Re: [f2fs-dev] [PATCH 3/7] f2fs: drop any block plugging

[Jaegeuk Kim]

On Sat, Jul 09, 2016 at 10:28:49AM +0800, Chao Yu wrote:
> Hi Jaegeuk,
> 
> On 2016/6/9 1:24, Jaegeuk Kim wrote:
> > In f2fs, we don't need to keep block plugging for NODE and DATA writes, since
> > we already merged bios as much as possible.
> 
> IMO, we can not remove block plug, this is because there are still many
> conditions which stops us merging r/w IOs into one bio as we expect,
> theoretically, block plug can hold bios as much as possible, then submitting
> them into queue in batch, it will reduce racing of grabbing queue->lock during
> bio submitting, if we drop them, when syncing nodes or flushing datas, we will
> suffer more lock racing.
> 
> Or there are something I am missing, do you suffer any performance issue on
> block plug?

In the latest patch, I've turned off plugging forcefully, only if the underlying
device is SMR drive.
And, still I removed other block plugging, since I couldn't see any performance
regression. Even in some workloads, I could have seen some inverted IOs due to
race condition between plugged and unplugged IOs.

Thanks,

> 
> Thanks,
> 
> > 
> > Signed-off-by: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
> > ---
> >  fs/f2fs/checkpoint.c |  4 ----
> >  fs/f2fs/data.c       | 17 ++++++++++-------
> >  fs/f2fs/gc.c         |  5 -----
> >  fs/f2fs/segment.c    |  7 +------
> >  4 files changed, 11 insertions(+), 22 deletions(-)
> > 
> > diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
> > index 5ddd15c..4179c7b 100644
> > --- a/fs/f2fs/checkpoint.c
> > +++ b/fs/f2fs/checkpoint.c
> > @@ -897,11 +897,8 @@ static int block_operations(struct f2fs_sb_info *sbi)
> >  		.nr_to_write = LONG_MAX,
> >  		.for_reclaim = 0,
> >  	};
> > -	struct blk_plug plug;
> >  	int err = 0;
> >  
> > -	blk_start_plug(&plug);
> > -
> >  retry_flush_dents:
> >  	f2fs_lock_all(sbi);
> >  	/* write all the dirty dentry pages */
> > @@ -938,7 +935,6 @@ retry_flush_nodes:
> >  		goto retry_flush_nodes;
> >  	}
> >  out:
> > -	blk_finish_plug(&plug);
> >  	return err;
> >  }
> >  
> > diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> > index 30dc448..5f655d0 100644
> > --- a/fs/f2fs/data.c
> > +++ b/fs/f2fs/data.c
> > @@ -98,10 +98,13 @@ static struct bio *__bio_alloc(struct f2fs_sb_info *sbi, block_t blk_addr,
> >  }
> >  
> >  static inline void __submit_bio(struct f2fs_sb_info *sbi, int rw,
> > -						struct bio *bio)
> > +			struct bio *bio, enum page_type type)
> >  {
> > -	if (!is_read_io(rw))
> > +	if (!is_read_io(rw)) {
> >  		atomic_inc(&sbi->nr_wb_bios);
> > +		if (current->plug && (type == DATA || type == NODE))
> > +			blk_finish_plug(current->plug);
> > +	}
> >  	submit_bio(rw, bio);
> >  }
> >  
> > @@ -117,7 +120,7 @@ static void __submit_merged_bio(struct f2fs_bio_info *io)
> >  	else
> >  		trace_f2fs_submit_write_bio(io->sbi->sb, fio, io->bio);
> >  
