Re: [f2fs-dev] [PATCH v2] f2fs: avoid congestion_wait when do_checkpointfor better performance

[Gu Zheng]

Hi Yuan,
On 10/08/2013 07:30 PM, Yuan Zhong wrote:

> Hi Gu,
> 
>> Hi Yuan,
>> On 10/08/2013 04:30 PM, Yuan Zhong wrote:
> 
>>> Previously, do_checkpoint() will call congestion_wait() for waiting the pages (previous submitted node/meta/data pages) to be written back.
>>> Because congestion_wait() will set a regular period (e.g. HZ / 50 ) for waiting.
>>> For this reason, there is a situation that after the pages have been written back, 
>>> but the checkpoint thread still wait for congestion_wait to exit.
> 
>> How do you confirm this issue? 
> 
>   I traced the execution path.
>   In f2fs_end_io_write, dec_page_count(p->sbi, F2FS_WRITEBACK) will be called.
>   And I found that, when pages of F2FS_WRITEBACK has been zero, but
>   checkpoint thread still congestion_wait for pages of F2FS_WRITEBACK to be zero.

Yes, it maybe. Congestion_wait add the task to a global wait queue which related to
all back devices, so if F2FS_WRITEBACK has been zero, but other io may be still going on.
Anyway, using a private wait queue to hold is a better choose.:)

	
>   So, I think this point could be improved.
>   And I wrote a simple test case and tested on Micro-SD card, the steps as following:
>       (a) create a fixed-size file (4KB)
>       (b) go on to sync the file 
>       (c) go back to step #a (fixed numbers of cycling:1024)	
>    The results indicated that the execution time is reduced greatly by using this patch.

Yes, the change is an improvement if the issue is existent.

  
> 
> 
>> I suspect that the block-core does not have a wake-up mechanism
>> when the back device is uncongested.
> 
> 
>   Yes, you are right.
>   So I wake up the checkpoint thread by myself, when pages of F2FS_WRITEBACK to be zero.
>   In f2fs_end_io_write, f2fs_writeback_wait is called.
>   you cloud find this code in my patch. 

Saw it.:)
But one problem is that the checkpoint routine always is singleton, so the wait queue just
services only one body, it seems not very worthy. How about just schedule and wake up it
directly? See the following one.

Signed-off-by: Gu Zheng <guz.fnst@xxxxxxxxxxxxxx>
---
 fs/f2fs/checkpoint.c |   11 +++++++++--
 fs/f2fs/f2fs.h       |    1 +
 fs/f2fs/segment.c    |    4 ++++
 3 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
index d808827..2a5999d 100644
--- a/fs/f2fs/checkpoint.c
+++ b/fs/f2fs/checkpoint.c
@@ -757,8 +757,15 @@ static void do_checkpoint(struct f2fs_sb_info *sbi, bool is_umount)
 	f2fs_put_page(cp_page, 1);
 
 	/* wait for previous submitted node/meta pages writeback */
-	while (get_pages(sbi, F2FS_WRITEBACK))
-		congestion_wait(BLK_RW_ASYNC, HZ / 50);
+	sbi->cp_task = current;
+	while (get_pages(sbi, F2FS_WRITEBACK)) {
+		set_current_state(TASK_UNINTERRUPTIBLE);
+		if (!get_pages(sbi, F2FS_WRITEBACK))
+			break;
+		io_schedule();
+	}
+	__set_current_state(TASK_RUNNING);
+	sbi->cp_task = NULL;
 
 	filemap_fdatawait_range(sbi->node_inode->i_mapping, 0, LONG_MAX);
 	filemap_fdatawait_range(sbi->meta_inode->i_mapping, 0, LONG_MAX);
diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index a955a59..408ace7 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -365,6 +365,7 @@ struct f2fs_sb_info {
 	struct mutex writepages;		/* mutex for writepages() */
 	int por_doing;				/* recovery is doing or not */
 	int on_build_free_nids;			/* build_free_nids is doing */
+	struct task_struct *cp_task;		/* checkpoint task */
 
 	/* for orphan inode management */
 	struct list_head orphan_inode_list;	/* orphan inode list */
diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
index bd79bbe..3b20359 100644
--- a/fs/f2fs/segment.c
+++ b/fs/f2fs/segment.c
@@ -597,6 +597,10 @@ static void f2fs_end_io_write(struct bio *bio, int err)
 
 	if (p->is_sync)
 		complete(p->wait);
+
+	if (!get_pages(p->sbi, F2FS_WRITEBACK) && p->sbi->cp_task)
+		wake_up_process(p->sbi->cp_task);
+
 	kfree(p);
 	bio_put(bio);
 }
-- 
1.7.7

