[PATCH v2] Btrfs: set keep_lock when necessary in btrfs_defrag_leaves

[PATCH v2] Btrfs: set keep_lock when necessary in btrfs_defrag_leaves

[Liu Bo]

path->keep_lock may force to lock tree root and higher nodes, and make
lock contention worse, thus it needs to be avoided as much as
possible.

In btrfs_degrag_leaves, path->keep_lock is set but @path immediatley
gets released, which is not necessary at all.

Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>
---
v2: update commit log with more details.

 fs/btrfs/tree-defrag.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/fs/btrfs/tree-defrag.c b/fs/btrfs/tree-defrag.c
index 3c0987ab587d..c12747904d4c 100644
--- a/fs/btrfs/tree-defrag.c
+++ b/fs/btrfs/tree-defrag.c
@@ -65,8 +65,6 @@ int btrfs_defrag_leaves(struct btrfs_trans_handle *trans,
 		memcpy(&key, &root->defrag_progress, sizeof(key));
 	}
 
-	path->keep_locks = 1;
-
 	ret = btrfs_search_forward(root, &key, path, BTRFS_OLDEST_GENERATION);
 	if (ret < 0)
 		goto out;
@@ -81,6 +79,7 @@ int btrfs_defrag_leaves(struct btrfs_trans_handle *trans,
 	 * a deadlock (attempting to write lock an already write locked leaf).
 	 */
 	path->lowest_level = 1;
+	path->keep_locks = 1;
 	wret = btrfs_search_slot(trans, root, &key, path, 0, 1);
 
 	if (wret < 0) {
-- 
1.8.3.1

Re: [PATCH v2] Btrfs: set keep_lock when necessary in btrfs_defrag_leaves

[Nikolay Borisov]

On 26.04.2018 04:27, Liu Bo wrote:
> path->keep_lock may force to lock tree root and higher nodes, and make
> lock contention worse, thus it needs to be avoided as much as
> possible.
> 
> In btrfs_degrag_leaves, path->keep_lock is set but @path immediatley
> gets released, which is not necessary at all.
> 

This is still very terse and still doesn't explain why keep_locks should
be set right before btrfs_search_slot and not the beginning of the
function. "when necessary" is perhaps the second most uninformative
reason of doing something, right after "because of reasons"
> Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>
> ---
> v2: update commit log with more details.
> 
>  fs/btrfs/tree-defrag.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/fs/btrfs/tree-defrag.c b/fs/btrfs/tree-defrag.c
> index 3c0987ab587d..c12747904d4c 100644
> --- a/fs/btrfs/tree-defrag.c
> +++ b/fs/btrfs/tree-defrag.c
> @@ -65,8 +65,6 @@ int btrfs_defrag_leaves(struct btrfs_trans_handle *trans,
>  		memcpy(&key, &root->defrag_progress, sizeof(key));
>  	}
>  
> -	path->keep_locks = 1;
> -
>  	ret = btrfs_search_forward(root, &key, path, BTRFS_OLDEST_GENERATION);
>  	if (ret < 0)
>  		goto out;
> @@ -81,6 +79,7 @@ int btrfs_defrag_leaves(struct btrfs_trans_handle *trans,
>  	 * a deadlock (attempting to write lock an already write locked leaf).
>  	 */
>  	path->lowest_level = 1;
> +	path->keep_locks = 1;
>  	wret = btrfs_search_slot(trans, root, &key, path, 0, 1);
>  
>  	if (wret < 0) {
> 

Re: [PATCH v2] Btrfs: set keep_lock when necessary in btrfs_defrag_leaves

[David Sterba]

On Thu, Apr 26, 2018 at 09:27:43AM +0800, Liu Bo wrote:
> path->keep_lock may force to lock tree root and higher nodes, and make
> lock contention worse, thus it needs to be avoided as much as
> possible.
> 
> In btrfs_degrag_leaves, path->keep_lock is set but @path immediatley
> gets released, which is not necessary at all.

So what is the effect of this? The locks are held throughout
btrfs_search_forward and released after that. Thi

> Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>
> ---
> v2: update commit log with more details.
> 
>  fs/btrfs/tree-defrag.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/fs/btrfs/tree-defrag.c b/fs/btrfs/tree-defrag.c
> index 3c0987ab587d..c12747904d4c 100644
> --- a/fs/btrfs/tree-defrag.c
> +++ b/fs/btrfs/tree-defrag.c
> @@ -65,8 +65,6 @@ int btrfs_defrag_leaves(struct btrfs_trans_handle *trans,
>  		memcpy(&key, &root->defrag_progress, sizeof(key));
>  	}
>  
> -	path->keep_locks = 1;
> -
>  	ret = btrfs_search_forward(root, &key, path, BTRFS_OLDEST_GENERATION);

What does btrfs_search_forward do as the first statement:

5115 int btrfs_search_forward(struct btrfs_root *root, struct btrfs_key *min_key,
5116                          struct btrfs_path *path,
5117                          u64 min_trans)
5118 {
.... declarations
5128
5129         path->keep_locks = 1;

So even if removed from above, there will be no change. The value of
keep_locks is preserved after btrfs_path_release.

