[PATCH] xfs: remove redundant batch variables for serialization

[PATCH] xfs: remove redundant batch variables for serialization

[alexjlzheng]

From: Jinliang Zheng <alexjlzheng@tencent.com>

Historically, when generic percpu counters were introduced in xfs for
free block counters by commit 0d485ada404b ("xfs: use generic percpu
counters for free block counter"), the counters use a custom batch size.
In xfs_mod_freecounter(), originally named xfs_mod_fdblocks(), this
patch attempts to serialize the program using a smaller batch size as a
parameter to the addition function as the counter approaches 0.

Commit 8c1903d3081a ("xfs: inode and free block counters need to use
__percpu_counter_compare") pointed out the error in commit 0d485ada404b
("xfs: use generic percpu counters for free block counter") mentioned
above and said that "Because the counters use a custom batch size, the
comparison functions need to be aware of that batch size otherwise the
comparison does not work correctly".

After commit 8c1903d3081a ("xfs: inode and free block counters need to
use __percpu_counter_compare"), the existence of the batch variable is
no longer necessary, so it was removed to simplify the code.

Signed-off-by: Jinliang Zheng <alexjlzheng@tencent.com>
---
 fs/xfs/xfs_mount.c | 17 +----------------
 1 file changed, 1 insertion(+), 16 deletions(-)

diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
index 0a0fd19573d8..72dab39376b7 100644
--- a/fs/xfs/xfs_mount.c
+++ b/fs/xfs/xfs_mount.c
@@ -1144,7 +1144,6 @@ xfs_mod_freecounter(
 	int64_t			lcounter;
 	long long		res_used;
 	uint64_t		set_aside = 0;
-	s32			batch;
 	bool			has_resv_pool;
 
 	ASSERT(counter == &mp->m_fdblocks || counter == &mp->m_frextents);
@@ -1177,20 +1176,6 @@ xfs_mod_freecounter(
 		return 0;
 	}
 
-	/*
-	 * Taking blocks away, need to be more accurate the closer we
-	 * are to zero.
-	 *
-	 * If the counter has a value of less than 2 * max batch size,
-	 * then make everything serialise as we are real close to
-	 * ENOSPC.
-	 */
-	if (__percpu_counter_compare(counter, 2 * XFS_FDBLOCKS_BATCH,
-				     XFS_FDBLOCKS_BATCH) < 0)
-		batch = 1;
-	else
-		batch = XFS_FDBLOCKS_BATCH;
-
 	/*
 	 * Set aside allocbt blocks because these blocks are tracked as free
 	 * space but not available for allocation. Technically this means that a
@@ -1204,7 +1189,7 @@ xfs_mod_freecounter(
 	 */
 	if (has_resv_pool)
 		set_aside = xfs_fdblocks_unavailable(mp);
-	percpu_counter_add_batch(counter, delta, batch);
+	percpu_counter_add_batch(counter, delta, XFS_FDBLOCKS_BATCH);
 	if (__percpu_counter_compare(counter, set_aside,
 				     XFS_FDBLOCKS_BATCH) >= 0) {
 		/* we had space! */
-- 
2.31.1

Re: [PATCH] xfs: remove redundant batch variables for serialization

[Darrick J. Wong]

On Thu, Sep 07, 2023 at 08:30:18PM +0800, alexjlzheng@gmail.com wrote:
> From: Jinliang Zheng <alexjlzheng@tencent.com>
> 
> Historically, when generic percpu counters were introduced in xfs for
> free block counters by commit 0d485ada404b ("xfs: use generic percpu
> counters for free block counter"), the counters use a custom batch size.
> In xfs_mod_freecounter(), originally named xfs_mod_fdblocks(), this
> patch attempts to serialize the program using a smaller batch size as a
> parameter to the addition function as the counter approaches 0.
> 
> Commit 8c1903d3081a ("xfs: inode and free block counters need to use
> __percpu_counter_compare") pointed out the error in commit 0d485ada404b
> ("xfs: use generic percpu counters for free block counter") mentioned
> above and said that "Because the counters use a custom batch size, the
> comparison functions need to be aware of that batch size otherwise the
> comparison does not work correctly".
> 
> After commit 8c1903d3081a ("xfs: inode and free block counters need to
> use __percpu_counter_compare"), the existence of the batch variable is
> no longer necessary, so it was removed to simplify the code.

It *was removed*?  It looks like this patch is _doing_ the removing.

I don't get it, what problem are you having?

--D

> Signed-off-by: Jinliang Zheng <alexjlzheng@tencent.com>
> ---
>  fs/xfs/xfs_mount.c | 17 +----------------
>  1 file changed, 1 insertion(+), 16 deletions(-)
> 
> diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
> index 0a0fd19573d8..72dab39376b7 100644
> --- a/fs/xfs/xfs_mount.c
> +++ b/fs/xfs/xfs_mount.c
> @@ -1144,7 +1144,6 @@ xfs_mod_freecounter(
>  	int64_t			lcounter;
>  	long long		res_used;
>  	uint64_t		set_aside = 0;
> -	s32			batch;
>  	bool			has_resv_pool;
>  
>  	ASSERT(counter == &mp->m_fdblocks || counter == &mp->m_frextents);
> @@ -1177,20 +1176,6 @@ xfs_mod_freecounter(
>  		return 0;
>  	}
>  
> -	/*
> -	 * Taking blocks away, need to be more accurate the closer we
> -	 * are to zero.
> -	 *
> -	 * If the counter has a value of less than 2 * max batch size,
> -	 * then make everything serialise as we are real close to
> -	 * ENOSPC.
> -	 */
> -	if (__percpu_counter_compare(counter, 2 * XFS_FDBLOCKS_BATCH,
> -				     XFS_FDBLOCKS_BATCH) < 0)
> -		batch = 1;
> -	else
> -		batch = XFS_FDBLOCKS_BATCH;
> -
>  	/*
>  	 * Set aside allocbt blocks because these blocks are tracked as free
>  	 * space but not available for allocation. Technically this means that a
> @@ -1204,7 +1189,7 @@ xfs_mod_freecounter(
>  	 */
>  	if (has_resv_pool)
>  		set_aside = xfs_fdblocks_unavailable(mp);
> -	percpu_counter_add_batch(counter, delta, batch);
> +	percpu_counter_add_batch(counter, delta, XFS_FDBLOCKS_BATCH);
>  	if (__percpu_counter_compare(counter, set_aside,
>  				     XFS_FDBLOCKS_BATCH) >= 0) {
>  		/* we had space! */
> -- 
> 2.31.1
>