Regards,
Gu 

> 
> 
>>> This is a problem here, especially, when sync a large number of small files or dirs.
>>> In order to avoid this, a wait_list is introduced, 
>>> the checkpoint thread will be dropped into the wait_list if the pages have not been written back, 
>>> and will be waked up by contrast.
> 
>> Please pay some attention to the mail form, this mail is out of format in my mail client.
> 
>> Regards,
>> Gu
> 
> Regards,
> Yuan
> 
>>>
>>> Signed-off-by: Yuan Zhong <yuan.mark.zhong@xxxxxxxxxxx>
>>> ---  
>>>  fs/f2fs/checkpoint.c |    3 +--
>>>  fs/f2fs/f2fs.h       |   19 +++++++++++++++++++
>>>  fs/f2fs/segment.c    |    1 +
>>>  fs/f2fs/super.c      |    1 +
>>>  4 files changed, 22 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
>>> index ca39442..5d69ae0 100644
>>> --- a/fs/f2fs/checkpoint.c
>>> +++ b/fs/f2fs/checkpoint.c
>>> @@ -758,8 +758,7 @@ static void do_checkpoint(struct f2fs_sb_info *sbi, bool is_umount)
>>>  	f2fs_put_page(cp_page, 1);
>>>  
>>>  	/* wait for previous submitted node/meta pages writeback */
>>> -	while (get_pages(sbi, F2FS_WRITEBACK))
>>> -		congestion_wait(BLK_RW_ASYNC, HZ / 50);
>>> +	f2fs_writeback_wait(sbi);
>>>  
>>>  	filemap_fdatawait_range(sbi->node_inode->i_mapping, 0, LONG_MAX);
>>>  	filemap_fdatawait_range(sbi->meta_inode->i_mapping, 0, LONG_MAX);
>>> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
>>> index 7fd99d8..4b0d70e 100644
>>> --- a/fs/f2fs/f2fs.h
>>> +++ b/fs/f2fs/f2fs.h
>>> @@ -18,6 +18,8 @@
>>>  #include <linux/crc32.h>
>>>  #include <linux/magic.h>
>>>  #include <linux/kobject.h>
>>> +#include <linux/wait.h>
>>> +#include <linux/sched.h>
>>>  
>>>  /*
>>>   * For mount options
>>> @@ -368,6 +370,7 @@ struct f2fs_sb_info {
>>>  	struct mutex fs_lock[NR_GLOBAL_LOCKS];	/* blocking FS operations */
>>>  	struct mutex node_write;		/* locking node writes */
>>>  	struct mutex writepages;		/* mutex for writepages() */
>>> +	wait_queue_head_t writeback_wqh;	/* wait_queue for writeback */
>>>  	unsigned char next_lock_num;		/* round-robin global locks */
>>>  	int por_doing;				/* recovery is doing or not */
>>>  	int on_build_free_nids;			/* build_free_nids is doing */
>>> @@ -961,6 +964,22 @@ static inline int f2fs_readonly(struct super_block *sb)
>>>  	return sb->s_flags & MS_RDONLY;
>>>  }
>>>  
>>> +static inline void f2fs_writeback_wait(struct f2fs_sb_info *sbi)
>>> +{
>>> +	DEFINE_WAIT(wait);
>>> +
>>> +	prepare_to_wait(&sbi->writeback_wqh, &wait, TASK_UNINTERRUPTIBLE);
>>> +	if (get_pages(sbi, F2FS_WRITEBACK))
>>> +		io_schedule();
>>> +	finish_wait(&sbi->writeback_wqh, &wait);
>>> +}
>>> +
>>> +static inline void f2fs_writeback_wake(struct f2fs_sb_info *sbi)
>>> +{
>>> +	if (!get_pages(sbi, F2FS_WRITEBACK))
>>> +		wake_up_all(&sbi->writeback_wqh);
>>> +}
>>> +
>>>  /*
>>>   * file.c
>>>   */
>>> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
>>> index bd79bbe..0708aa9 100644
>>> --- a/fs/f2fs/segment.c
>>> +++ b/fs/f2fs/segment.c
>>> @@ -597,6 +597,7 @@ static void f2fs_end_io_write(struct bio *bio, int err)
>>>  
>>>  	if (p->is_sync)
>>>  		complete(p->wait);
>>> +	f2fs_writeback_wake(p->sbi);
>>>  	kfree(p);
>>>  	bio_put(bio);
>>>  }
>>> diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
>>> index 094ccc6..3ac6d85 100644
>>> --- a/fs/f2fs/super.c
>>> +++ b/fs/f2fs/super.c
>>> @@ -835,6 +835,7 @@ static int f2fs_fill_super(struct super_block *sb, void *data, int silent)
>>>  	mutex_init(&sbi->gc_mutex);
>>>  	mutex_init(&sbi->writepages);
>>>  	mutex_init(&sbi->cp_mutex);
>>> +	init_waitqueue_head(&sbi->writeback_wqh);
>>>  	for (i = 0; i < NR_GLOBAL_LOCKS; i++)
>>>  		mutex_init(&sbi->fs_lock[i]);
>>>  	mutex_init(&sbi->node_write);N‹§²æìr¸›yúèšØb²X¬¶Ç§vØ^–)Þº{.nÇ+‰·¥Š{±‘êçzX§¶›¡Ü¨}©ž²Æ zÚ&j:+v‰¨¾«‘êçzZ+€Ê+zf£¢·hšˆ§~†­†Ûiÿûàz¹®w¥¢¸?™¨è­Ú&¢)ߢf”ù^jÇ«y§m…á@A«a¶Úÿ0¶ìh®å’i


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