>  	if (ret < 0)
>  		goto out;
> @@ -81,6 +79,7 @@ int btrfs_defrag_leaves(struct btrfs_trans_handle *trans,
>  	 * a deadlock (attempting to write lock an already write locked leaf).
>  	 */
>  	path->lowest_level = 1;
> +	path->keep_locks = 1;
>  	wret = btrfs_search_slot(trans, root, &key, path, 0, 1);
>  
>  	if (wret < 0) {
> -- 
> 1.8.3.1
> 
> --
> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

Re: [PATCH v2] Btrfs: set keep_lock when necessary in btrfs_defrag_leaves

[Liu Bo]

On Fri, Apr 27, 2018 at 2:06 AM, David Sterba <dsterba@suse.cz> wrote:
> On Thu, Apr 26, 2018 at 09:27:43AM +0800, Liu Bo wrote:
>> path->keep_lock may force to lock tree root and higher nodes, and make
>> lock contention worse, thus it needs to be avoided as much as
>> possible.
>>
>> In btrfs_degrag_leaves, path->keep_lock is set but @path immediatley
>> gets released, which is not necessary at all.
>
> So what is the effect of this? The locks are held throughout
> btrfs_search_forward and released after that. Thi
>
>> Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>
>> ---
>> v2: update commit log with more details.
>>
>>  fs/btrfs/tree-defrag.c | 3 +--
>>  1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/fs/btrfs/tree-defrag.c b/fs/btrfs/tree-defrag.c
>> index 3c0987ab587d..c12747904d4c 100644
>> --- a/fs/btrfs/tree-defrag.c
>> +++ b/fs/btrfs/tree-defrag.c
>> @@ -65,8 +65,6 @@ int btrfs_defrag_leaves(struct btrfs_trans_handle *trans,
>>               memcpy(&key, &root->defrag_progress, sizeof(key));
>>       }
>>
>> -     path->keep_locks = 1;
>> -
>>       ret = btrfs_search_forward(root, &key, path, BTRFS_OLDEST_GENERATION);
>
> What does btrfs_search_forward do as the first statement:
>
> 5115 int btrfs_search_forward(struct btrfs_root *root, struct btrfs_key *min_key,
> 5116                          struct btrfs_path *path,
> 5117                          u64 min_trans)
> 5118 {
> .... declarations
> 5128
> 5129         path->keep_locks = 1;
>
> So even if removed from above, there will be no change. The value of
> keep_locks is preserved after btrfs_path_release.
>

It will set it back,

out:
        path->keep_locks = keep_locks;


thanks,
liubo
>>       if (ret < 0)
>>               goto out;
>> @@ -81,6 +79,7 @@ int btrfs_defrag_leaves(struct btrfs_trans_handle *trans,
>>        * a deadlock (attempting to write lock an already write locked leaf).
>>        */
>>       path->lowest_level = 1;
>> +     path->keep_locks = 1;
>>       wret = btrfs_search_slot(trans, root, &key, path, 0, 1);
>>
>>       if (wret < 0) {
>> --
>> 1.8.3.1
>>
>> --
>> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
>> the body of a message to majordomo@vger.kernel.org
>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> --
> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

Re: [PATCH v2] Btrfs: set keep_lock when necessary in btrfs_defrag_leaves

[Liu Bo]

