Re: [PATCH] Btrfs: avoid syncing log in the fast fsync path when not necessary

[Qu Wenruo <quwenruo@cn.fujitsu.com>]

To Chris:

Would you please consider including this patch to 4.1 merge window?
As it fixes a performance regression in 4.0.

Thanks,
Qu

-------- Original Message  --------
Subject: [PATCH] Btrfs: avoid syncing log in the fast fsync path when 
not necessary
From: Filipe Manana <fdmanana@suse.com>
To: <linux-btrfs@vger.kernel.org>
Date: 2015年04月01日 09:00

> Commit 3a8b36f37806 ("Btrfs: fix data loss in the fast fsync path") added
> a performance regression for that causes an unnecessary sync of the log
> trees (fs/subvol and root log trees) when 2 consecutive fsyncs are done
> against a file, without no writes or any metadata updates to the inode in
> between them and if a transaction is committed before the second fsync is
> called.
>
> Huang Ying reported this to lkml (https://lkml.org/lkml/2015/3/18/99)
> after a test sysbench test that measured a -62% decrease of file io
> requests per second for that tests' workload.
>
> The test is:
>
>    echo performance > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
>    echo performance > /sys/devices/system/cpu/cpu1/cpufreq/scaling_governor
>    echo performance > /sys/devices/system/cpu/cpu2/cpufreq/scaling_governor
>    echo performance > /sys/devices/system/cpu/cpu3/cpufreq/scaling_governor
>    mkfs -t btrfs /dev/sda2
>    mount -t btrfs /dev/sda2 /fs/sda2
>    cd /fs/sda2
>    for ((i = 0; i < 1024; i++)); do fallocate -l 67108864 testfile.$i; done
>    sysbench --test=fileio --max-requests=0 --num-threads=4 --max-time=600 \
>      --file-test-mode=rndwr --file-total-size=68719476736 --file-io-mode=sync \
>      --file-num=1024 run
>
> A test on kvm guest, running a debug kernel gave me the following results:
>
> Without 3a8b36f378060d:             16.01 reqs/sec
> With 3a8b36f378060d:                 3.39 reqs/sec
> With 3a8b36f378060d and this patch: 16.04 reqs/sec
>
> Reported-by: Huang Ying <ying.huang@intel.com>
> Tested-by: Huang, Ying <ying.huang@intel.com>
> Signed-off-by: Filipe Manana <fdmanana@suse.com>
> ---
>   fs/btrfs/file.c         |  9 ++++++---
>   fs/btrfs/ordered-data.c | 14 ++++++++++++++
>   fs/btrfs/ordered-data.h |  3 +++
>   3 files changed, 23 insertions(+), 3 deletions(-)
>
> diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
> index 309dd57..379275c 100644
> --- a/fs/btrfs/file.c
> +++ b/fs/btrfs/file.c
> @@ -1878,6 +1878,7 @@ int btrfs_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
>   	struct btrfs_log_ctx ctx;
>   	int ret = 0;
>   	bool full_sync = 0;
> +	const u64 len = end - start + 1;
>
>   	trace_btrfs_sync_file(file, datasync);
>
> @@ -1906,7 +1907,7 @@ int btrfs_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
>   		 * all extents are persisted and the respective file extent
>   		 * items are in the fs/subvol btree.
>   		 */
> -		ret = btrfs_wait_ordered_range(inode, start, end - start + 1);
> +		ret = btrfs_wait_ordered_range(inode, start, len);
>   	} else {
>   		/*
>   		 * Start any new ordered operations before starting to log the
> @@ -1978,8 +1979,10 @@ int btrfs_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
>   	 */
>   	smp_mb();
>   	if (btrfs_inode_in_log(inode, root->fs_info->generation) ||
> -	    (full_sync && BTRFS_I(inode)->last_trans <=
> -	     root->fs_info->last_trans_committed)) {
> +	    (BTRFS_I(inode)->last_trans <=
> +	     root->fs_info->last_trans_committed &&
> +	     (full_sync ||
> +	      !btrfs_have_ordered_extents_in_range(inode, start, len)))) {
>   		/*
>   		 * We'v had everything committed since the last time we were
>   		 * modified so clear this flag in case it was set for whatever
> diff --git a/fs/btrfs/ordered-data.c b/fs/btrfs/ordered-data.c
> index 157cc54..72b6f0d 100644
> --- a/fs/btrfs/ordered-data.c
> +++ b/fs/btrfs/ordered-data.c
> @@ -838,6 +838,20 @@ out:
>   	return entry;
>   }
>
> +bool btrfs_have_ordered_extents_in_range(struct inode *inode,
> +					 u64 file_offset,
> +					 u64 len)
> +{
> +	struct btrfs_ordered_extent *oe;
> +
> +	oe = btrfs_lookup_ordered_range(inode, file_offset, len);
> +	if (oe) {
> +		btrfs_put_ordered_extent(oe);
> +		return true;
> +	}
> +	return false;
> +}
> +
>   /*
>    * lookup and return any extent before 'file_offset'.  NULL is returned
>    * if none is found
> diff --git a/fs/btrfs/ordered-data.h b/fs/btrfs/ordered-data.h
> index e96cd4c..9ba7209 100644
> --- a/fs/btrfs/ordered-data.h
> +++ b/fs/btrfs/ordered-data.h
> @@ -191,6 +191,9 @@ btrfs_lookup_first_ordered_extent(struct inode * inode, u64 file_offset);
>   struct btrfs_ordered_extent *btrfs_lookup_ordered_range(struct inode *inode,
>   							u64 file_offset,
>   							u64 len);
> +bool btrfs_have_ordered_extents_in_range(struct inode *inode,
> +					 u64 file_offset,
> +					 u64 len);
>   int btrfs_ordered_update_i_size(struct inode *inode, u64 offset,
>   				struct btrfs_ordered_extent *ordered);
>   int btrfs_find_ordered_sum(struct inode *inode, u64 offset, u64 disk_bytenr,
>