);
>                 nr_pages = max_reclaim >> PAGE_CACHE_SHIFT;
> -               writeback_inodes_sb_nr_if_idle(root->fs_info->sb, nr_pages,
> -                                              WB_REASON_FS_FREE_SPACE);
> +               if (!bdi_write_congested(root->fs_info->sb->s_bdi))
> +                       writeback_inodes_sb_nr_if_idle(root->fs_info, nr_pages,
> +                                                      WB_REASON_FS_FREE_SPACE);

You don't pass the same argument to writeback_inodes_sb_nr_if_idle in
the changed code, this would not compile (root->fs_info->sb), but it's
a minor thing.

I'm more interested in the background motivation of the change, it's
clear that it tries to avoid writing data if the devices are congested,
have you measured an improvement against original behaviour?

writeback_inodes_sb_nr_if_idle checks if writeback is in progress and
does not start if this is true. That way this will not hammer the device
unnecessarily.

Your code tries to avoid even more hammering of the device when the
writes do not come from writeback. This may or may not be a good thing
(and hard to guess without a few tests, or at least I don't see that).

Shrink delalloc starts the writeback for a given number of pages and
then hopes they'll be flushed so the reserved space can be reclaimed
back. If the device is congested, this will not start the writeback and
it would be very unlikely that total delalloc bytes shrinks. The rest of
the function relies on asynchronous behaviour, it's even less clear what
it would do without the writeback call. In the worst case it could block
on 'wait_event' or at 'btrfs_wait_ordered_extents' in some case, though
this is just more of a speculation.


david

Re: [PATCH] Btrfs: shrink_delalloc check bdi write congested

[Itaru Kitayama]

Hi David,
Thank you for your comments. I wanted to fix a lockdep warning on a
possible deadlock
case encountered during the xfstests with a scratch space almost full.

You are right I encountered the worst scenario you described below, I
drop this patch and
I'll look at btrfs_congested_fn more to examine the mechanisms
implemented there are
working as expected.

Thanks,

Itaru

On Mon, Oct 1, 2012 at 7:29 AM, David Sterba <dave@jikos.cz> wrote:
> Hi again,
>
> On Sun, Sep 30, 2012 at 11:11:10AM +0900, Itaru Kitayama wrote:
>> This is the correct one.
>>
>> Signed-off-by: Itaru Kitayama <kitayama@cl.bb4.ne.jp>
>>
>> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
>> index efb044e..c032dbe 100644
>> --- a/fs/btrfs/extent-tree.c
>> +++ b/fs/btrfs/extent-tree.c
>> @@ -3712,8 +3712,9 @@ static void shrink_delalloc(struct btrfs_root
>> *root, u64 to_reclaim, u64 orig,
>>         while (delalloc_bytes && loops < 3) {
>>                 max_reclaim = min(delalloc_bytes, to_reclaim);
>>                 nr_pages = max_reclaim >> PAGE_CACHE_SHIFT;
>> -               writeback_inodes_sb_nr_if_idle(root->fs_info->sb, nr_pages,
>> -                                              WB_REASON_FS_FREE_SPACE);
>> +               if (!bdi_write_congested(root->fs_info->sb->s_bdi))
>> +                       writeback_inodes_sb_nr_if_idle(root->fs_info, nr_pages,
>> +                                                      WB_REASON_FS_FREE_SPACE);
>
> You don't pass the same argument to writeback_inodes_sb_nr_if_idle in
> the changed code, this would not compile (root->fs_info->sb), but it's
> a minor thing.
>
> I'm more interested in the background motivation of the change, it's
> clear that it tries to avoid writing data if the devices are congested,
> have you measured an improvement against original behaviour?
>
> writeback_inodes_sb_nr_if_idle checks if writeback is in progress and
> does not start if this is true. That way this will not hammer the device
> unnecessarily.
>
> Your code tries to avoid even more hammering of the device when the
> writes do not come from writeback. This may or may not be a good thing
> (and hard to guess without a few tests, or at least I don't see that).
>
> Shrink delalloc starts the writeback for a given number of pages and
> then hopes they'll be flushed so the reserved space can be reclaimed
> back. If the device is congested, this will not start the writeback and
> it would be very unlikely that total delalloc bytes shrinks. The rest of
> the function relies on asynchronous behaviour, it's even less clear what
> it would do without the writeback call. In the worst case it could block
> on 'wait_event' or at 'btrfs_wait_ordered_extents' in some case, though
> this is just more of a speculation.
>
>
> david

Re: [PATCH] Btrfs: shrink_delalloc check bdi write congested

[David Sterba]

On Mon, Oct 01, 2012 at 08:55:28AM +0900, Itaru Kitayama wrote:
> Thank you for your comments. I wanted to fix a lockdep warning on a
> possible deadlock
> case encountered during the xfstests with a scratch space almost full.

This is the known cleaner/writeback deadlock and avoiding writeback
under device congestion may avoid it, but not in all cases and we want
to fix it properly (Miao sent the patches, but they're not merged so
far).

> You are right I encountered the worst scenario you described below, I
> drop this patch and
> I'll look at btrfs_congested_fn more to examine the mechanisms
> implemented there are
> working as expected.

I'm not sure what you want to achieve with the congestion checks (other
than avoid the cleaner deadlock).

david