On Fri, Apr 27, 2018 at 2:06 AM, David Sterba <dsterba@suse.cz> wrote:
> On Thu, Apr 26, 2018 at 09:27:43AM +0800, Liu Bo wrote:
>> path->keep_lock may force to lock tree root and higher nodes, and make
>> lock contention worse, thus it needs to be avoided as much as
>> possible.
>>
>> In btrfs_degrag_leaves, path->keep_lock is set but @path immediatley
>> gets released, which is not necessary at all.
>
> So what is the effect of this? The locks are held throughout
> btrfs_search_forward and released after that. Thi
>
>> Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>
>> ---
>> v2: update commit log with more details.
>>
>>  fs/btrfs/tree-defrag.c | 3 +--
>>  1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/fs/btrfs/tree-defrag.c b/fs/btrfs/tree-defrag.c
>> index 3c0987ab587d..c12747904d4c 100644
>> --- a/fs/btrfs/tree-defrag.c
>> +++ b/fs/btrfs/tree-defrag.c
>> @@ -65,8 +65,6 @@ int btrfs_defrag_leaves(struct btrfs_trans_handle *trans,
>>               memcpy(&key, &root->defrag_progress, sizeof(key));
>>       }
>>
>> -     path->keep_locks = 1;
>> -
>>       ret = btrfs_search_forward(root, &key, path, BTRFS_OLDEST_GENERATION);
>
> What does btrfs_search_forward do as the first statement:
>
> 5115 int btrfs_search_forward(struct btrfs_root *root, struct btrfs_key *min_key,
> 5116                          struct btrfs_path *path,
> 5117                          u64 min_trans)
> 5118 {
> .... declarations
> 5128
> 5129         path->keep_locks = 1;
>
> So even if removed from above, there will be no change. The value of
> keep_locks is preserved after btrfs_path_release.
>

FYI, btrfs_search_forward() doesn't need keep_locks's semantics as all
of its callers only access path->nodes[0], thus I'm planning to remove
keep_locks setting inside it, too.

thanks,
liubo

>>       if (ret < 0)
>>               goto out;
>> @@ -81,6 +79,7 @@ int btrfs_defrag_leaves(struct btrfs_trans_handle *trans,
>>        * a deadlock (attempting to write lock an already write locked leaf).
>>        */
>>       path->lowest_level = 1;
>> +     path->keep_locks = 1;
>>       wret = btrfs_search_slot(trans, root, &key, path, 0, 1);
>>
>>       if (wret < 0) {
>> --
>> 1.8.3.1
>>
>> --
>> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
>> the body of a message to majordomo@vger.kernel.org
>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> --
> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

Re: [PATCH v2] Btrfs: set keep_lock when necessary in btrfs_defrag_leaves

[David Sterba]

On Fri, Apr 27, 2018 at 11:23:23AM +0800, Liu Bo wrote:
> >> ---
> >> v2: update commit log with more details.
> >>
> >>  fs/btrfs/tree-defrag.c | 3 +--
> >>  1 file changed, 1 insertion(+), 2 deletions(-)
> >>
> >> diff --git a/fs/btrfs/tree-defrag.c b/fs/btrfs/tree-defrag.c
> >> index 3c0987ab587d..c12747904d4c 100644
> >> --- a/fs/btrfs/tree-defrag.c
> >> +++ b/fs/btrfs/tree-defrag.c
> >> @@ -65,8 +65,6 @@ int btrfs_defrag_leaves(struct btrfs_trans_handle *trans,
> >>               memcpy(&key, &root->defrag_progress, sizeof(key));
> >>       }
> >>
> >> -     path->keep_locks = 1;
> >> -
> >>       ret = btrfs_search_forward(root, &key, path, BTRFS_OLDEST_GENERATION);
> >
> > What does btrfs_search_forward do as the first statement:
> >
> > 5115 int btrfs_search_forward(struct btrfs_root *root, struct btrfs_key *min_key,
> > 5116                          struct btrfs_path *path,
> > 5117                          u64 min_trans)
> > 5118 {
> > .... declarations
> > 5128
> > 5129         path->keep_locks = 1;
> >
> > So even if removed from above, there will be no change. The value of
> > keep_locks is preserved after btrfs_path_release.
> >
> 
> It will set it back,
> 
> out:
>         path->keep_locks = keep_locks;

Oh, right. So much for reading the whole function.

Re: [PATCH v2] Btrfs: set keep_lock when necessary in btrfs_defrag_leaves

[David Sterba]

On Fri, Apr 27, 2018 at 01:36:35PM +0800, Liu Bo wrote:
> > What does btrfs_search_forward do as the first statement:
> >
> > 5115 int btrfs_search_forward(struct btrfs_root *root, struct btrfs_key *min_key,
> > 5116                          struct btrfs_path *path,
> > 5117                          u64 min_trans)
> > 5118 {
> > .... declarations
> > 5128
> > 5129         path->keep_locks = 1;
> >
> > So even if removed from above, there will be no change. The value of
> > keep_locks is preserved after btrfs_path_release.
> >
> 
> FYI, btrfs_search_forward() doesn't need keep_locks's semantics as all
> of its callers only access path->nodes[0], thus I'm planning to remove
> keep_locks setting inside it, too.

Ok.

Please update the changelog of this patch and note something about
btrfs_search_forward that sets the path lock on it's own and that
there's no change in defrag leaves.