> > -	__submit_bio(io->sbi, fio->rw, io->bio);
> > +	__submit_bio(io->sbi, fio->rw, io->bio, fio->type);
> >  	io->bio = NULL;
> >  }
> >  
> > @@ -235,7 +238,7 @@ int f2fs_submit_page_bio(struct f2fs_io_info *fio)
> >  		return -EFAULT;
> >  	}
> >  
> > -	__submit_bio(fio->sbi, fio->rw, bio);
> > +	__submit_bio(fio->sbi, fio->rw, bio, fio->type);
> >  	return 0;
> >  }
> >  
> > @@ -1040,7 +1043,7 @@ got_it:
> >  		 */
> >  		if (bio && (last_block_in_bio != block_nr - 1)) {
> >  submit_and_realloc:
> > -			__submit_bio(F2FS_I_SB(inode), READ, bio);
> > +			__submit_bio(F2FS_I_SB(inode), READ, bio, DATA);
> >  			bio = NULL;
> >  		}
> >  		if (bio == NULL) {
> > @@ -1083,7 +1086,7 @@ set_error_page:
> >  		goto next_page;
> >  confused:
> >  		if (bio) {
> > -			__submit_bio(F2FS_I_SB(inode), READ, bio);
> > +			__submit_bio(F2FS_I_SB(inode), READ, bio, DATA);
> >  			bio = NULL;
> >  		}
> >  		unlock_page(page);
> > @@ -1093,7 +1096,7 @@ next_page:
> >  	}
> >  	BUG_ON(pages && !list_empty(pages));
> >  	if (bio)
> > -		__submit_bio(F2FS_I_SB(inode), READ, bio);
> > +		__submit_bio(F2FS_I_SB(inode), READ, bio, DATA);
> >  	return 0;
> >  }
> >  
> > diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
> > index 4a03076..67fd285 100644
> > --- a/fs/f2fs/gc.c
> > +++ b/fs/f2fs/gc.c
> > @@ -777,7 +777,6 @@ static int do_garbage_collect(struct f2fs_sb_info *sbi,
> >  {
> >  	struct page *sum_page;
> >  	struct f2fs_summary_block *sum;
> > -	struct blk_plug plug;
> >  	unsigned int segno = start_segno;
> >  	unsigned int end_segno = start_segno + sbi->segs_per_sec;
> >  	int seg_freed = 0;
> > @@ -795,8 +794,6 @@ static int do_garbage_collect(struct f2fs_sb_info *sbi,
> >  		unlock_page(sum_page);
> >  	}
> >  
> > -	blk_start_plug(&plug);
> > -
> >  	for (segno = start_segno; segno < end_segno; segno++) {
> >  		/* find segment summary of victim */
> >  		sum_page = find_get_page(META_MAPPING(sbi),
> > @@ -830,8 +827,6 @@ static int do_garbage_collect(struct f2fs_sb_info *sbi,
> >  		f2fs_submit_merged_bio(sbi,
> >  				(type == SUM_TYPE_NODE) ? NODE : DATA, WRITE);
> >  
> > -	blk_finish_plug(&plug);
> > -
> >  	if (gc_type == FG_GC) {
> >  		while (start_segno < end_segno)
> >  			if (get_valid_blocks(sbi, start_segno++, 1) == 0)
> > diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> > index 7b58bfb..eff046a 100644
> > --- a/fs/f2fs/segment.c
> > +++ b/fs/f2fs/segment.c
> > @@ -379,13 +379,8 @@ void f2fs_balance_fs_bg(struct f2fs_sb_info *sbi)
> >  			excess_prefree_segs(sbi) ||
> >  			excess_dirty_nats(sbi) ||
> >  			(is_idle(sbi) && f2fs_time_over(sbi, CP_TIME))) {
> > -		if (test_opt(sbi, DATA_FLUSH)) {
> > -			struct blk_plug plug;
> > -
> > -			blk_start_plug(&plug);
> > +		if (test_opt(sbi, DATA_FLUSH))
> >  			sync_dirty_inodes(sbi, FILE_INODE);
> > -			blk_finish_plug(&plug);
> > -		}
> >  		f2fs_sync_fs(sbi->sb, true);
> >  		stat_inc_bg_cp_count(sbi->stat_info);
> >  	}
> >